What is the crop coefficient (Kc) in irrigation?

The crop coefficient (Kc) is a multiplier that adjusts reference ET (ETo) to the actual water needs of a specific plant type. Cool-season turf grasses (tall fescue, Kentucky bluegrass, ryegrass) have a Kc of approximately 0.80, meaning they use 80 percent of the reference ET rate. Warm-season turf (bermuda, zoysia, St. Augustine) has a Kc around 0.65 because they are more drought-tolerant and water-use-efficient. Ornamental shrubs and established landscape plants typically have Kc values of 0.40 to 0.50. Vegetable gardens have Kc values of 0.80 to 1.0 depending on crop type and growth stage. The watering runtime calculator applies the appropriate Kc for your selected plant type automatically.

What is cycle-and-soak irrigation scheduling?

Cycle-and-soak is an irrigation scheduling technique used when sprinkler precipitation rate exceeds soil infiltration rate. When sprinklers apply water faster than soil can absorb it, water pools on the surface and runs off, especially on slopes, causing water waste and erosion. The cycle-and-soak method breaks the total runtime into shorter cycles with rest periods between them, allowing water to infiltrate before the next cycle begins. For example: if clay soil absorbs water at 0.15 inches per hour and fixed spray heads deliver 1.5 inches per hour, the sprinklers apply water 10 times faster than soil can absorb it. A 30-minute total runtime would need 3 cycles of 10 minutes each, with 45-minute soak periods between cycles. The calculator identifies when cycle-and-soak is needed and calculates the cycle length and soak time.

What ET rate should I use for my region?

Peak summer reference ET (ETo) rates by US region: Desert Southwest (Arizona, Nevada, inland Southern California) 0.28 to 0.35 inches per day, Southern California Coast 0.12 to 0.16 inches per day, Mountain West (Colorado, Utah, Idaho) 0.22 to 0.28 inches per day, Pacific Northwest (Washington, Oregon) 0.10 to 0.15 inches per day, Texas and Oklahoma 0.22 to 0.28 inches per day, Central Plains (Kansas, Nebraska, Iowa) 0.20 to 0.25 inches per day, Southeast and Gulf Coast 0.18 to 0.22 inches per day, Florida 0.16 to 0.20 inches per day, Northeast 0.13 to 0.17 inches per day, and Mid-Atlantic 0.15 to 0.19 inches per day. These are peak summer values; spring and fall ET is roughly 60 to 65 percent of the summer peak. The ET reference presets in the calculator use the middle values of these ranges.

How do I measure my sprinkler precipitation rate at home?

The catch-can test is the standard method: place 6 to 8 straight-sided containers (tuna cans are ideal because they have uniform depth and are free) in different locations within the irrigation zone, spacing them evenly. Run the zone for 15 minutes at the same time of day you normally irrigate. Measure the water depth in each can using a ruler, average the measurements, and multiply by 4 to get inches per hour. Example: average can depth after 15 minutes = 0.35 inches. Precipitation rate = 0.35 x 4 = 1.40 inches per hour. If can depths vary widely (say, 0.2 to 0.6 inches), your zone has a distribution uniformity problem and you should consider replacing spray heads or adjusting coverage overlap. Test in no-wind conditions for accurate results.

How many gallons per week does my lawn need?

To calculate weekly gallons: multiply the weekly irrigation depth in inches by your zone area in square feet and by 0.623 (the gallons equivalent of 1 inch of water over 1 square foot). Weekly irrigation depth = daily ET times 7 days (adjusted for plant type using the crop coefficient). For a 2,000 square foot cool-season turf zone in Denver with an ET rate of 0.24 inches per day: weekly need = 0.24 x 0.80 (turf Kc) x 7 = 1.34 inches. Gallons per week = 1.34 x 2,000 x 0.623 = 1,670 gallons. The watering runtime calculator computes this automatically once you enter your zone area, plant type, ET rate, and sprinkler precipitation rate.

Should I water less in spring and fall than summer?

Yes, ET rates in spring and fall are typically 60 to 70 percent of peak summer values, because days are shorter, temperatures are lower, and humidity is higher. If your summer lawn ET is 0.20 inches per day, your spring and fall ET is approximately 0.13 to 0.14 inches per day. Smart irrigation controllers (WiFi-enabled controllers using weather data) adjust automatically for these seasonal changes. If you use a manual timer, reduce run times by 30 to 40 percent in spring and fall compared to your summer schedule. Most US landscape professionals recommend a spring and fall audit of irrigation schedules to avoid overwatering, which promotes fungal disease and shallow root systems in cooler months.

What soil type do I have and why does it matter for irrigation?

The four main US soil texture types and their water characteristics: Sandy soil absorbs water very quickly (1.0 inch per hour infiltration rate) but has low water-holding capacity, requiring more frequent irrigation of shallower depth. Loam soil (the ideal garden soil) absorbs at 0.5 inches per hour and holds water well. Clay loam absorbs at 0.25 inches per hour and holds water for longer periods, needing less frequent but deeper irrigation. Clay soil absorbs very slowly at 0.10 to 0.20 inches per hour and is highly prone to runoff when sprinklers apply water faster than clay can absorb it, making cycle-and-soak irrigation essential. Check the USDA NRCS Web Soil Survey at websoilsurvey.nrcs.usda.gov to find your soil type from satellite-based soil mapping data.

What is the best time of day to run irrigation?

The best time to run irrigation in the US is between 4:00 AM and 8:00 AM. Early morning provides the most efficient irrigation because wind speeds are lowest (less drift and evaporation loss), temperatures are cooler (less evaporation from soil surface immediately after watering), foliage has time to dry before nightfall (reducing fungal disease pressure), and water pressure is typically highest in municipal systems before peak demand hours begin at 7 AM. Avoid irrigating after 10 AM when evaporation rates rise and wind increases, and avoid late afternoon or evening irrigation, which leaves foliage wet overnight and dramatically increases fungal disease risk, particularly powdery mildew and gray mold on turf and ornamentals.

What is a smart irrigation controller and should I get one?

A smart irrigation controller (also called a WiFi controller or weather-based controller) connects to local weather data or an onsite sensor and automatically adjusts irrigation schedules based on actual ET conditions. When it rains, it skips scheduled watering. During a heat wave, it increases runtime. EPA WaterSense-labeled controllers have been tested to deliver the right amount of water, and rebates for smart controllers are available from many US water utilities (typically 50 to 150 dollars off). Popular US brands include Rachio, Rain Bird WiFi, and Hunter Hydrawise. Studies by the US EPA estimate smart controllers reduce outdoor water use by 15 to 30 percent compared to manual timers, which is significant since outdoor irrigation represents 30 to 60 percent of residential water consumption in many US states.

💧 ET-Based • Crop Coefficients • Cycle-and-Soak • 13 US Regions • Season Adjusted

Watering Runtime and ET Calculator: Minutes, Gallons, and Schedule

Enter your irrigation zone area, plant type, sprinkler type, and regional ET rate to get exact minutes per session, weekly gallons, efficiency rating, and cycle-and-soak schedule for clay soils. Built on evapotranspiration science. Free, no signup.

🌡️ 13 US Region ET Presets 🌿 Crop Coefficient Adjustment 📅 3 Season Modes 🚰 Cycle-and-Soak 📊 Efficiency Rating 📄 PDF Schedule

💧 Watering Runtime and ET Calculator

Select your US region and plant type, choose sprinkler type, enter ET rate, and get runtime per session, weekly gallons, and a cycle-and-soak schedule if your soil needs it.

Current Season

Zone Area (sq ft) Measure the area covered by this irrigation zone or sprinkler.

Plant / Zone Type

Sprinkler Type

Sprinkler Precipitation Rate (in/hr) Auto-filled by type above. Override with catch-can test result.

US Region (fills ET rate below)

Daily Reference ET Rate (in/day) Summer peak value from region above, or find at your state’s Cooperative Extension.

Soil Type Clay soils need cycle-and-soak. Check NRCS Web Soil Survey for your soil type.

Watering Days per Week

💧

Your Runtime and Schedule Will Appear Here

Select your region, enter zone area and ET rate, then click Calculate.

Irrigation Science for US Homeowners

Why Evapotranspiration Is the Key to Efficient US Lawn Irrigation

Most American homeowners set their irrigation timers based on what looks right or what the previous owner did, then never change the schedule. The result is an irrigation program that may be perfect for one week in June but wildly wrong for October, for a drought year, and for the actual plant types in each zone. The alternative is ET-based irrigation: setting runtimes based on how much water the landscape is actually losing, calculated from real weather conditions.

Evapotranspiration (ET) is the combined rate of water loss from soil evaporation and plant transpiration. On a hot, dry, windy August day in Phoenix, ET might reach 0.35 inches of water per day. On a cool, overcast October day in Seattle, ET might be 0.05 inches. When you know the ET rate, you know exactly how much water your landscape needs to replace, and you can calculate how long your sprinklers need to run to deliver that exact amount based on their precipitation rate.

The core formula: Runtime (minutes) = Target water depth (in) / Sprinkler precipitation rate (in/hr) x 60. Target depth = Daily plant ET x days between sessions. Plant ET = Reference ET x crop coefficient (Kc). The calculator handles all three steps simultaneously.

Reference ET vs Plant ET and Crop Coefficients

Reference ET (ETo or ET0) is calculated from weather station data using a standardized equation (the Penman-Monteith equation is the current ASCE and USDA standard) for a well-watered cool-season reference crop. This gives a universal benchmark comparable across US weather stations. Actual plant water needs are calculated by multiplying reference ET by a plant-specific crop coefficient (Kc).

Cool-season turf grasses like tall fescue and Kentucky bluegrass have a Kc of approximately 0.80, meaning they use 80 percent of reference ET. Warm-season grasses (bermuda, zoysia, St. Augustine) are more water-efficient with a Kc around 0.65. Established shrubs and ornamental landscape plants have Kc values of 0.40 to 0.50 because they are often drought-adapted and have lower water needs per square foot than actively growing turf. Vegetable gardens in peak production have Kc values of 0.80 to 1.05 because vegetables are actively transpiring large volumes of water through their leaf surface throughout the growing day.

Sprinkler Precipitation Rates: The Other Half of the Formula

The runtime formula needs two data points: how much water to apply (from ET) and how fast your sprinklers apply it (precipitation rate). Fixed spray heads are the most common in US residential lawns and deliver approximately 1.0 to 2.0 inches per hour: they are fast and efficient for small zones but prone to runoff on slopes and clay soils. Rotary heads (MP Rotators, Hunter rotors, Rain Bird rotary nozzles) deliver water much more slowly (0.4 to 0.9 inches per hour) at lower precipitation rates that match the infiltration rate of most soil types better, dramatically reducing runoff. In the US Southwest, the shift from fixed spray to rotary nozzles is a common water conservation upgrade subsidized by utilities.

The most accurate way to know your precipitation rate is the catch-can test: place several tuna cans in the zone, run for 15 minutes, measure the average water depth, and multiply by 4. This reveals both your precipitation rate and your distribution uniformity (how evenly water is applied across the zone), which is just as important as the total amount delivered.

Cycle-and-Soak: The Clay Soil Solution

Clay soils have a very low water infiltration rate (0.10 to 0.25 inches per hour), while most sprinklers apply water at 0.6 to 2.0 inches per hour. When sprinklers apply water faster than clay soil can absorb it, the water pools on the surface and runs off downslope, carrying soil and fertilizer with it. On flat lawns, pooled water that does not run off sits on the surface and creates anaerobic conditions near the root zone.

The cycle-and-soak method breaks total irrigation runtime into multiple short cycles (typically 5 to 15 minutes each) with 30 to 60 minute rest periods between cycles. During the rest period, surface water infiltrates into the clay soil profile before the next cycle begins. This technique dramatically reduces runoff on clay-heavy soils and slopes, and most modern irrigation controllers have a cycle-and-soak or “smart cycle” feature built in. The calculator identifies when your soil and sprinkler combination requires cycle-and-soak and calculates the exact cycle length, number of cycles, and recommended soak time between them.

How the Calculator Works

How the Watering Runtime Calculator Converts ET to Minutes and Gallons

The calculator chains four calculations: reference ET adjustment for season, crop coefficient adjustment for plant type, target depth calculation per session, and conversion from depth to runtime based on sprinkler precipitation rate.

Season Adjustment

The ET rate you enter is assumed to be the peak summer rate for your region. Spring and fall ET is typically 65 percent of the summer peak (shorter days, cooler temperatures, more cloud cover). Winter ET is approximately 30 percent of summer peak in mild climates. Select the current season toggle before calculating, and the calculator applies the appropriate seasonal factor to the ET rate automatically.

Crop Coefficient Calculation

Plant ET (in/day) = Reference ET (adjusted for season) times Kc for selected plant type. For a cool-season lawn at 0.20 inches/day summer ET in spring: plant ET = 0.20 times 0.65 (season) times 0.80 (Kc) = 0.104 inches/day. This is the actual water demand for this type of plant in this season.

Target Depth and Runtime

Target depth per session (inches) = plant ET times days between sessions (7 divided by watering days per week). Runtime = target depth divided by precipitation rate, times 60 minutes per hour.

Gallons per Week

Gallons per week = weekly applied depth (inches) times zone area (sq ft) times 0.623. The 0.623 factor converts the unit “square feet times inches” to gallons, because 1 inch of water over 1 square foot is 0.0833 cubic feet, and 1 cubic foot of water is 7.48 gallons, giving 0.0833 times 7.48 = 0.623 gallons.

Cycle-and-Soak Calculation

If your sprinkler precipitation rate exceeds the soil’s infiltration rate (standard for fixed spray on clay), the calculator triggers cycle-and-soak. Maximum cycle length = (soil infiltration rate divided by sprinkler precipitation rate) times 60 minutes. Number of cycles = ceiling of (total runtime divided by max cycle length). Soak time is set to 30 to 60 minutes depending on soil type.

Real US Irrigation Examples

Three Real US Irrigation Zone Examples Calculated by ET Rate

Three common US residential irrigation scenarios showing the calculator applied to real zone types and regional climates.

Tucson AZ, Peak Summer, Clay Soil

Cool-Season Turf, Fixed Spray, 3x/Week

Zone area1,500 sq ft

Reference ET0.32 in/day (desert)

Plant ET (Kc 0.80)0.256 in/day

Target depth0.60 in every 2.3 days

Runtime per session24 min

Weekly gallons1,687 gal

Cycle-and-soak10 min x 3 cycles, 45 min soak

Nashville TN, Summer, Loam Soil

Vegetable Garden, Drip Emitters, 3x/Week

Zone area400 sq ft

Reference ET0.19 in/day (Appalachian)

Plant ET (Kc 0.85)0.162 in/day

Target depth0.38 in every 2.3 days

Precip rate (drip)0.20 in/hr

Runtime per session114 min

Weekly gallons288 gal

Portland OR, Spring, Sandy Loam

Established Shrubs, Rotary Heads, 2x/Week

Zone area800 sq ft

Reference ET (spring)0.12 x 0.65 = 0.078 in/day

Plant ET (Kc 0.45)0.035 in/day

Target depth0.123 in every 3.5 days

Precip rate (rotary)0.60 in/hr

Runtime per session12 min

Weekly gallons49 gal

Expert Irrigation Tips

Six Expert Tips for Efficient Irrigation Scheduling in US Climates

Run the Catch-Can Test Before Setting Any Timer

Before you set or adjust any irrigation timer, spend 20 minutes running the catch-can test: place 6 to 8 tuna cans evenly across each zone, run the system for 15 minutes, and measure the average water depth. Multiply by 4 for inches per hour. This single test tells you both your actual precipitation rate (to plug into this calculator) and your distribution uniformity. If some cans show 0.3 inches and others show 0.8 inches, your coverage is uneven and adding more runtime will just over-water the already-wet areas while under-watering the dry ones. Fix coverage problems with head adjustments or new nozzles before optimizing runtime.

Find Your Real ET Rate From State Extension Data

The most accurate ET data for your location comes from your state’s Cooperative Extension irrigation schedule publications or a nearby agricultural weather station. California has CIMIS (cimis.water.ca.gov), Colorado has CoAgMet, Texas has Texas ET Network (texaset.tamu.edu), and many other states have similar public resources. These networks publish daily and weekly ET data by weather station. Using real local ET data rather than regional averages can improve irrigation precision by 20 to 30 percent. The EPA WaterSense website (epa.gov/watersense) lists resources for finding ET data in every US state.

Reduce Spring and Winter Runtimes by 35 Percent

Most US homeowners set their irrigation timer in May and leave it unchanged through October. This single habit wastes hundreds of gallons weekly, especially in September and October when days shorten, temperatures drop, and ET falls to 60 to 70 percent of the summer peak. Use the season toggle in this calculator to see your spring and fall runtimes alongside your summer schedule. Then actually adjust your controller. Even better: invest in a smart controller that adjusts automatically. Water utilities in many US cities offer rebates of 50 to 150 dollars for certified smart controllers, often enough to cover most of the purchase price.

Consider Upgrading Spray to Rotary Nozzles on Clay Soils

If the calculator shows cycle-and-soak is required for your soil and sprinkler combination, the most permanent fix is replacing fixed spray nozzles with rotary (MP Rotator-style) nozzles, which apply water at 0.40 to 0.70 inches per hour rather than 1.0 to 2.0 inches per hour. This lower precipitation rate matches or falls below most soil infiltration rates, eliminating the runoff problem entirely without needing cycle-and-soak programming. Rotary nozzle retrofit kits are available at Home Depot and Lowe’s for about 2 to 5 dollars per nozzle and typically screw directly into existing spray head bodies. Many US water utilities offer free rotary nozzle kits as a conservation measure.

Check Soil Moisture Before Each Irrigation Session

A soil probe or even a simple long screwdriver is the most reliable irrigation feedback tool available. Push it into your lawn or plant bed: if it slides in easily past 4 to 6 inches, the soil has adequate moisture and you can skip the next irrigation session. If it meets resistance at 1 to 2 inches, the soil is dry and watering is needed. This simple physical check catches situations where unexpected rainfall, cool cloudy periods, or seasonal ET changes have made your calculator-based schedule temporarily unnecessary. Skipping even one irrigation session per week during shoulder-season months saves 100 to 300 gallons per zone.

Audit Your Irrigation System Every Spring

A 30-minute annual irrigation audit before setting your seasonal schedule can save 20 to 40 percent of irrigation water. Walk each zone while it runs: look for clogged or broken nozzles that are not rotating or spraying, heads tilted or sunk that are wasting water into soil rather than spray coverage area, and heads positioned to spray pavement, driveways, or fences. Check the rain sensor or smart controller weather station to ensure it is still connected and functioning. Inspect drip emitter zones for clogged emitters (plants wilting despite the system running) and cracked or crushed tubing. The EPA WaterSense program estimates that a professional irrigation audit reduces outdoor water use by an average of 15 percent annually.

Quick Reference

Quick Reference: ET Rates and Sprinkler Types for US Regions

Use this table for quick reference before using the full calculator. ET rates shown are peak summer reference values. Multiply by the crop coefficient for your plant type to get plant-specific daily water demand.

US Region / Climate	Summer ET (in/day)	Common Sprinkler	Typical Precip Rate	Runtime for 0.25″ depth
Desert SW (AZ, NV, inland CA)	0.28 to 0.35	Rotary (water-saving)	0.60 in/hr	25 min
Texas / Oklahoma	0.22 to 0.28	Fixed spray or rotary	1.0 in/hr avg	15 min
Mountain West (CO, UT)	0.22 to 0.26	Rotary / impact	0.60 in/hr	25 min
Central Plains (KS, NE)	0.20 to 0.24	Fixed spray or oscillating	1.0 in/hr avg	15 min
Southeast / Gulf Coast	0.18 to 0.22	Fixed spray	1.5 in/hr	10 min
Mid-Atlantic / Northeast	0.14 to 0.18	Fixed spray or rotary	1.0 in/hr avg	15 min
Pacific Northwest (WA, OR)	0.10 to 0.14	Rotary or drip	0.60 in/hr	25 min
Drip systems (any region)	Use full ET value	Drip emitters	0.20 in/hr	75 min
Crop coefficient (Kc) reference	Cool turf: 0.80	Warm turf: 0.65	Shrubs: 0.45	Veggies: 0.85

Sources: ET reference rates based on USDA NRCS and state Cooperative Extension irrigation scheduling guides. Crop coefficients from University of Maryland Extension turfgrass management publications and University of California Cooperative Extension irrigation management guidelines. Sprinkler precipitation rates are typical values; verify with a catch-can test for your specific equipment. See EPA WaterSense for additional US irrigation efficiency resources.

Frequently Asked Questions

16 FAQs About Watering Runtime, ET, and Irrigation Scheduling

How do I calculate irrigation run time?▼

Runtime (minutes) = Target water depth (in) divided by precipitation rate (in/hr), times 60. Target depth = plant ET per day times days between sessions. For cool-season turf in a moderate US climate: ET = 0.20 in/day, Kc = 0.80, plant ET = 0.16 in/day. Watering every 3 days: target = 0.48 inches. With fixed spray at 1.5 in/hr: runtime = 0.48 / 1.5 x 60 = 19 minutes. The calculator chains all these steps automatically once you enter ET rate, plant type, sprinkler precipitation rate, and watering frequency.

What is evapotranspiration (ET) in irrigation?▼

Evapotranspiration (ET) is the combined rate of water loss from soil evaporation and plant transpiration. It represents the total water the landscape needs to replace each day to maintain healthy plant water status. Reference ET (ETo) is calculated from weather data using the ASCE Penman-Monteith equation and represents the water use of a well-watered cool-season reference crop. To get actual plant water needs, multiply reference ET by the crop coefficient (Kc) for your plant type. ET data for your area is published by your state’s Cooperative Extension irrigation scheduling program or from public weather networks like CIMIS (California) and CoAgMet (Colorado).

What is sprinkler precipitation rate and how do I find mine?▼

Precipitation rate is the depth of water (inches per hour) a sprinkler system delivers. Fixed spray heads: 1.0 to 2.0 in/hr. Rotary or MP Rotator heads: 0.4 to 0.9 in/hr. Oscillating sprinklers: 0.5 to 1.5 in/hr. Impact sprinklers: 0.3 to 0.7 in/hr. Drip systems: 0.1 to 0.3 in/hr. To measure yours: place tuna cans in the zone, run 15 minutes, average the water depth, multiply by 4. This is the most accurate approach and also reveals distribution uniformity problems that affect how well even runtime is distributed across the zone.

What is the crop coefficient (Kc)?▼

The crop coefficient (Kc) adjusts reference ET to the actual water use of a specific plant type. Cool-season turf (fescue, bluegrass): Kc = 0.80. Warm-season turf (bermuda, zoysia): Kc = 0.65. Shrubs and established landscape: Kc = 0.45. Vegetable garden in production: Kc = 0.85. Trees: Kc = 0.50. A landscape with mixed plantings uses an average Kc around 0.65. Multiplying reference ET by the Kc gives the actual daily water demand: at ET = 0.20 in/day and Kc = 0.80, cool-season turf needs 0.20 x 0.80 = 0.16 inches per day to maintain good health. This is the value used to calculate the target depth per irrigation session.

What is cycle-and-soak irrigation?▼

Cycle-and-soak breaks total irrigation runtime into short cycles with rest periods between them, preventing runoff when sprinklers apply water faster than soil can absorb it. Clay soil absorbs water at approximately 0.10 to 0.25 inches per hour, while fixed spray heads deliver 1.0 to 2.0 inches per hour: water applied faster than soil absorbs it runs off, wasting water and carrying fertilizer into storm drains. The fix: run a 10-minute cycle, wait 45 to 60 minutes for water to infiltrate, repeat until total runtime is reached. Most modern controllers have a built-in cycle-and-soak feature (often called Smart Cycle on Rain Bird). The calculator shows maximum cycle length, number of cycles, and soak time for your specific soil and sprinkler combination.

How often should I water my lawn in summer?▼

In most US climates during peak summer, cool-season turf needs watering 2 to 4 times per week. Warm-season turf needs 1 to 3 times per week. Deep, infrequent watering (less frequent, longer sessions) promotes deeper root growth and drought resilience compared to shallow daily watering. In the Desert Southwest (Arizona, Nevada), daily or near-daily watering of cool-season turf may be necessary in July and August when ET exceeds 0.30 inches per day. In the Pacific Northwest and coastal California, 2 times per week is often sufficient even in summer. The calculator shows the efficiency of your chosen frequency relative to actual ET demand, so you can adjust frequency and runtime together to achieve the best balance.

What ET rate should I use for my US region?▼

Peak summer reference ET by region: Desert Southwest (AZ, NV, inland CA) 0.28 to 0.35 in/day. Mountain West (CO, UT, ID) 0.22 to 0.26 in/day. Texas and Oklahoma 0.22 to 0.28 in/day. Central Plains (KS, NE) 0.20 to 0.24 in/day. Southeast and Gulf Coast 0.18 to 0.22 in/day. Northeast 0.13 to 0.17 in/day. Pacific Northwest 0.10 to 0.14 in/day. For more precise values, use your state Cooperative Extension irrigation scheduling guides or a local agricultural weather station ET network. California: CIMIS (cimis.water.ca.gov). Colorado: CoAgMet. Texas: Texas ET Network at Texas A and M (texaset.tamu.edu). Many other states have similar public ET networks.

How many gallons per week does a lawn need?▼

Weekly gallons = weekly water depth (in) x zone area (sq ft) x 0.623. Weekly depth = plant ET per day x 7 days. For a 2,000 sq ft cool-season lawn in Denver (ET 0.24 in/day, Kc 0.80): plant ET = 0.192 in/day, weekly depth = 1.34 inches. Gallons = 1.34 x 2,000 x 0.623 = 1,670 gallons per week. In Phoenix (ET 0.32, same lawn): weekly depth = 1.79 inches, gallons = 2,230 per week. The 0.623 constant converts square feet and inches to gallons: 1 inch over 1 square foot is 0.0833 cubic feet, times 7.48 gallons per cubic foot = 0.623 gallons. Enter your area in the calculator to get the exact weekly gallon total for your zone.

Should I water less in spring and fall?▼

Yes. Spring and fall ET rates are 60 to 70 percent of summer peak values because days are shorter, temperatures cooler, and humidity often higher. If summer runtime is 20 minutes per session, spring and fall runtime should be around 13 minutes. Most homeowners who set their timer in May and leave it unchanged through October are significantly overwatering by September. In spring, soil is often naturally moist from winter precipitation, making irrigation unnecessary or minimal until the soil dries. Smart controllers adjust automatically with weather data. For manual timers, use this calculator’s season toggle to get your spring and fall schedules and update your controller in mid-April and again in late September.

What soil type do I have and why does it matter?▼

Soil type determines how fast water infiltrates and how long it stays available to plant roots. Sandy soil (common in Florida, Gulf Coast, and parts of the Desert Southwest): absorbs water quickly (1.0 in/hr) but drains fast, needing more frequent watering. Loam (the ideal soil for most landscape plants): absorbs at 0.5 in/hr and holds water well. Clay loam and clay (common in the US Southeast, Midwest, and Pacific Northwest lowlands): absorb water very slowly (0.15 to 0.25 in/hr), hold it for a long time, and are highly prone to runoff when sprinklers exceed the infiltration rate. Use the USDA NRCS Web Soil Survey at websoilsurvey.nrcs.usda.gov to find your exact soil type by entering your address.

What is the best time of day to irrigate?▼

Early morning (4:00 AM to 8:00 AM) is the best time for irrigation in all US climates. Reasons: lowest wind speeds reduce drift and evaporation, lower temperatures mean less immediate evaporation loss from soil surface, foliage dries before nightfall (reducing fungal disease risk by 30 to 50 percent), and municipal water pressure is highest before the 7 AM residential demand peak. Avoid irrigating between 10 AM and 6 PM (high evaporation loss and wind drift, especially in the Desert Southwest) and after 8 PM (leaves stay wet all night, dramatically increasing mildew and brown patch disease risk). Program controllers to finish before 8 AM whenever possible.

How do I know if I am overwatering or underwatering?▼

Signs of overwatering: soft spongy turf, mushrooms and algae growth, persistently yellow or pale grass despite fertilization, runoff visible during irrigation, and a soil probe that meets no resistance at any depth. Push a soil probe (or long screwdriver) into the turf: if it slides in easily past 6 inches, moisture is adequate. Signs of underwatering: grass blades folded lengthwise (the drought curl), footprints remaining visible 30 minutes after walking on turf, blue-gray color developing in turf, and a soil probe meeting firm resistance at 1 to 2 inches depth. The efficiency indicator in this calculator shows whether your schedule matches, exceeds, or falls short of your plant’s ET demand, providing a numerical overwatering or underwatering signal.

What is a smart irrigation controller?▼

A smart irrigation controller connects to local weather data (via WiFi using a nearby weather station or an onsite sensor) and automatically adjusts irrigation schedules based on actual ET conditions, rainfall, and soil moisture. Popular US models include Rachio 3, Rain Bird WiFi ST8I, and Hunter Hydrawise. EPA WaterSense-labeled smart controllers have been independently tested for water efficiency. Rebates of 50 to 150 dollars are available from many US water utilities (check your utility’s website or ewaterusa.com for rebate listings). Research published by the EPA estimates smart controllers reduce outdoor water use by 15 to 30 percent compared to manual timers, and they pay for themselves in water savings within 1 to 2 seasons in moderate to high-ET US climates.

How does drip irrigation runtime differ from spray heads?▼

Drip systems deliver water at very low precipitation rates (0.10 to 0.30 in/hr) directly to the root zone, so runtime must be much longer to deliver the same total water depth: delivering 0.5 inches takes 150 to 300 minutes with drip versus 20 to 30 minutes with spray. However, drip is highly efficient because water goes directly to roots with minimal evaporation and no overhead spray losses to wind or drift. Drip runtime also does not trigger cycle-and-soak requirements because the application rate is always below any soil’s infiltration rate. Vegetable gardens and shrub beds with drip systems typically need 1 to 3 hours of daily or every-other-day runtime in peak summer conditions in warm US climates. Enter your drip system’s precipitation rate in the calculator for accurate runtime recommendations.

Can I use this calculator for commercial irrigation systems?▼

The calculator applies correctly to any scale of irrigation that uses the same precipitation-rate-to-ET formula. For commercial or large-scale systems, the runtime calculation is identical: the only difference is that zone areas may be much larger, requiring more precise precipitation rate measurement and ET data. Commercial irrigation schedules often use professional Landscape Irrigation Auditor (CLCA) or Certified Irrigation Designer (CID) services to conduct zone-by-zone catch-can tests, uniformity measurements, and system hydraulic analysis. For residential zones, the calculator provides results accurate to within 10 to 20 percent of professionally audited schedules, which is sufficient as a starting point for manual timer adjustment.

How accurate is this watering runtime calculator?▼

The calculator produces mathematically exact results for the inputs provided. Real-world accuracy depends on the accuracy of the ET rate (use data from your state ET network for best results) and the precipitation rate (use the catch-can test for your specific sprinkler heads rather than the default values). The efficiency comparison (ET demand vs irrigation applied) flags whether your schedule is matched, over, or under the plant’s water need. Using the regional ET presets gives results within 10 to 20 percent of precision data, which is appropriate for setting an initial irrigation schedule. Refine the schedule based on soil moisture checks, visual plant health, and a professional irrigation audit every 2 to 3 years for best long-term efficiency.

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Mulch and Topsoil Depth Calculator

A 3-inch layer of mulch in your garden beds reduces evapotranspiration from soil by 25 to 50 percent. Calculate mulch quantities for beds alongside irrigated zones.

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Garden Path Stepping Stone Calculator

Calculate stepping stones, sand base, and weed barrier for garden paths. Hardscaped paths reduce irrigation zone area and lower your total water demand.

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Seed Starting Date Calculator

Plan indoor start dates for 31 crops by USDA zone. New transplant zones need more frequent irrigation at reduced depth until plants establish root systems.

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Fertilizer Dilution PPM Calculator

Convert N-P-K fertilizer to PPM in US gallons. Pair your irrigation schedule with a fertigation program to deliver nutrients efficiently at the correct application depth.

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Potting Mix DIY Recipe Calculator

Build potting mixes with drainage, retention, and aeration scores. Better-draining mixes in containers reduce overwatering risk and complement ET-based drip schedules.

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Hydroponic Reservoir Calculator

Size DWC, NFT, Kratky, and Ebb and Flow reservoirs by plant count and stage. Hydroponic systems use ET principles without soil: solution EC and daily top-off replace soil moisture monitoring.

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Grow Light Distance Calculator

Get LED, HPS, and T5 hanging heights by wattage and stage. Higher light intensity increases plant ET in indoor environments, directly affecting drip irrigation runtime.

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Garden Calculators Hub

All 8 free garden planning tools: seed starting, fertilizer PPM, grow lights, potting mix, hydroponics, mulch, stepping stones, and this watering runtime and ET calculator.

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Legal Disclaimer and Editorial Transparency. The watering runtime and ET calculator uses the formula Runtime (min) = Target depth (in) / Precipitation rate (in/hr) x 60, where target depth equals daily plant ET times days between sessions. Daily plant ET equals reference ET times the crop coefficient (Kc) for the selected plant type times a seasonal adjustment factor. ET regional presets represent typical summer peak values based on published USDA and Cooperative Extension irrigation scheduling data. Actual ET varies daily with temperature, humidity, wind, and solar radiation. Crop coefficients are approximate values based on published ASCE and University of California ET guidelines and vary by growth stage, species, and local microclimate. Cycle-and-soak calculations use standard soil infiltration rate benchmarks by texture class and trigger when sprinkler precipitation rate exceeds soil infiltration rate. All results are starting-point estimates for irrigation scheduling and should be verified by periodic soil moisture checks, visual plant health monitoring, and professional irrigation audits. For precise ET data for your location, consult your state’s Cooperative Extension irrigation scheduling resources or the EPA WaterSense program. Check the USDA NRCS Web Soil Survey for your site’s soil type. No advertising, no data collection, no signup required. Content reviewed August 2025.