Free French Drain Percolation and Sizing Calculator for US Residential and Commercial Projects

All four are drainage solutions but they work differently and serve different situations. A French drain (also called a perimeter drain or sub-drain) is a gravel-filled trench with a perforated pipe that intercepts and infiltrates subsurface or surface water along its entire length. A curtain drain is a French drain installed specifically across a slope to intercept hillside groundwater before it reaches a downslope structure; it is a French drain by design, just with a specific interceptor purpose. A swale is an open, shallow grassy channel that conveys surface water over land to a collection point; it does not infiltrate, it redirects. A dry well is a large vertical pit filled with gravel or a perforated plastic chamber that stores a volume of water and releases it slowly into the surrounding soil at a single point. French drains and dry wells are often connected: the French drain collects water along its length and routes it to a dry well for final disposal.

For most residential French drains in the US, a 4-inch perforated HDPE or PVC pipe is the standard. A 4-inch pipe can handle several hundred gallons per minute of flow at the typical slopes used in residential drainage, far more than any French drain in average soil conditions needs to convey. For drains serving larger impervious areas (parking lots, commercial pads, or long runs exceeding 200 feet), a 6-inch perforated pipe is commonly specified to reduce head losses along the pipe and ensure water can enter freely even when the pipe is not perfectly clean. Never use corrugated HDPE pipe with perforations in fine-grained soils without a high-quality filter sock: the corrugations collect fine particles that eventually seal the perforations from the outside. Smooth-wall perforated PVC with a non-woven filter sock is the most clog-resistant combination for residential installations in silty or clayey soils.

A properly designed and installed French drain with quality filter fabric and washed aggregate should last 30 to 40 years in most US soil conditions. The primary failure mechanisms are biological clogging (iron bacteria forming a gel in the gravel), root intrusion through older PVC joints, fabric degradation in highly acidic soils, and soil migration through damaged or incorrectly installed fabric. Systems installed without filter fabric in silty or clayey soils may begin losing capacity within 5 to 10 years as fines migrate into the gravel. Annual inspection of the outlet for flow during storms, and periodic inspection of any accessible cleanout ports, is the best way to detect early clogging before it becomes a full replacement project. Jetting the pipe with a sewer cleaning machine every 5 to 10 years in high-clogging-risk soils significantly extends service life.

OSHA Standard 1926.652 requires that any trench 5 feet or deeper must be protected against cave-in by sloping the trench walls, shoring the walls with timber or hydraulic shoring, or using a trench box or shield. For a French drain in Type C soil (granular sand, gravel, or any soil that is wet, subject to water infiltration, or has a fissured structure), the required slope is 1.5 horizontal to 1 vertical (34 degrees from horizontal), meaning a 6-foot-deep trench requires at least 9 feet of additional horizontal clearance on each side of the bottom width. In practice, most residential French drains stay at 4 to 5 feet deep specifically to avoid the additional cost and space requirements of OSHA-compliant shoring. When a drain must be deeper, hydraulic trench boxes rented from equipment suppliers are the most practical option for small contractors. OSHA trench collapses are almost always fatal and kill roughly 50 workers per year in the US; this is not a paperwork formality.

Yes, but the design approach differs for each. For intercepting subsurface groundwater (the typical application near wet basements or on saturated hillsides), the trench must be deep enough to cut into the saturated zone and the pipe must be below the seasonal high groundwater table. For collecting surface runoff (yard drainage, parking lot drainage), the trench can be shallower and should have a perforated or slotted cap at the surface level to allow surface water to enter. For combined systems handling both surface and subsurface water, the trench is typically designed to the deeper subsurface requirement, and the upper portion of the gravel fill acts as a surface collection zone. This calculator uses the surface runoff rational method as the primary sizing driver, which is conservative and appropriate for both surface-dominant and mixed systems.

The runoff coefficient C represents the fraction of rainfall that becomes surface runoff rather than infiltrating into the ground or evaporating. A C of 0.90 for a rooftop means 90 percent of rain runs off; a C of 0.15 for a sandy lawn means only 15 percent runs off and 85 percent infiltrates. Correct C selection is critical for drain sizing: using 0.90 instead of 0.35 for a lawn triples the calculated runoff and dramatically oversizes the drain. The Rational Method C values in this calculator follow ASCE 7 and standard US drainage engineering practice. When a drainage area has multiple surface types (part lawn, part paved driveway), compute a weighted average: if 3,000 sq ft is lawn (C=0.35) and 2,000 sq ft is paved (C=0.85), the composite C = (3000×0.35 + 2000×0.85) / 5000 = 0.55.

Water infiltrates from the gravel into the surrounding soil through all surfaces in contact with that soil: the trench bottom and both vertical sides. The trench top is sealed (covered with soil and turf), so it does not contribute to infiltration. Using only the bottom area is overly conservative and results in unnecessarily long drains. Using the full perimeter (bottom plus two sides) is the standard approach used in USDA agricultural drainage design, most state extension service sizing guides, and typical civil engineering practice for French drain sizing. For very wide trenches relative to their depth (width much greater than depth), the bottom area dominates and the side contribution is proportionally less significant. For narrow, deep trenches, the side area dominates. The depth-to-width ratio of your trench design significantly affects how efficiently the system uses its length, which is why trench geometry is a key input in this calculator.

The return period (also called recurrence interval) describes the statistical frequency of a storm event. A 10-year storm has a 10 percent probability of occurring in any given year; a 25-year storm has a 4 percent annual probability. A French drain sized for the 10-year storm handles most storms you will experience in a typical decade but overflows during rarer, more intense events. The appropriate design storm depends on the consequences of system overflow. For a typical residential yard drainage system where overflow means temporary puddling, the 10-year storm is usually sufficient and is what most US municipalities require for residential drainage permits. For a French drain protecting a basement from flooding, or serving a commercial parking lot where ADA compliance and customer safety are concerns, the 25-year storm is more appropriate. Look up your location-specific storm intensities at NOAA Atlas 14 PFDS for accurate values by duration and return period.

You can install one, but it will be very long or may not be feasible if the clay is extensive. In fat clay with a perc rate of 360 minutes per inch or slower, a 2-foot-wide by 3-foot-deep trench with a safety factor of 2.0 has an infiltration capacity of only about 0.35 gallons per hour per linear foot. To handle a modest 500-square-foot drainage area during a 1-inch-per-hour storm, you would need over 1,200 linear feet of trench, which is impractical for most residential lots. In these situations, a French drain must be combined with a storage component: a dry well, underground cistern, or detention pond that holds the runoff volume and releases it slowly over many hours after the storm passes. Alternatively, a French drain can still be used in clay to intercept seepage from foundation walls or groundwater, where the flow rate is much lower than surface storm runoff and even a slow perc rate is sufficient to handle the trickle of subsurface water.

The safety factor in French drain sizing accounts for the fact that the soil’s infiltration capacity decreases over time as biological films, root intrusion, and fine particle migration partially clog the gravel-soil interface. A freshly installed French drain in clean sandy loam might have a measured perc rate of 20 minutes per inch, but after 10 years of operation the effective rate at the trench wall may be 40 minutes per inch as the soil-gravel interface develops a bio-clogging layer. A safety factor of 2.0 is the minimum commonly used in residential drainage design to account for this degradation. For systems expected to remain in service for 20 to 30 years without maintenance access, a factor of 3.0 to 4.0 is more appropriate. For commercial systems or those in iron-rich soils where iron bacteria clogging is aggressive, a factor of 4.0 to 6.0 is sometimes used by civil engineers designing municipal systems. Using a higher safety factor results in a longer required trench, which provides resilience against aging without redesign.

A French drain outlet must discharge to a legal point that can receive the flow without creating new drainage problems. Acceptable outlets include: a roadside ditch or storm drain (with municipal permission), a retention or detention pond on the property, a dry well or infiltration gallery at a lower elevation, a natural watercourse such as a stream or creek (subject to state and Army Corps of Engineers jurisdiction for waters of the US), or a defined easement with a recorded drainage right on a downslope property. Never discharge a French drain directly onto a neighbor’s property, into a septic system, into a sanitary sewer (illegal in virtually all US jurisdictions), or into a wetland without permits. Many jurisdictions require that stormwater be managed on-site through infiltration or detention before reaching the street, so verify local stormwater ordinances before planning your outlet location.

The current version of this calculator uses a single composite runoff coefficient C for the entire drainage area. If your site has multiple surface types, compute the area-weighted composite C before entering it. For example, a drainage area of 6,000 total square feet with 4,000 sq ft of lawn (C = 0.35) and 2,000 sq ft of concrete driveway (C = 0.85) has a composite C = (4000 x 0.35 + 2000 x 0.85) / 6000 = (1400 + 1700) / 6000 = 0.517. Enter 0.517 as your runoff coefficient, but since the dropdown uses pre-set values, select the closest option (mixed residential at 0.40 is the closest lower bound, or choose commercial at 0.70 if you want to be conservative). For complex multi-use drainage areas on commercial projects, a licensed civil engineer should develop a full stormwater management plan using TR-55 or other NRCS methods approved by your local municipality.

The effective void storage volume is the actual water-holding capacity of the gravel-filled trench, calculated as total trench volume times the gravel void ratio. This storage acts as a buffer that absorbs the peak rush of runoff during the intense early minutes of a storm, giving the soil time to infiltrate it over the following hours. A French drain that has exactly enough infiltration capacity to match the peak storm flow with zero storage will overflow the moment runoff slightly exceeds design intensity. Storage provides resilience against storm intensity variations. The storm volume check in this calculator compares total runoff volume over the storm duration against the system’s effective storage plus its infiltration capacity over that same period. If the total storm volume exceeds what the system can handle through infiltration alone, the storage volume absorbs the surplus and releases it after the storm peak passes.

Permit requirements vary significantly by jurisdiction, but the general rule in the US is: if it touches a public right-of-way, discharges to a public storm system, disturbs more than 1 acre of land (triggering EPA NPDES permit requirements), or involves trench excavation near utilities, a permit is likely required. Most states also require Miss Utility or 811 locates before any excavation regardless of permit status. Many cities and counties in the Pacific Northwest, California, and the Great Lakes states have adopted local stormwater ordinances that require engineered drainage plans for any system serving more than 500 to 1,000 square feet of impervious surface. The safest approach is to call your county building department and ask two specific questions: do I need a permit for a French drain, and does it need to be designed by a licensed engineer? Getting a clear answer takes 15 minutes and prevents permit violation fines that can run into thousands of dollars on residential sites.

Visit NOAA’s Precipitation Frequency Data Server (PFDS) at hdsc.nws.noaa.gov. Click on your state on the map, then click on your approximate project location. The tool returns a table of precipitation frequency estimates for storm durations from 5 minutes to 60 days and return periods from 1 year to 1000 years. For French drain sizing using the Rational Method, select the 1-hour duration column and the 10-year return period row as a starting point for residential drainage. The value shown (in inches per hour) is what you enter in this calculator as the design storm intensity. For basement protection or commercial sites where more conservative sizing is appropriate, use the 10-year or 25-year return period. NOAA Atlas 14 is the authoritative precipitation frequency reference for the continental US and is accepted by all state DOTs and municipal stormwater programs.

Partially. A dry creek bed (also called a rock channel or bioswale) conveys surface water as open channel flow, which follows Manning’s equation rather than Darcy’s Law for infiltration. This calculator handles the infiltration-based French drain sizing component, which is useful if you plan to combine a dry creek bed with an underlying French drain trench (a common design where the rock channel conveys overflow while the subsurface French drain handles normal flows). For the open channel flow capacity of the rock surface itself, you would need a Manning’s equation calculation based on the channel cross-section, slope, and roughness coefficient of the rock material. That analysis is separate from this tool. As a combined system, size the underground French drain for the routine storm using this calculator, then verify the surface channel can convey the extreme storm overflow without eroding the channel banks using Manning’s equation with roughness coefficient n of 0.035 to 0.050 for loose rock channels.