Pool Volume in Gallons: How to Measure Any Pool Shape
A standard 16×32 rectangular pool at 5-foot average depth holds approximately 19,200 gallons. The formula for rectangular pools: Length x Width x Average Depth x 7.5 = gallons. Round pools: Pi x Radius x Radius x Average Depth x 7.5. Knowing your pool volume in gallons is essential for accurate chemical dosing, pump sizing, and water change calculations.
Guessing your pool volume leads to over- or under-dosing chemicals. Every chemical treatment on your pool label lists dosing per 10,000 gallons: if you do not know your gallons, the label is useless. Getting the number right takes 5 minutes with a measuring tape and the right formula.
The pool volume calculator handles rectangular, round, oval, and kidney shapes with variable depth zones.
Open the Pool Calculator →Pool Volume Formula by Shape
| Pool shape | Formula | Key measurement |
|---|---|---|
| Rectangular / square | L x W x D x 7.5 | Length, width, average depth |
| Round / circular | 3.14 x r x r x D x 7.5 | Radius (half diameter), average depth |
| Oval | 0.785 x L x W x D x 7.5 | Longest length, widest width, average depth |
| Kidney / figure-8 | 0.45 x (A + B) x L x D x 7.5 | A and B: widths at each lobe; L: length |
| L-shaped | Sum of two rectangles x D x 7.5 | Treat as two rectangles |
Pool Volume in Gallons for Common US Pool Sizes
| Pool size | Gallons (5-ft avg) | Gallons (4-ft avg) | Gallons (6-ft avg) |
|---|---|---|---|
| 10×20 (above-ground) | 7,500 | 6,000 | 9,000 |
| 12×24 | 10,800 | 8,640 | 12,960 |
| 14×28 | 14,700 | 11,760 | 17,640 |
| 16×32 (common) | 19,200 | 15,360 | 23,040 |
| 18×36 | 24,300 | 19,440 | 29,160 |
| 20×40 | 30,000 | 24,000 | 36,000 |
| 24×48 (large) | 43,200 | 34,560 | 51,840 |
| Round pool diameter | Gallons (4-ft depth) | Gallons (3.5-ft depth) |
|---|---|---|
| 12 feet | 3,393 | 2,969 |
| 15 feet | 5,301 | 4,638 |
| 18 feet | 7,634 | 6,680 |
| 21 feet | 10,390 | 9,091 |
| 24 feet | 13,565 | 11,869 |
| 27 feet | 17,170 | 15,023 |
Pool volume in gallons affects everything from weekly chemical costs to annual water bills. A 20,000-gallon pool in Phoenix loses approximately 250 gallons weekly to evaporation in summer. For irrigation management on the surrounding lawn, our guide on how long to water your lawn by zone covers the zone runtime calculations that often run alongside pool operation. For yard drainage around a new pool installation, our french drain installation guide covers drainage structures that prevent pool-area flooding.
How to Calculate Average Depth Correctly
Most residential pools have a shallow end and a deep end connected by a slope. For most standard pools where the slope starts at the midpoint: average depth = (shallow + deep) / 2. For play pools where the slope starts one-third from the shallow end: average depth = ((Shallow x 1/3) + (Deep x 2/3)). When in doubt, use (shallow + deep) / 2; it is accurate enough for chemical dosing purposes.
Why Pool Volume in Gallons Matters
Chemical dosing
Every pool chemical is dosed per gallon. The EPA recommends chlorine levels of 1 to 3 parts per million (ppm). For a 20,000-gallon pool, raising chlorine by 1 ppm requires approximately 1.3 pounds of dichlor granules. Miscalculating pool volume by 30 percent leads to chronic over- or under-treatment that costs more to fix than the measurement takes to perform.
Pump sizing
Pool pumps are sized to turn over the pool volume in 6 to 8 hours. A 20,000-gallon pool needs a pump capable of 40 to 55 gallons per minute (GPM). GPM needed = Pool volume / (turnover hours x 60 minutes). Over-sizing the pump wastes electricity; under-sizing produces poor water circulation.
Three Real Pools Measured
Example 1: Rectangular inground pool in Scottsdale, Arizona
A homeowner with a 16×34 rectangular pool. Shallow end: 3 feet. Deep end: 7 feet. Slope at midpoint: average depth = (3 + 7) / 2 = 5 feet. Volume: 16 x 34 x 5 x 7.5 = 20,400 gallons. Weekly chlorine at 1.5 ppm target: approximately 2.5 lbs of trichlor per week. Annual chemical budget: approximately $650.
Example 2: Above-ground round pool in Houston, Texas
A family with an 18-foot diameter round pool at 3.5-foot water depth. Volume: 3.14 x 9 x 9 x 3.5 x 7.5 = 6,681 gallons. A 0.75 HP pump at 20 GPM provides a 5.6-hour turnover, correctly sized. Many above-ground pool kits include over-sized pumps that shorten pump life and waste electricity: knowing your actual volume helps you identify this before purchase.
Example 3: Kidney-shaped inground pool in Tampa, Florida
A homeowner with a kidney-shaped pool: length 30 feet, widths A=12 feet and B=15 feet. Average depth 4.5 feet. Volume: 0.45 x (12 + 15) x 30 x 4.5 x 7.5 = 12,278 gallons. The homeowner had been treating as a 15,000-gallon pool, over-dosing by 22 percent and wasting approximately $130 per year in unnecessary chemicals.
Enter your pool shape and dimensions. The pool volume calculator gives you gallons, turnover rate, and chemical dosing reference.
Calculate Pool Gallons →Understanding Pool Volume Impact on Water Bills and Seasonal Costs
Knowing your pool volume in gallons is not just a chemistry exercise. It has direct dollar implications on every utility bill through the pool season.
Water fill and top-off costs
Filling a residential pool for the first time or after a partial drain uses a significant volume of municipal water. At the national average municipal water rate of approximately $0.005 per gallon in 2026 (roughly $5 per 1,000 gallons, though rates vary widely by city), filling a 20,000-gallon pool from empty costs approximately $100 in water charges. In drought-restricted cities like Los Angeles, San Diego, or Phoenix, where water rates can reach $0.008 to $0.015 per gallon, the same fill costs $160 to $300. Knowing the exact volume in gallons helps you budget for seasonal refills and identify whether a leak is causing abnormal water loss beyond normal evaporation.
Leak detection using the bucket test
A pool losing more water than evaporation accounts for may have a leak. The bucket test: fill a bucket with pool water and set it on the pool steps (so it is at the same temperature as the pool). Mark the water level in both the bucket and the pool. After 24 hours, compare the drop. If the pool lost significantly more water than the bucket, there is likely a leak. Knowing your pool volume in gallons helps you calculate the leak rate: if the pool lost 0.5 inches overnight, and the pool surface area is 512 sq ft (a 16×32 pool), that is 512 x (0.5/12) x 7.5 = 160 gallons per day, well above normal evaporation (50 to 100 gallons per day in summer). A 160-gallon daily leak at current water rates in a typical US city costs $0.80 per day or about $24 per month, plus the chemical replenishment costs of the fresh water diluting your chemistry.
Chemical cost by pool volume in gallons
Annual pool chemical costs scale directly with pool volume. A typical 20,000-gallon residential pool in the Sun Belt spends approximately $400 to $700 per year on chlorine, algaecide, pH stabilizer, and shock. A 10,000-gallon above-ground pool might spend $200 to $350. The relationship is roughly linear with volume: know your gallons, and you can benchmark your chemical spend against what a correctly maintained pool should cost. Significantly higher chemical costs often indicate a dosing error from an incorrect volume estimate, a chemistry imbalance driving excess chlorine demand, or a leak introducing fresh water that dilutes and requires constant re-treatment.
Frequently Asked Questions About Pool Volume
How many gallons is a standard 16×32 pool?
A 16×32 rectangular pool at 5-foot average depth holds approximately 19,200 gallons. At 4-foot average depth: 15,360 gallons. At 6 feet: 23,040 gallons. Formula: Length x Width x Average Depth x 7.5 = gallons.
How do I calculate pool volume in gallons?
Rectangular: L x W x Average Depth x 7.5. Round: 3.14 x radius squared x Depth x 7.5. Oval: 0.785 x L x W x Depth x 7.5. Kidney: 0.45 x (A + B) x Length x Depth x 7.5. Average depth for pools with slope at midpoint = (shallow + deep) / 2.
How many gallons is an 18-foot round pool?
An 18-foot diameter round pool at 4-foot water depth holds approximately 7,634 gallons. At 3.5-foot depth: 6,680 gallons. Formula: 3.14 x 9 x 9 x Depth x 7.5. The 9 is the radius (18-foot diameter divided by 2).
How many gallons does it take to fill a pool?
A 16×32 pool needs approximately 19,200 gallons to fill. At a garden hose flow rate of 9 gallons per minute, that is about 35 hours. Many municipalities offer lower bulk water rates for pool fills; check with your local utility before scheduling a fill.
How do I know how many gallons my pool holds without measuring?
Check original construction permits or manufacturer documentation. If unavailable, measure with a tape measure: length, width, shallow end depth, deep end depth. Never estimate; inaccurate volume causes chronic chemical dosing problems that cost more to fix than the measurement takes.
How many gallons evaporate from a pool per day?
Pool evaporation averages 0.5 to 1 inch of depth per week in moderate climates and 1 to 2 inches per week in hot, dry, or windy climates. For a 16×32 pool, 1 inch of depth equals approximately 512 gallons. A pool cover reduces evaporation by 90 to 95 percent, saving significant water and chemical costs in warm climates.
Does pool volume affect pump size?
Yes. Pump GPM needed = Volume / (6 hours x 60 minutes). For a 20,000-gallon pool: 55.6 GPM minimum. For a 10,000-gallon pool: 27.8 GPM minimum. Many above-ground pool packages include over-sized pumps; knowing your volume helps verify correct sizing.
How often should I replace pool water?
A partial water change (25 to 50 percent drain and refill) every 3 to 7 years dilutes dissolved solids that accumulate over time and cannot be chemically removed. Full draining is rarely necessary for properly maintained pools. Check local regulations before draining in drought-restricted areas.
Related Calculators and Guides
For pool water chemistry, the CDC Healthy Swimming guidelines and the EPA WaterSense program provide authoritative US references for pool water quality standards and outdoor water conservation guidance.