How to Calculate Pool Volume
Every pool volume formula is the same idea: find the cubic feet, then convert to gallons. The only thing that changes is how the shape affects the math.
To calculate pool volume, multiply length by width by average depth to get cubic feet, then multiply by 7.48 to get gallons. Curved shapes use smaller multipliers: 6.7 for oval, 5.9 times diameter squared for round, and 7.0 for kidney or freeform pools. A 20 by 40 foot pool with 5.5 feet of average depth holds 32,914 gallons.
Where the 7.48 comes from
A US gallon is defined as 231 cubic inches. A cubic foot is 12 x 12 x 12 = 1,728 cubic inches. Divide 1,728 by 231 and you get 7.48052: the number of gallons in one cubic foot. So for a rectangular pool, the volume in gallons is simply length x width x average depth (cubic feet) x 7.48052.
This also explains why pool formulas work in any consistent unit. In meters, one cubic meter holds 264.172 gallons. In liters, one cubic meter holds 1,000 liters. The geometry is the same; only the conversion factor changes.
The four pool volume formulas
| Shape | Formula (all in feet) | Example |
|---|---|---|
| Rectangle | L x W x avg depth x 7.48 | 20 x 40 x 5.5 x 7.48 = 32,914 gal |
| Oval | L x W x avg depth x 6.7 | 15 x 30 x 4.25 x 6.7 = 12,814 gal |
| Round | Diameter x diameter x avg depth x 5.9 | 24 x 24 x 4 x 5.9 = 13,594 gal |
| Kidney / freeform | L x W x avg depth x 7.0 | 28 x 14 x 4.5 x 7.0 = 12,348 gal |
The curved-shape multipliers are the 7.48 figure scaled down for the water the curves remove. An oval pool holds about 90 percent of the rectangle with the same overall dimensions (6.7 / 7.48 = 0.896). A round pool uses diameter squared because the area of the circle is already folded into the 5.9 multiplier. Kidney and freeform pools use 7.0 as a middle-ground estimate, since no two freeform pools curve the same way.
Worked example: rectangular pool, start to finish
Take a 20 by 40 foot pool, 3 feet in the shallow end, 8 feet in the deep end.
- Average depth: (3 + 8) / 2 = 5.5 feet.
- Cubic feet: 20 x 40 x 5.5 = 4,400 cubic feet.
- Gallons: 4,400 x 7.48052 = 32,914 gallons.
- Liters: 32,914 x 3.78541 = 124,594 liters.
That is the entire method. Every rectangular pool on earth reduces to those four lines.
Worked example: oval pool
A 15 by 30 foot oval, averaging 4.25 feet deep: 15 x 30 x 4.25 = 1,912.5 cubic feet if it were a rectangle. Multiply by the oval factor 6.7 instead of 7.48: 1,912.5 x 6.7 = 12,814 gallons. The straight side walls of an oval hold more water than a true ellipse would, which is why the oval multiplier (6.7) is higher than the pure ellipse math (about 5.87) would suggest. See oval pool volume for a full chart of oval sizes.
Converting between units
- Gallons to liters: multiply gallons by 3.78541. A 20,000 gallon pool holds about 75,708 liters.
- Liters to gallons: divide liters by 3.78541.
- Cubic meters to gallons: multiply by 264.172. A 100 cubic meter pool holds 26,417 gallons.
- Cubic feet to gallons: multiply by 7.48052.
- Gallons to pounds of water: multiply by 8.34. That 32,914 gallon pool weighs about 274,500 pounds, or roughly 137 tons.
Common mistakes
- Using the deep end instead of the average depth. This is the single biggest error. A pool that runs 3 to 8 feet deep averages 5.5 feet, and using 8 feet inflates the volume by 45 percent.
- Using wall height for above ground pools. Water sits 6 to 8 inches below the top rail. A 52 inch wall holds about 4 feet of water.
- Mixing radius and diameter. The round formula takes diameter squared. If you measured the radius, double it first.
- Forgetting the shape factor. Length x width x depth alone gives cubic feet, not gallons. The x 7.48 step is what converts to gallons.
Run the numbers without the hand math: the free pool volume calculator handles all four shapes and shows gallons, liters, and water weight.
Frequently asked questions
How do I calculate the volume of a swimming pool?
Multiply length by width by average depth in feet to get cubic feet, then multiply by 7.48 to get gallons. For curved shapes use 6.7 for oval, 5.9 times diameter squared for round, or 7.0 for kidney pools as an estimate.
What is the volume of a 16x32 pool?
About 19,150 gallons at a 5 foot average depth, or about 21,065 gallons at a 5.5 foot average depth. The calculation is 16 x 32 x average depth x 7.48.
How many gallons is a 40x20 pool?
About 32,914 gallons at a 5.5 foot average depth (40 x 20 x 5.5 x 7.48). That is the same as a 20 by 40 pool; the order of the dimensions does not matter.
How many gallons are in a 15x30 pool?
As a rectangle at a 5 foot average depth, about 16,831 gallons (15 x 30 x 5 x 7.48). As an oval at 4.5 feet of average depth, about 13,568 gallons (15 x 30 x 4.5 x 6.7).
Skip the hand math. Enter your pool shape and dimensions for an instant gallon count, plus liters and water weight.
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