How do you calculate pool volume?
A pool’s volume is the area of its water surface times its average depth. That gives cubic feet. To turn cubic feet into gallons, multiply by 7.48.
Cubic feet = surface area x average depth
Gallons = cubic feet x 7.48
Liters = gallons x 3.785
Where does 7.48 come from? A cubic foot is a box 1 foot long, 1 foot wide, and 1 foot tall. A foot is 12 inches, so that box holds 12 x 12 x 12 = 1,728 little 1-inch cubes, called cubic inches. A US gallon is 231 cubic inches. Divide 1,728 by 231 and you get about 7.48 gallons in every cubic foot.
Some pool guides round this up to 7.5 to make the math easier. That counts about 3 extra gallons for every 1,000, so their answers come out a little high.
Why does the number matter? Almost everything you add to a pool, like chlorine, shock, or pH balancer, is measured by how many gallons of water you have. The directions on the product tell you how much to add for your pool’s size.
If you think your pool holds more water than it really does, you will add more than it needs and waste money. If you think it holds less, you will add too little, and the chlorine may not be enough to keep the water clean and safe to swim in.
Pool volume formulas by shape
The surface area is the only part that changes with the shape. Here is how to work out the surface for each shape.
| Shape | Surface area | What to measure |
|---|---|---|
| Rectangle | length x width | Inside length and width |
| Round | 3.14 x radius x radius | Diameter, then halve it |
| Oval | 3.14 x length x width / 4 | Longest length and widest width |
| Kidney | 0.45 x (A + B) x length | Length, plus the widest point of each end (A and B) |
| Freeform | 0.785 x length x width | Longest length and widest width |
Oval and freeform pools use the same estimate. A freeform pool rarely matches a true oval, so its result is a rougher guide than the others. If your pool is close to two rectangles joined together, measure each part as a rectangle and add the gallons.
How to find the average depth
Most in-ground pools slope from a shallow end to a deep end. Measure both, add them, and divide by 2.
Average depth = (shallow end + deep end) / 2
A pool that is 3 ft deep at one end and 8 ft at the other has an average depth of (3 + 8) / 2 = 5.5 ft. Round above-ground pools usually have one depth all the way across, so pick “Same all over” for those.
Example: a 32 x 16 ft in-ground pool
Say your pool is a 32 x 16 ft rectangle. The water is 3.5 ft deep at the shallow end and 6 ft at the deep end.
Surface area = 32 x 16 = 512 sq ft
Average depth = (3.5 + 6) / 2 = 4.75 ft
Cubic feet = 512 x 4.75 = 2,432 cu ft
Gallons = 2,432 x 7.48 = about 18,193 gallons
In liters, that is about 68,867. With the rounded 7.5 factor, you would get 18,240 gallons instead, about 47 gallons more.
Pool gallons chart
These charts use the formulas above. Your own pool may differ if its water depth or shape is not quite the same.
Round pools
| Diameter | Water surface | Gallons at 4 ft deep | Gallons at 4.5 ft deep |
|---|---|---|---|
| 12 ft | 113 sq ft | 3,384 | 3,807 |
| 15 ft | 177 sq ft | 5,288 | 5,949 |
| 18 ft | 254 sq ft | 7,614 | 8,566 |
| 21 ft | 346 sq ft | 10,364 | 11,659 |
| 24 ft | 452 sq ft | 13,536 | 15,228 |
| 27 ft | 573 sq ft | 17,132 | 19,274 |
| 30 ft | 707 sq ft | 21,151 | 23,795 |
Rectangle pools at a 5 ft average depth
| Size | Water surface | Gallons |
|---|---|---|
| 12 x 24 ft | 288 sq ft | 10,772 |
| 15 x 30 ft | 450 sq ft | 16,831 |
| 16 x 32 ft | 512 sq ft | 19,150 |
| 18 x 36 ft | 648 sq ft | 24,237 |
| 20 x 40 ft | 800 sq ft | 29,922 |
See all Pool & Spa calculators.
Limitations
Every shape here is an estimate of the real outline. Kidney, oval, and freeform pools curve in ways a single factor cannot match exactly, so treat those results as close, not exact.
The average depth method assumes a steady slope from end to end. Pools with a steep drop into a deep diving well, built-in steps, benches, or sun shelves will hold a different amount of water. For dosing, follow the product label and test your water after treating it.
Sources
- NIST Handbook 44 Appendix C: General Tables of Units of Measurement
- NIST Guide to the SI, Appendix B.9: Conversion factors
Frequently asked questions
How many gallons are in a cubic foot of pool water?
About 7.48 gallons. Picture a box 1 foot long, 1 foot wide, and 1 foot tall. Filled with water, it holds about 7.48 gallons. So to get your pool's gallons, multiply its cubic feet by 7.48. Some pool guides round this up to 7.5 for easier math, which counts about 3 extra gallons for every 1,000.
How do I find the average depth of a pool?
Add the shallow end depth and the deep end depth, then divide by 2. A pool that is 3 ft at one end and 8 ft at the other has an average depth of 5.5 ft. Measure the water itself, not the wall, since the water line sits below the top.
How many gallons is a 24 ft round above ground pool?
A 24 ft round pool with 4 ft of water holds about 13,500 gallons. The water surface is 3.14 x 12 x 12, or about 452 sq ft. Times 4 ft of depth, that is about 1,810 cubic feet, which works out to 13,536 gallons.
How do I measure a kidney shaped pool?
Measure the full length, then the width across the widest part of each rounded end. The usual kidney estimate adds the two widths, multiplies by the length, and then by 0.45. A 32 ft pool with ends 12 ft and 16 ft wide has about 403 sq ft of water surface.