How to measure your pool
Measure the water, not the shell. Run the tape across the surface at the water line for length and width (or diameter), then measure depth from the water line straight down to the floor. A pool that’s 4 feet from coping to floor but filled to the middle of the skimmer only has about 3½ feet of water in it, and that half foot is 12% of the volume.
- Sloped pools: take one depth at the shallow end and one at the deep end. The calculator averages them, which is exact for a straight, even slope.
- Short hopper deep ends: if the floor stays shallow for most of the length and then drops into a small deep well, a simple average overstates the volume. Take depths every few feet along the pool and average those instead, then enter the result as one depth.
- Above-ground pools: use the inside diameter and the water depth, which usually sits 6 to 8 inches below the top rail.
- Odd shapes: choose “Other” and enter the surface area. Your builder’s plans, a safety cover order or a liner invoice usually lists it.
The formula
Gallons = surface area (ft²) × average depth (ft) × 7.48
Litres = surface area (m²) × average depth (m) × 1,000
One cubic foot holds 7.48 US gallons (NIST). One cubic metre is 1,000 litres.The area changes with the shape:
| Shape | Surface area |
|---|---|
| Rectangle | length × width |
| Round | π × diameter² ÷ 4 (about 0.785 × diameter²) |
| Oval with straight sides | (length − width) × width + π × width² ÷ 4 |
| Kidney | about 0.45 × (wide end + narrow end) × length |
The oval formula treats the pool as a rectangle with a half circle on each end, which is how most above-ground ovals are built. The kidney factor is an industry rule of thumb, so expect it to be within about 10%.
Worked examples
A 16 × 32 ft rectangle, 3½ to 6 ft deep
- Area = 32 × 16 = 512 ft²
- Average depth = (3.5 + 6) ÷ 2 = 4.75 ft
- Volume = 512 × 4.75 × 7.48 = 18,193 gallons (68,867 litres)
A 24 ft round above-ground pool with 4 ft of water
- Area = 0.785 × 24² = 452 ft²
- Volume = 452 × 4 × 7.48 = 13,536 gallons
A 30 ft kidney, 15 ft and 10 ft wide, 3½ to 6 ft deep
- Area ≈ 0.45 × (15 + 10) × 30 = 338 ft²
- Volume ≈ 338 × 4.75 × 7.48 = about 12,000 gallons
Volume of common pool sizes
These use the formulas above. Check yours with real measurements, because “16 × 32” is often the size of the hole, not the water.
| In-ground, 3½ to 6 ft deep | Surface | Gallons |
|---|---|---|
| 12 × 24 ft | 288 ft² | 10,233 |
| 14 × 28 ft | 392 ft² | 13,929 |
| 15 × 30 ft | 450 ft² | 15,990 |
| 16 × 32 ft | 512 ft² | 18,193 |
| 18 × 36 ft | 648 ft² | 23,025 |
| 20 × 40 ft | 800 ft² | 28,426 |
| Above-ground round | 3½ ft of water | 4 ft of water |
|---|---|---|
| 12 ft round | 2,961 | 3,384 |
| 15 ft round | 4,627 | 5,288 |
| 18 ft round | 6,662 | 7,614 |
| 21 ft round | 9,068 | 10,364 |
| 24 ft round | 11,844 | 13,536 |
| 27 ft round | 14,991 | 17,132 |
| 30 ft round | 18,507 | 21,151 |
How depth changes volume
Show the numbers
| Average depth (ft) | 12 × 24 | 16 × 32 | 20 × 40 |
|---|---|---|---|
| 3 | 6,463 | 11,490 | 17,953 |
| 4 | 8,618 | 15,320 | 23,938 |
| 5 | 10,772 | 19,150 | 29,922 |
| 6 | 12,926 | 22,980 | 35,906 |
| 7 | 15,081 | 26,810 | 41,891 |
Check your number another way
If you already run a salt cell, you can measure volume with salt. Test the salt level, add a known amount, run the pump for a day and test again. Then:
Gallons ≈ pounds of salt added × 119,826 ÷ rise in ppm
119,826 turns pounds per gallon into milligrams per litre, which is what ppm means. Salt strips read in steps of a few hundred ppm, so add enough salt for a clear rise.Filling with a metered supply works too: note the meter before and after a fill from empty. A water truck delivery ticket also gives you the number.
Why the volume matters
Every dose on this site scales with volume. If you think you have 15,000 gallons and you really have 20,000, every chlorine, acid and salt dose comes out a quarter too small, and you will chase readings that never quite reach target. Save the result once: this site remembers it on your device and offers it in every other calculator.
Frequently asked questions
How many gallons is a 16 × 32 pool?
With a 3½ ft shallow end and 6 ft deep end, about 18,193 gallons (68,867 litres). A pool that size at a constant 4 ft holds about 15,320 gallons.
How many gallons is a 24 ft round above-ground pool?
About 13,536 gallons with 4 ft of water, or 11,844 gallons with 3½ ft. Most 52-inch wall pools are filled to about 4 ft.
Where does 7.48 come from?
One cubic foot holds 7.48 US gallons. A US gallon is defined as 231 cubic inches, and a cubic foot is 1,728 cubic inches, so 1,728 ÷ 231 = 7.48.
Should I measure depth to the top of the wall?
No. Measure to the water line. The gap between the water and the coping or top rail is often 4 to 8 inches, which can be 10% or more of the water in a shallow pool.
Do steps, benches and sun shelves count?
They take up space that would otherwise hold water. For a few steps the difference is small. A large tanning ledge or a beach entry is worth measuring separately and subtracting.
How do I convert gallons to litres?
Multiply US gallons by 3.785. If you use UK (imperial) gallons, multiply by 4.546 instead. The result panel shows litres and UK gallons for you.
How accurate is a kidney or freeform estimate?
Treat it as within about 10%. If you need better, enter the surface area from your plans using the Other shape, or measure with the salt method above.
Sources
- NIST Guide to the SI, Appendix B: Conversion Factors. U.S. National Institute of Standards and Technology.
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.