Your pool’s volume in gallons comes down to three things: its surface area, its average depth, and the 7.48 gallons in every cubic foot of water.
Step 1 — surface area (sq ft):
- Rectangle / square: length × width
- Round / circular: diameter × diameter × 0.785
- Oval: length × width × 0.785
Step 2 — average depth (ft): for a sloped pool, add the shallow-end and deep-end depths and
divide by two: (shallow + deep) ÷ 2. A constant-depth pool just uses that one depth.
Step 3 — gallons: surface area × average depth × 7.48.
The pool volume calculator does all three steps for you — pick the shape, enter the size and both depths, and it returns the gallons plus the surface area and water volume in cubic feet.
Typical pool sizes and gallons
- 15 ft round, ~4 ft deep: ≈ 5,300 gallons
- 18 ft round, ~4 ft deep: ≈ 7,600 gallons
- 24 ft round, ~4 ft deep: ≈ 13,500 gallons
- 16 × 32 in-ground, ~5.5 ft avg: ≈ 21,000 gallons
- 20 × 40 in-ground, ~6 ft avg: ≈ 35,900 gallons
These are estimates — actual fill is a little lower because pools aren’t filled to the very top, and attached spas, beach entries, or curved walls change the math.
Why volume matters
Nearly every pool chemical is dosed per gallon — chlorine, shock, algaecide, stabilizer, and pH adjusters all reference your total volume. Get the number wrong and you either waste product or leave the water under-treated. Volume also drives heater and pump sizing, so it’s worth nailing down once and writing on the inside of your equipment cabinet.