The formulas
All measurements are in feet. Use the depth of the water, not the pool wall.
| Pool shape | Gallons |
|---|---|
| Rectangle or square | length × width × average depth × 7.48 |
| Round | diameter × diameter × average depth × 5.9 |
| Oval | length × width × average depth × 5.9 |
The 7.48 comes from the number of US gallons in one cubic foot. The 5.9 is 7.48 × π ÷ 4, which accounts for the curved shape.
Finding the average depth
If the pool has a shallow and a deep end with an even slope between them, add the two depths and divide by 2. For a pool that's 3½ ft at the shallow end and 8 ft at the deep end, the average depth is (3.5 + 8) ÷ 2 = 5.75 ft.
If the deep end is a short hopper rather than an even slope, split the pool into sections, work out each one separately, and add them together. For a free-form pool, estimate an average length and width, or split it into simpler shapes.
Worked examples
| Pool | Math | Gallons |
|---|---|---|
| Rectangle, 16 × 32 ft, 3½–8 ft deep | 16 × 32 × 5.75 × 7.48 | about 22,000 |
| Round, 24 ft, 52 in of water | 24 × 24 × 4.33 × 5.9 | about 14,700 |
| Round, 18 ft, 48 in of water | 18 × 18 × 4 × 5.9 | about 7,650 |
| Oval, 15 × 30 ft, 52 in of water | 15 × 30 × 4.33 × 5.9 | about 11,500 |
The pool volume calculator does the math for each shape and converts to liters.
Why the number matters
- Chemical doses. Chlorine, shock, pH adjusters and other products are dosed per 10,000 gallons. Overestimating the volume means overdosing.
- Pump and filter sizing. Pumps are usually sized so the whole volume circulates through the filter several times a day. Divide the volume by the hours you want for one turnover to get the gallons per hour needed.
- Heating. Heater size and run time depend on how much water you're warming.
- Filling. Water utilities and delivery services quote by the gallon.
Checking it with a water meter
If you're filling the pool from a hose on a metered supply, read the water meter before and after. Meters often read in cubic feet, so multiply the difference by 7.48 to get gallons. Some utilities can also give you a temporary credit on sewer charges for pool fills, so it's worth asking.
Tips
- Measure the water depth, which is usually a few inches below the top of the wall on above-ground pools.
- Round your answer to the nearest few hundred gallons. It's an estimate, not a meter reading.
- Write the volume on the inside of the pump-house door or in your phone for every chemical top-up.
- Follow the product label for chemical doses and safety precautions.