How the pool volume calculator works
A pool's volume is its surface area times its average depth. In feet that gives cubic feet, and each cubic foot holds 7.48 US gallons. The North Carolina Division of Public Health uses exactly this method when it reviews pool plans: length × width × average depth × 7.48, with π × radius × radius in place of length × width for a round pool.
1. Pick the shape
- Rectangle or square: length × width.
- Round: π × radius × radius, where the radius is half the diameter. That is where the 5.9 shortcut comes from: π ÷ 4 × 7.48 = 5.9, applied to diameter × diameter.
- Oval with straight sides: most above-ground ovals are a rectangle with a half circle on each end. The area is width × (length − width) plus π × (width ÷ 2)².
- Oval, true ellipse: π × half the length × half the width, the same as Leslie's 3.14 × length × width × 0.25.
- Kidney: Leslie's formula, (A + B) × length × 0.45, where A and B are the widths at the two widest points.
- Freeform: Leslie's shortcut, longest length × widest width × average depth × 5.9. It treats the pool like an oval that fits inside those two measurements.
2. Measure the water depth at each end
Measure from the water surface to the floor at the shallow end and at the deep end. Add them and divide by 2. This is the method in the North Carolina sheet, the City of Akron guide and the CPO pool math workbook. For a flat-bottom above-ground pool, use the water depth, which is lower than the wall height.
3. Convert
Cubic feet × 7.48 gives gallons, and gallons × 3.785 gives liters (NIST Handbook 44). In meters, cubic meters × 1,000 gives liters.
Worked example
North Carolina's plan review sheet uses a pool 25 ft long and 14 ft wide, 3 ft deep at one end and 5 ft at the other. The average depth is (3 + 5) ÷ 2 = 4 ft.
| Surface area: 25 × 14 | 350 sq ft |
|---|---|
| Volume: 350 × 4 | 1,400 cu ft |
| Gallons: 1,400 × 7.48 | 10,472 gal |
| Liters: 10,472 × 3.785 | 39,641 L |
| 10% liquid chlorine for 1 ppm | 14 fl oz |
A 24 ft round above-ground pool with 4 ft of water holds π × 12 × 12 × 4 × 7.48 = 13,535 gallons. A 15 × 30 ft straight-sided oval with 4 ft of water holds 12,019 gallons. If that same oval were a true ellipse it would hold only 10,575, which is why the shape choice matters.
Where the sources differ
- 7.5 or 7.48. The exact figure is 7.4805 gallons per cubic foot (NIST). Leslie's, the City of Akron and the CPO workbook round it to 7.5, which adds about 0.3%. North Carolina uses 7.48, and so does this calculator.
- Ovals. Akron and Leslie's treat an oval as an ellipse, a 5.9 multiplier on length × width × depth. If you have seen a 6.7 multiplier for ovals, that is about what a straight-sided oval twice as long as it is wide works out to: the 12,019 gallons above is 6.68 × 30 × 15 × 4. Pick the oval that matches your pool.
- Freeform pools. Akron uses 7.5 for freeform pools, the same as a rectangle around the whole pool. Leslie's uses 5.9. A rectangle drawn around the pool is the most it could hold, so 7.5 runs high. If your pool has a big bump-out, split it into a rectangle and a half circle, work each one out and add them.
- Steps and benches. The North Carolina sheet notes that the length × width × depth answer does not subtract the water taken up by steps. Its example plan listed 10,366 gallons against 10,472 from the formula.
Sources
- NC Division of Public Health: Basic Pool Plan Review Calculations sheet (pool volume, 7.48 gal per cubic foot)
- NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement
- Leslie's: How to Calculate Your Pool Volume (oval, kidney and irregular formulas)
- City of Akron: How to calculate the gallons in a swimming pool (shape multipliers)
- CPO Pool Math Workbook (Chad Bland, CPO instructor)
- Indiana Department of Health: Adjusting Chemical Levels in a Swimming Pool (chlorine per ppm)