Pool Volume Calculator

Why you need your pool's volume

Every chemical dose is worked out per gallon or litre of water. The label on a tub of chlorine says "per 10,000 gallons", a salt chlorinator manual says how many kilograms per 10,000 litres, and a heater's warm-up time depends on how much water it has to heat. If the volume you use is 30% too high, every dose is 30% too strong. So the first number every pool owner should know is the volume, and it only takes a tape measure and a few minutes.

Volume (US gallons) = length × width × average depth (feet) × 7.48
Volume (litres) = length × width × average depth (metres) × 1,000
Average depth = (shallow end + deep end) ÷ 2

One cubic foot holds 7.48 US gallons and one cubic metre holds exactly 1,000 litres. For shapes that are not rectangles, the area of the water surface changes, but the idea is the same: surface area × average depth.

Formulas for each shape

The kidney formula is an approximation used throughout the pool trade, because real freeform pools vary. If your pool is very irregular, split it into a rectangle plus half-circles, work out each part with this calculator and add them together.

Worked examples

• Rectangle 32 × 16 ft, 3.5 ft shallow end, 6 ft deep end: average depth 4.75 ft. Volume = 32 × 16 × 4.75 × 7.48 = about 18,190 US gallons (68,870 litres).
• Round above-ground pool 24 ft across with 4 ft of water: area = π × 12² = 452.4 sq ft. Volume = 452.4 × 4 × 7.48 = about 13,540 gallons (51,240 litres).
• Metric rectangle 8 × 4 m, 1.0 m to 1.8 m deep: average 1.4 m. Volume = 8 × 4 × 1.4 = 44.8 m³ = 44,800 litres (about 11,835 US gallons).
• Square hot tub 2.1 × 2.1 m with 0.9 m of water: 3.97 m³ before seats. Molded seats and footwells take a lot of space, so the real volume is usually well below the box figure; use the manufacturer's stated capacity when you have it.

How to measure accurately

1. Measure the water, not the shell: depth from the water line to the floor, not from the coping.
2. For a sloped floor, measure depth at the shallow end and the deep end. If the floor has a steep hopper (a sudden drop), take the average of several readings along the length.
3. Measure length and width at the water line, inside the walls.
4. Above-ground pools: use the water depth, which is usually a few inches below the wall height.

Other ways to find the volume

• Manufacturer's data. Above-ground pools and hot tubs nearly always list capacity in the manual or on the box.
• Water meter. Read the meter before and after filling. This is the most accurate method for irregular pools.
• Salt test. On a salt pool, if you add a known amount of salt and the reading rises less or more than expected, your volume estimate is off.

Common mistakes

• Using wall height instead of water depth. Six inches of difference on a 4 ft pool is a 12% error.
• Mixing gallons. This calculator uses US gallons (3.785 L). A UK (imperial) gallon is 4.546 L, about 20% larger.
• Forgetting the deep end. Using only the shallow depth underestimates volume and leads to under-dosing.

With your volume, go straight to the pool chemical calculator for chlorine, alkalinity, hardness, stabilizer and salt doses, or the pool heater size calculator to see how long heating will take.

Estimates only. Always follow the dosing directions on the product label.

Frequently asked questions

Sources

• NIST Handbook 44, Appendix C: 1 US gallon = 231 cubic inches = 3.785411784 litres; 1 cubic foot = 7.48052 US gallons.
• Pool & Hot Tub Alliance (PHTA). Pool and spa owner resources on water volume and chemical dosing.

Last reviewed 28 October 2026. Results are estimates; see our disclaimer.