Pool volume in gallons = length x width x avg depth x 7.48 (rectangular) or pi x r² x avg depth x 7.48 (round). Get gallons and cubic feet for any pool shape.
Gallons = volume in cu ft x 7.48052. Assumes uniform average depth.
Rectangular pools: length (ft) x width (ft) x average depth (ft) x 7.48 = gallons. Round pools: 3.14159 x radius² x average depth x 7.48 = gallons. Average depth = (shallow end + deep end) / 2.
Getting the gallon count right matters more than most pool owners realize. Every chemical treatment, from chlorine shock to pH adjusters to algaecide, is dosed per 10,000 gallons. Overestimate by 25 percent and your chemicals are too dilute. Underestimate and you are overdosing the water.
Pools slope from the shallow end to the deep end. Using only the deep end overstates the volume; using only the shallow end understates it. The average of the two gives an accurate approximation for most residential pools with a single slope. For pools with a flat bottom and a drop-off step, use a weighted average based on the length of each zone.
Knowing gallons also helps estimate heating costs and fill time. A standard garden hose delivers about 9 gallons per minute. A 25,000 gallon pool takes roughly 46 hours to fill. For pool construction or resurfacing, the cubic feet calculator can help size concrete shell volumes and gravel sub-bases. For larger landscaping projects around the pool, see the gravel calculator.
Rectangular pool: length x width x avg depth x 7.48 = gallons. Round pool: 3.14159 x r² x avg depth x 7.48 = gallons. Average depth = (shallow + deep) / 2.
At a 5 ft average depth: 40 x 20 x 5 x 7.48 = 29,920 gallons.
A rectangular pool about 16 ft x 32 ft with 2.6 ft average depth, or a 18 ft diameter round pool at 3.4 ft average depth, holds roughly 10,000 gallons.
Quick reference: 12x24 ft at 4 ft avg = 8,614 gal; 16x32 ft at 5 ft = 19,149 gal; 20x40 ft at 5.5 ft = 32,957 gal; 24 ft round at 4 ft = 13,564 gal.
Add shallow end depth to deep end depth and divide by 2. A pool with 3 ft shallow and 8 ft deep = (3 + 8) / 2 = 5.5 ft average depth.
Every chemical is dosed per 10,000 gallons. An error of 20 percent on volume means every treatment is off by the same margin. Getting it right once saves money every maintenance cycle.
Chris Terry is the editor of Encore Editorial and oversees content, sourcing, and the accuracy of everything published here. His background spans business operations, market research, and making complicated things readable.