Pool Volume (Gallons) Calculator

How to use the pool volume calculator

1. Pick the shape: round, oval, rectangle or kidney.
2. Enter the size: the diameter of a round pool, the length and width of the others. For a kidney, the length and the width of each lobe at its widest.
3. Enter the water’s depth at the shallow end and the deep end. A flat bottom takes the same number in both. Measure the water, not the wall.

You get the US gallons, the litres, the water’s weight and the gallons in one inch of depth, which is what a top-up or a drain-down takes. The button under the answer opens the pool salt and chlorine calculator with your gallons filled in.

The formulas, and where they come from

Pentair’s engineering data (its 2025–2026 pool product catalog, “Estimating total gallons in a pool or spa”): “Length X Width X Average Depth X Gal.cu.ft. = Gallons”, with 7.5 gallons per cubic foot for a rectangle and 6.7 for an oval. For a round pool: “Dia. x Dia. x Av Dp x 5.9 = Gallons”, and its example “24 x 24 x 3.5 x 5.9 = 11,894 Gals.”

Hayward prints the same three in its AquaRite salt-cell manual: × 7.5 rectangular, × 5.9 round, × 6.7 oval.

Kidney. Pentair draws a kidney with its two widths, A and B, and its length C: “AREA = .45 X (A + B) C”. The calculator multiplies that area by the average depth and 7.5. (Pentair’s table also lists a 7.0 factor for a kidney, a shortcut that uses one width.)

Average depth. Pentair’s worked example adds a 3 ft shallow end and an 8 ft deep end and halves them: 40 × 20 = 800 sq ft, × 5.5 = 4,400 cubic feet, × 7.5 = 33,000 gallons.

Why 7.5 and 5.9. A US gallon is 231 cubic inches, so a cubic foot holds 7.48 gallons; the makers round it to 7.5. A circle fills 78.5% of the square around it, and 7.5 × 0.785 ≈ 5.9. The water weighs 8.34 lb a gallon, the figure Pentair’s heater sizing uses.

Round pools: gallons by diameter and water depth

Diameter × diameter × depth × 5.9. The 24 ft row at 3.5 ft is Pentair’s own example.

Rectangular pools: gallons by size and average depth

Length × width × average depth × 7.5. A pool 3 ft at the shallow end and 8 ft at the deep end averages 5.5 ft.

Worked examples

11,894 gallons in a 24 ft round pool, 3.5 ft deep: Pentair’s round example, to the gallon. One inch of depth is 283 gallons.

33,000 gallons in a 40 × 20 ft rectangular pool, 3 to 8 ft deep: Pentair’s formula A example. The water weighs about 138 tons.

13,568 gallons in a 30 × 15 ft oval pool, 4.5 ft deep: 30 × 15 × 4.5 × 6.7.

12,023 gallons in a 30 × 15/10 ft kidney pool, 3.5 to 6 ft deep: 0.45 × (15 + 10) × 30 = 337.5 sq ft, × 4.75 ft × 7.5.

Common mistakes

• Using the wall height as the depth. An above-ground pool is never filled to the top. Intex prints its frame pools’ capacity at 90% of the wall and its Easy Set pools’ at 80%.
• Using the deep end as the depth. A pool 3 ft at one end and 8 ft at the other averages 5.5 ft; using 8 adds nearly half again.
• Radius for diameter. The 5.9 formula takes the diameter twice. With the radius, use radius × radius × 3.14 × depth × 7.5, Pentair’s other round formula.
• Trusting a formula over the label. Intex prints its 15 ft × 48″ Metal Frame at 4,440 gallons; the round formula at 3.6 ft gives 4,779. Where the maker prints a capacity, dose by it.
• One formula for a free-form pool. Pentair: the area “can be determined by using one or more of the above formulas”. Measure an L-shape or a pool with a spa as two parts and add them.

More on pools at Dreamy Home Style

• Backyard pool designs
• Above ground pool ideas
• Pool ideas for small backyards
• Semi-inground pool ideas
• Pool deck ideas

Choosing a pool for the yard? The above-ground pool size calculator fits the sizes Intex and Doughboy sell to your space. Planning the yard around it? Draw it to scale in the free 2D floor planner.