Pool Volume Calculator: Gallons for Any Pool Shape

Pool shape

RectangleRoundOvalKidney

Lengthft

Widthft

Depth

One depth throughoutShallow and deep ends (averaged)

Depthft

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Pick the shape, enter its measurements in feet, and one depth or both ends; the tool gives gallons and liters. Every dosing calculator on the site starts from this number, so it is worth measuring once and keeping the answer somewhere you can find it again. A tape measure is enough. The arithmetic is the same whether the walls are vinyl, plaster or fiberglass, and the calculator shows the surface area and the cubic feet as well, so each step can be checked on its own.

Worked example: CircuPool's 30 by 15 foot pool

This is the case the calculator opens with, written out as an example rather than as a recommendation for any particular pool. A rectangle 30 feet long and 15 feet wide, 4.5 feet deep throughout, works out at 15,148 gal (57,342 L). Its surface is 450 square feet, so the water is 2,025 cubic feet.

CircuPool's CORE manual works the same pool: "15’ wide x 30’ long x 4.5’ Average Depth x 7.5 = 15187 Gallons". The two answers are 39 gallons apart, and the whole of that gap sits in the gallons per cubic foot each one uses: NIST's 7.480520 here against the rounded 7.5 there, which is 0.26% higher. The measurements, the surface area and the cubic feet are identical either way.

Worked example: a 24-foot round pool

A round pool 24 feet across and 4 feet deep comes to 13,536 gal. The calculator takes the area as π × 12 × 12, which is about 452.4 square feet, then multiplies by the depth and by 7.480520.

CircuPool's manual keeps a shortcut for the same shape: "Pool Diameter x Pool Diameter x Average Depth x 5.9 = Pool Gallons". For this pool that is 24 × 24 × 4 × 5.9, or 13,593.6 gallons. The 5.9 is doing two jobs in one number, and the arithmetic behind it is easy to reconstruct: π ÷ 4 turns diameter × diameter into the area of a circle, and π ÷ 4 × 7.5 comes to 5.890, rounded to 5.9. Hayward's AquaRite manual gives a round pool the same 5.9.

Two ovals

PHTA treats an oval as an ellipse: "Volume Oval (gallons) = Short Radius x Long Radius x 3.14 x Avg. Depth x 7.5". The salt-system manuals use something else. CircuPool writes it as "Pool Width x Pool Length x Average Depth x 6.7 = Pool Gallons", and Hayward's AquaRite manual gives an oval the same 6.7.

The two part company by enough to matter. At 30 by 15 feet and 4.5 feet deep, the calculator returns 11,897 gal as an ellipse and 13,532 gal by the manuals' oval, 1,635 gallons apart. The reason shows up when the two factors are set against 7.5: π ÷ 4 leaves 0.785 of the rectangle that boxes the pool in, while 6.7 ÷ 7.5 leaves 0.893 of it. The second is the fuller shape, a rectangle with its corners rounded off rather than sides that curve the whole way.

So pick Ellipse (PHTA) for a true ellipse and Hayward/CircuPool oval for a rectangle with rounded ends; measuring the actual curve at the ends of the pool is the only way to tell the two apart. Hayward and CircuPool don't say where 6.7 comes from.

Kidney pools

PHTA gives a kidney its own line: "Volume Kidney (gallons) = (WidthA + WidthB) x Length x 0.45 x Avg. Depth x 7.5". Width A and width B are the two lobes, each measured across its widest part, and the length runs the long way. A kidney 30 feet long with lobes of 10 and 14 feet, 4.5 feet deep, comes out at 10,907 gal in this calculator.

PHTA doesn't say where 0.45 comes from. Treat the result as an estimate: two kidneys that measure the same three ways can still hold different amounts of water, because the factor cannot know how deeply the waist is cut. Where a dose would be uncomfortable to overshoot, it is worth checking the figure against how much water the pool actually took to fill.

Depth

A pool with a shallow end and a deep end has no single depth to type in. PHTA's rule is "Average Depth = (Shallow Depth + Deep Depth) ÷ 2", and choosing Shallow and deep ends (averaged) under Depth applies exactly that. A 32 by 16 foot pool running from 3.5 feet at the steps to 8 feet at the drain averages 5.75 feet and holds 22,023 gallons. The surface area is untouched by the choice; the cubic feet and the gallons both move with the averaged depth.

Why 7.480520

NIST SP 811 puts a US gallon at 3.785412 liters, and a cubic foot at 0.02831685 cubic meters. A cubic foot is therefore 28.31685 liters, and dividing that by 3.785412 leaves 7.480520 gallons per cubic foot. The site derives that figure from the two NIST factors in code rather than typing it in, and uses it for every shape on this page.

PHTA rounds it and says so plainly: "1 Cubic Foot Water = 7.5 gallons". The salt-system manuals round it the same way, which is the entire reason CircuPool's worked example lands 39 gallons above this one. Neither figure is wrong for a bucket of chlorine; the difference only becomes visible on a pool large enough for a quarter of a percent to be a real number of gallons.

Once you have the volume, the pool chemical calculator and the pool shock calculator size their doses from it.