Round Pool Calculator

How many gallons a round pool holds

A round pool calculator finds the water volume of a circular pool from its diameter and depth. A round pool is a cylinder lying on its base, so the tool uses the cylinder volume formula and converts cubic feet to gallons.

The field labels matter. The first field is height or depth in feet. The second is the diameter in inches, so a 15 ft pool is entered as 180.

The formula in plain words

1. Radius in feet = diameter in inches ÷ 24. The 24 comes from halving the diameter and dividing by 12 inches per foot.
2. Cubic feet = π × radius × radius × depth in feet.
3. Gallons = cubic feet × 7.48, since a cubic foot holds 7.48 gallons.
4. Cost = gallons × price per gallon.

Worked example: a 15 ft pool at 4 ft

• Radius: 180 ÷ 24 = 7.5 ft
• Cubic feet: π × 7.5² × 4 = 706.86 ft³
• Gallons: 706.86 × 7.48 = 5,287.30
• Fill cost at $0.01 per gallon: $52.87

Worked example: a 21 ft pool at 4.5 ft

A pool is 21 ft across, which is 252 in. Water depth is 4.5 ft.

• Radius: 252 ÷ 24 = 10.5 ft
• Cubic feet: π × 10.5² × 4.5 = 1,558.62 ft³
• Gallons: 1,558.62 × 7.48 = 11,658.50
• Fill cost at $0.01 per gallon: $116.59

Gallons by diameter at 4 ft deep

Pool diameter	Diameter to enter	Gallons
12 ft	144 in	3,383.87
15 ft	180 in	5,287.30
18 ft	216 in	7,613.71
24 ft	288 in	13,535.49

Going from 12 ft to 24 ft doubles the diameter but multiplies the water by four. The radius is squared in the formula.

How to measure

Measure the diameter across the middle of the pool at the water line, inside edge to inside edge. Convert feet to inches by multiplying by 12. Then measure the depth of water in the middle with a stick or tape.

For a pool with a sloped floor, take a few readings and average them. Because the tool assumes a flat bottom, an average gives a better answer than the deepest point.

If the pool is not truly round, such as an oval, a different tool is a better fit. The pool gallons calculator handles rectangular shapes.

Why the number matters

Chemical dosing is tied to volume. A bottle of sanitizer or shock lists how much to add per 10,000 gallons, and the wrong volume gives the wrong dose. Too little leaves the water unprotected, and too much wastes product and can irritate swimmers. Always read the product label.

Volume also decides how long the pump takes to turn over the water and how much heat a heater has to supply. The pool heat pump sizing calculator uses surface area for that job.

Weight of the water

Water weighs about 8.34 lb per gallon. The 15 ft pool above holds 5,287.30 gallons, so the water alone is roughly 44,000 lb, or more than 20 tons. That load sits on a circle only 15 ft across. The ground under an above-ground pool should be level, compacted, and free of sod and stones. Check the pool maker’s site-prep instructions before you fill.

Common mistakes

• Typing the diameter in feet. The field expects inches, and a 15 gives a tiny answer.
• Using the wall height as depth. The water is lower than the wall.
• Entering radius for diameter. That cuts the volume by a factor of four.
• Forgetting the weight. A full round pool is heavy, so prepare a level base.
• Leaving out top-ups. Evaporation and splash-out mean you add water through the season.

Related calculators

For a rectangular or kidney-shaped pool, use the pool gallons calculator. A small round tub is better handled by the stock tank pool calculator, and heater sizing lives in the pool heat pump sizing calculator.