Circle Pool Volume Calculator: Formula and Examples

A circle pool volume calculator estimates how much water a round pool holds from its diameter and average water depth. Enter both measurements in the same unit system to get the volume in cubic feet and gallons, or cubic metres and litres.

Formula

A round pool with vertical sides is approximated as a cylinder. Its volume is the area of its circular base multiplied by its water depth:

V = πr²h = π(d ÷ 2)²h

• V is volume in cubic feet when the measurements are in feet.
• π is approximately 3.14159.
• r is the pool’s radius in feet: half its diameter.
• d is the inside diameter of the pool, measured across its centre, in feet.
• h is the average water depth in feet.

The calculator converts cubic feet to US gallons using 7.48052 gallons per cubic foot. It converts cubic feet to litres using 28.3168 litres per cubic foot. These are volume conversions; they do not change the calculated pool size.

Worked Example

With the defaults, the pool diameter is 18 ft and its average water depth is 4 ft. The radius is half the diameter:

r = 18 ÷ 2 = 9 ft

Find the circular base area, then multiply by the depth:

V = π × 9² × 4 = π × 81 × 4 = 1,017.876 ft³

Convert to US gallons:

1,017.876 × 7.48052 = 7,615.84 US gal

Rounded to the calculator’s zero decimal places, the headline result is 7,616 US gallons. The same volume is approximately 1,017.88 ft³ or 28,827 litres.

How to Measure a Round Pool

Measure the pool’s inside diameter, not the distance around its rim. The diameter is a straight line from one inside wall to the opposite inside wall through the centre. If the pool is not perfectly round, take measurements across the centre in two directions and use their average as an approximation.

Measure water depth from the water surface to the pool floor. If the floor is level, one depth measurement may be enough. For a sloped, dished or otherwise uneven bottom, measure at several representative points and average them. For example, if the shallow and deep areas are roughly 3 ft and 5 ft, their simple average is 4 ft; a more uneven pool may need more measurements to get a useful estimate.

Use the pool’s inside dimensions at the waterline where possible. A wall that slopes inward or outward can make the water’s surface diameter differ from the diameter at the bottom, so the cylinder formula will be less exact.

When to Use the Estimate

Pool volume is useful when estimating how much water is needed to fill a pool, comparing pool sizes, or determining a starting point for water-treatment instructions that depend on volume. The result is an estimate, not a direct measurement of the water currently in the pool. The actual amount can differ because of wall shape, steps, benches, a sloped floor, and the fill level.

For more complicated round pools, split the shape into simpler parts if their dimensions are known. For example, a pool with a level cylindrical section and a distinct sloped or rounded bottom can be estimated by calculating the sections separately and adding their volumes. Manufacturer specifications can also help when the pool has a nonstandard shape.

Units, Accuracy, and Common Mistakes

The calculator accepts feet. If your measurements are in inches, convert them before entering them: divide each measurement by 12. For metric measurements, calculate in metres using the same cylinder formula, then multiply cubic metres by 1,000 to get litres. Do not enter a diameter in feet and a depth in inches; mixing units produces a result that is wrong by a large factor.

• Do not use circumference as diameter. If you measured around the pool, divide the circumference by π to estimate its diameter.
• Do not use the radius as the diameter. The radius is half the width across the pool; using it as the diameter makes the calculated base area one-quarter as large.
• Use water depth, not wall height. A pool is usually not filled to the very top.
• Keep extra digits until the final conversion. Rounding the radius, area or depth too early can make the final estimate less accurate.
• Check the shape. The formula assumes a circular base and vertical sides. It cannot exactly represent tapered walls, a sloped bottom or built-in features.

A result in US gallons is not the same as imperial gallons. The calculator’s gallon output is specifically US gallons. For metric pool volumes, litres are often easier to interpret; 1 cubic metre equals 1,000 litres.

Limits of the Cylinder Formula

The calculation assumes that every horizontal slice of the pool has the same circular area. That is a good approximation for a round pool with near-vertical sides and a level floor, but not for a pool that narrows toward the bottom or has a substantial slope. The average-depth method improves an estimate for an uneven floor, but it does not account perfectly for how much of the pool has each depth.

Even accurate dimensions do not guarantee the exact amount of water on a particular day. The pool may be only partly filled, and internal features displace some water. Treat the result as a planning estimate and use the pool’s documented capacity when a precise capacity figure is required.

Frequently Asked Questions

How do I calculate the volume of a round pool?
Divide the diameter by two to get the radius, square the radius, and multiply by π and the average water depth: V = πr²h. Convert the resulting cubic units to gallons or litres if needed.

How many gallons are in a 18-foot round pool?
Diameter alone is not enough to determine pool volume; you also need the average water depth. For an 18 ft diameter and a 4 ft average water depth, the cylinder estimate is about 7,616 US gallons.

Should I measure pool depth to the rim or to the water?
Measure from the water surface to the floor. The space between the water and the rim is not part of the water volume.

Can I use this calculator for a pool with a sloped bottom?
Yes, as an estimate if you enter a representative average water depth. For a pronounced slope or an irregular floor, measure several depths or use documented capacity information for a closer figure.

How do I convert cubic feet to pool gallons?
For US gallons, multiply cubic feet by 7.48052. For example, 100 ft³ is about 748 US gallons.

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in d | Pool diameter | ft | 18 | 0
in h | Average water depth | ft | 4 | 0
out gallons | Pool volume | US gal | pi * (d / 2)^2 * h * 7.48052 | 0
out cubic_feet | Pool volume | ft³ | pi * (d / 2)^2 * h | 2
out litres | Pool volume | L | pi * (d / 2)^2 * h * 28.3168 | 0