Circular Pool Volume Calculator and Formula

A circular pool volume calculator estimates how much water a round pool holds from its inside diameter and average water depth. Enter both measurements in feet or metres to get the volume in cubic feet or cubic metres, plus an approximate capacity in US gallons or litres.

Formula

A circular pool is treated as a cylinder. Its volume is the area of the circular base multiplied by the water depth:

V = π × (d ÷ 2)² × h

• V is volume in cubic feet (ft³).
• d is the pool’s inside diameter in feet (ft).
• h is the average depth of the water in feet (ft).
• π is approximately 3.14159.

The calculator then converts cubic feet to US gallons using 1 ft³ ≈ 7.48052 US gallons, and to litres using 1 ft³ ≈ 28.3168 L. These are unit conversions, not separate measurements of the pool. The gallon output means US liquid gallons; an Imperial gallon is larger.

Worked Example

Using the calculator’s default values, the inside diameter is 15 ft and the average water depth is 4 ft.

1. Find the radius: 15 ÷ 2 = 7.5 ft.
2. Find the circular base area: π × 7.5² = π × 56.25 ≈ 176.7146 ft².
3. Multiply by depth: 176.7146 × 4 ≈ 706.8583 ft³, which rounds to 707 ft³.
4. Convert to US gallons: 706.8583 × 7.48052 ≈ 5,287.19, or 5,287 US gallons to the nearest gallon.
5. Convert to litres: 706.8583 × 28.3168 ≈ 20,015.7, or 20,016 L to the nearest litre.

The first output is rounded to a whole cubic foot, so conversions based on the unrounded volume can differ slightly from conversions made using the displayed rounded figure.

How to Measure a Round Pool

Measure the inside diameter from one inner wall to the opposite inner wall, passing through the centre. Use the pool’s water depth, not its total wall height: water is commonly below the rim, and the space above the water does not contribute to the current volume.

If the bottom is level, measure the water depth from the water surface to the floor. If the floor slopes or has a deep central area, take representative depth readings and use a reasonable average. For example, a pool that is 3 ft deep at one side and 5 ft at the opposite side may be approximated with an average depth of 4 ft if the slope is gradual and reasonably uniform. An uneven or sculpted floor can make a simple average less exact.

For metric measurements, use metres for both diameter and depth in the cylinder formula. The result is cubic metres; multiply by 1,000 to convert m³ to litres. One metre is 100 centimetres, so convert centimetres to metres before calculating. Do not enter a diameter in feet and a depth in metres together.

When to Use the Estimate

Volume is useful when estimating how much water a pool holds, comparing pool sizes, or working out a theoretical amount for filling or draining. It also provides a starting-point capacity for product instructions that are based on pool volume. Follow the relevant product label and pool equipment guidance: the calculator does not determine a treatment dose, pump flow, or required operating time.

The result is the geometric volume of water up to the measured waterline. It does not account for water displaced by swimmers, steps, ladders, seats, or other large objects. In a typical open pool, these details may be small relative to total capacity, but built-in features or an unusual interior can cause a more noticeable difference.

Common Errors and Limits

• Entering radius as diameter: The input asks for the full width across the pool. If you measured from the centre to the wall, double that radius to get the diameter.
• Using wall height instead of water depth: Measure the actual water column. Using the full wall height overstates the volume when the pool is not filled to the rim.
• Mixing units: Convert all lengths to the same unit before applying the formula. Volume units are cubed, so a small length-unit mismatch can create a large error.
• Rounding too early: Keep extra digits during the calculation and round the final display. Calculator outputs are estimates, not a reason to measure pool dimensions to unrealistic precision.
• Assuming every round pool is a perfect cylinder: Sloped sides, a sloping floor, rounded corners, and irregular shapes make the true volume differ from this model.

The calculator assumes a vertical-sided cylinder with a flat base and consistent circular cross-section. If the sides taper substantially, the top and bottom diameters differ, or the pool has a complex shape, the cylinder formula may be a rough estimate rather than a close capacity figure. A manufacturer’s capacity specification or a more detailed geometric model may be more suitable when available.

Frequently Asked Questions

How do I calculate the volume of a round pool?
Square half the inside diameter, multiply by π, then multiply by the average water depth. Keep diameter and depth in the same units; the answer is in cubic units.

How many gallons are in a circular pool?
Calculate its volume in cubic feet, then multiply by about 7.48052 for US gallons. The result depends on the pool’s actual inside diameter and water depth.

Should I use the pool’s diameter or radius?
Use the diameter as the calculator input: the full distance across the circular pool through its centre. The formula divides that measurement by two to obtain the radius.

How do I calculate a round pool in litres?
Use metres for diameter and water depth to get cubic metres, then multiply by 1,000 for litres. Alternatively, calculate cubic feet and multiply by about 28.3168.

Does this formula work for an above-ground pool?
It can estimate an above-ground pool’s volume if its interior is approximately circular, has near-vertical sides, and has a reasonably level bottom. Use the inside diameter and actual water depth, not the exterior width or wall height.

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in d | Pool inside diameter | ft | 15 | 0
in h | Average water depth | ft | 4 | 0
out v | Pool volume | ft^3 | pi * (d / 2)^2 * h | 0
out gal | Approximate volume | US gal | v * 7.48052 | 0
out liters | Approximate volume | L | v * 28.3168 | 0