Above Ground Pool Water Volume Calculator and Formula

An above ground pool water volume calculator estimates how many gallons or liters a pool holds when you enter its shape and inside dimensions. This calculator uses pool diameter, average water depth, and a shape factor to return an estimated volume in US gallons and liters.

Formula

For a round pool, volume is the area of its circular footprint multiplied by average water depth. For an oval pool, the calculator approximates the footprint using a shape factor.

Volume in cubic feet = π × (D ÷ 2)² × H × S

Volume in US gallons = π × (D ÷ 2)² × H × S × 7.48052

Volume in liters = gallons × 3.78541

• D = inside diameter for a round pool, in feet. For an oval pool, enter its inside length as the diameter value; the shape factor approximates its narrower width.
• H = average water depth, in feet, not the pool wall height.
• S = shape factor: 1 for a round pool; about 0.85 for an oval pool as a rough estimate.
• π ≈ 3.14159; 7.48052 converts cubic feet to US gallons, and 3.78541 converts US gallons to liters.

The oval factor is only an approximation. If you know both inside length and width, a more direct oval calculation is π × (length ÷ 2) × (width ÷ 2) × average depth. The calculator’s shape-factor field lets you represent the oval footprint as a fraction of a circle whose diameter matches the entered length.

Worked Example

Using the calculator’s default values—an 18-foot inside diameter, 4-foot average water depth, and a round-pool shape factor of 1:

1. Radius = 18 ÷ 2 = 9 ft.
2. Footprint area = π × 9² = 254.469 ft² (approximately).
3. Volume = 254.469 × 4 × 1 = 1,017.876 ft³.
4. Gallons = 1,017.876 × 7.48052 = 7,613.9 US gal, which rounds to 7,614 US gal.
5. Liters = 7,613.9 × 3.78541 = 28,818.2 L, which rounds to 28,818 L.

The calculator rounds its gallon and liter outputs to whole units. The cubic-foot output is shown to two decimal places.

How to Measure Your Pool

Measure the water-holding interior, rather than relying only on the pool’s advertised size. A nominal 18-foot pool may have a different usable inside diameter, and the water surface is usually below the top of the wall.

• For a round pool, measure across the inside from one side to the other through the center.
• For an oval pool, measure the inside length and width. If using this calculator’s single diameter field, use the inside length and adjust the shape factor to reflect the narrower width.
• Measure water depth at several locations and use the average. For a flat-bottomed pool, depth may be fairly uniform; for a sloped or dished bottom, take readings across the area.
• Convert measurements to feet before entering them. For inches, divide by 12; for centimeters, divide by 30.48.

For example, a water depth of 42 inches is 3.5 feet, not 42 feet. A small measurement error can make a noticeable difference because the diameter is squared in the formula.

When the Estimate Is Useful

Knowing approximate water volume can help you compare a pool’s capacity with its manual, estimate how much water is needed to fill it, and interpret product directions that specify quantities per gallon. It can also help when discussing filtration or water-care needs, though equipment and product instructions should be followed for the specific pool and water conditions.

Volume is not the same as the amount of water you must order or add. A pool may not be completely drained before refilling, and actual fill level, bottom shape, and dimensions affect the total. Use a water meter or supplier’s measured delivery total if you need a more precise record.

Common Errors and Limits

• Using wall height as water depth: The water level is often lower than the wall, so use the actual water depth.
• Entering radius as diameter: The field requests the full width across the pool. If you enter the radius, the calculated footprint becomes one quarter of the intended area.
• Mixing units: Do not enter inches in a field marked feet. Convert first, or convert all measurements to the same unit before applying the formula.
• Assuming an oval factor is exact: Oval proportions vary. Measure both axes and use the ellipse formula when accuracy matters.
• Ignoring a sloped or irregular bottom: The formula assumes a consistent average depth and a simple round or oval footprint. It is an estimate for pools with complex floors or unusual shapes.

Rounding the final answer is sensible, but rounding the diameter or depth too early can compound error. Keep measured values as precise as practical, then round the displayed volume to match the estimate’s purpose.

Frequently Asked Questions

How many gallons are in an 18-foot round pool?
It depends on average water depth. At 4 feet deep, the estimate is about 7,614 US gallons. A shallower fill holds less, so use the actual water depth in the formula.

How do I calculate the volume of an oval above ground pool?
Measure its inside length, inside width, and average water depth. Calculate π × (length ÷ 2) × (width ÷ 2) × depth in cubic feet, then multiply by 7.48052 for US gallons. The calculator’s shape factor is a convenient approximation if you do not enter both axes.

Should I use pool height or water depth?
Use water depth: the vertical distance from the pool floor to the water surface. Pool wall height is not necessarily the depth of the water.

How do I convert pool gallons to liters?
Multiply US gallons by 3.78541. For example, 1,000 US gallons is about 3,785 liters.

Why does my calculated volume differ from the pool manual?
The manual may give nominal capacity at a particular fill level, while your measurements may reflect a different waterline, interior dimensions, or bottom profile. Check that you used inside measurements and average water depth, and that all dimensions use the same units.

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in diameter | Inside diameter | ft | 18 | 0
in depth | Average water depth | ft | 4 | 0
in shape | Shape factor (round=1, oval=0.85) | - | 1 | 0
out gallons | Estimated water volume | US gal | pi * (diameter / 2)^2 * depth * shape * 7.48052 | 0
out liters | Estimated water volume | L | gallons * 3.78541 | 0
out cubicfeet | Estimated water volume | ft^3 | pi * (diameter / 2)^2 * depth * shape | 2