How Many Gallons Are in an Above-Ground Pool?

 

To calculate pool volume, multiply the water surface area by the actual water depth. For a flat-bottom above-ground pool, choose the formula matching its inside shape, then convert the result to US gallons or liters.

Use the Pool Volume Calculator

1. Measure the water, not the outside frame

Measure inside length and width at the waterline. For a circle, measure inside diameter. Use actual filled water depth, not wall height or a nominal pool model size. This method assumes vertical sides and a flat bottom of constant depth.

2. Choose the correct area formula

• Rectangle: length x width.
• Circle: pi x diameter squared / 4.
• True ellipse: pi x overall length x overall width / 4.
• Oblong with two semicircular ends: (overall length - width) x width + pi x width squared / 4.

An ellipse is different from an oblong. For irregular outlines or other oval shapes, do not assume either formula fits; consult manufacturer data or measure separate sections.

3. Use consistent depth units

When area is in square feet, divide water depth in inches by 12 before multiplying. With area in square meters, divide depth in centimeters by 100. Decimal feet are different from feet-and-inches notation: 12 ft 6 in equals 12.5 ft.

4. Convert volume

One cubic foot is approximately 7.48051948 US gallons. One cubic meter is 1,000 liters. One US gallon is exactly 3.785411784 liters. These are US liquid gallons, not Imperial gallons.

Example: an 18-foot round pool

For an actual inside diameter of 18 ft and water depth of 48 inches (4 ft), volume = pi x 18 squared / 4 x 4. The result is approximately 1,017.88 cubic feet, 7,614.24 US gallons, 28,823.04 liters, or 28.82 cubic meters. Nominal wall size and actual filled capacity can differ.

What if my pool has a sloped bottom?

A single constant-depth model does not describe a complex bottom. Averaging minimum and maximum depths is not generally valid for flat shallow sections, deep wells, steps, or irregular slopes. Use an appropriate sectional calculation or manufacturer fill-volume data.

Use the result as an estimate

Rounded corners, curved walls, steps, and fittings can change real capacity. Verify capacity for the intended fill level. This volume calculator does not determine chemical dosing, equipment sizing, structural suitability, or safe operation.

Check an oval pool's outline before calculating

Two pools can share the same overall length and width yet contain different amounts of water. Consider inside measurements of 10 meters by 4 meters with a constant depth of 1 meter. A true ellipse has an area of approximately 31.42 square meters and a volume of about 31,416 liters. An oblong with straight sides and two semicircular ends has an area of approximately 36.57 square meters and holds about 36,566 liters. Check the actual outline or manufacturer drawing before choosing a formula.

Why a small depth error matters

Volume changes directly with water depth when the surface area stays the same. In the 18 ft round-pool example, entering 52 inches instead of an actual 48 inches overstates volume by about 8.3%. This can happen when wall height is used in place of the filled water level. Measure the water depth appropriate to the constant-depth model and check it again if the fill level changes.

Compare your result with stated capacity

If the manufacturer lists a capacity, check the conditions attached to it. A published figure may refer to a particular fill line or different inside dimensions from the nominal model name. A small difference can come from rounded measurements; a larger difference deserves a check of shape, depth, and units. Do not force the measurements to match a published number without understanding which assumptions differ.

Convert a metric example step by step

A rectangular pool with inside dimensions of 5 meters by 3 meters and water depth of 120 centimeters has a depth of 1.2 meters. Multiply 5 x 3 x 1.2 to get 18 cubic meters. That equals 18,000 liters, or approximately 4,755.10 US gallons. Entering 120 as meters instead of converting centimeters would produce a very different result, so always read the depth label next to the input.

Keep a record of the measurements

Save the selected shape, inside dimensions, water depth, and units with the estimated volume. This makes the result easier to review when comparing it with manufacturer information or discussing a water delivery. If the pool is irregular or has a nonuniform floor, use a method suited to that geometry instead of treating additional decimal places as additional accuracy.

Compare common above-ground pool sizes

For worked examples using actual inside diameters and a stated water depth, see how many gallons are in an above-ground pool. The examples also explain why nominal wall height and different oval outlines can change the estimate.