Round, elliptical and capsule-shaped pools are different
“Oval” is a useful everyday description, but it does not identify one exact geometric shape. An ellipse curves continuously around its outline. A capsule, sometimes called a stadium shape, has two straight parallel sides joined by semicircular ends. Their areas differ, so a single oval multiplier cannot be exact for both.
Begin with a circle
A circular pool's plan area is π × diameter² ÷ 4. This is the same as π × radius² because the radius is half the diameter. For an 18-foot inside diameter, the area is π × 18² ÷ 4, or about 254.47 square feet.
At a uniform water depth of 4 feet, the estimated volume is about 1,017.88 cubic feet. Converting the result gives about 7,614 US gallons. Entering the 18-foot diameter as a radius would make the area four times too large, so confirm which measurement the field requests.
A true ellipse uses both full dimensions
For an ellipse with full inside length L and full inside width W, area is π × L × W ÷ 4. The quarter factor appears because the ellipse formula uses two half-dimensions, often called semiaxes.
An ideal elliptical pool measuring 30 feet by 15 feet has an area of about 353.43 square feet. At a flat water depth of 4 feet, it holds about 1,413.72 cubic feet, or 10,575 US gallons.
This model assumes that the measured outline actually follows an ellipse. A kidney-shaped pool, a bowed rectangle or an outline with irregular corners does not become an ellipse simply because its greatest length and width are known.
A capsule includes a central rectangle
For a capsule with overall length L and width W, where L is at least W, the middle rectangle is (L − W) long and W wide. The two semicircular ends together make one circle of diameter W. Area is therefore W × (L − W) + π × W² ÷ 4.
For the same 30-by-15-foot outer dimensions, the rectangle contributes 225 square feet and the two ends contribute about 176.71 square feet. Total area is about 401.71 square feet. At 4 feet deep, volume is about 1,606.86 cubic feet, or 12,020 US gallons.
That is about 1,445 gallons more than the ellipse example despite identical overall length, width and depth. Choosing the matching outline matters far more than the last digit of a conversion factor.
Confirm the model before using the number
These are geometry-derived formulas, not claims that every manufacturer uses one of these shapes. Compare the actual interior outline with a sketch, product drawing or verified plan. Use nonoverlapping simpler regions when the outline does not match.
Convert the resulting cubic volume with the standard NIST unit relationships, and round only the displayed answer. If the floor changes depth, use an area-weighted depth model or separate zones. Always retain the shape assumption alongside the volume estimate so the result can be checked when better dimensions become available.
Put the example to work
Open the pool volume calculator and replace the example values with your own measurements. Keep the assumptions and result together using Download CSV or Print.
Sources & further reading
Source material checked November 9, 2026. This guide is an original explanation; source publishers do not endorse this website.