How the round pool volume calculator works
The calculator halves your diameter to get the radius, squares it, then multiplies by 3.1416 for the surface area. That area times the water depth gives cubic feet, and cubic feet times 7.48 gives US gallons.
Most round pools are above ground with a flat floor, so a single water depth is all you need. Measure to the water line rather than the top rail, since the last few inches of wall are never filled.
The formula
Volume = 3.1416 x radius squared x depth
Measure: Diameter or radius, depth.
Worked example
An 18 foot round pool has a 9 foot radius. Area is 3.1416 x 9 x 9, which is 254.5 square feet.
With 4 feet of water that is 1,018 cubic feet, or about 7,616 gallons and 28,830 litres.
Tips before you measure
Measure the diameter through the centre of the pool, not across a chord near the edge. A foot of error on an 18 foot pool changes the volume by roughly 850 gallons.
If the liner bulges outward at the bottom, use the widest water line measurement. Chemical doses are safer slightly over-measured than under-measured.
Round / Circular pool volume FAQs
How many gallons is a 24 foot round pool?
With 4 feet of water a 24 foot round pool holds about 13,540 gallons. With 4.5 feet it holds roughly 15,230 gallons.
Do I use radius or diameter?
Either works. The calculator lets you pick, and it halves the diameter for you if that is the measurement you have.
Does the pool wall height matter?
No. Only the water depth counts, and most round pools are filled six inches below the top rail.
Other pool shapes
Not quite the right outline? The main pool volume calculator covers all 20 shapes with the same units and formulas.