Pool Volume Calculator

What the pool volume calculator does

Knowing how many gallons your pool holds is the number behind everything else, from how much chlorine to add to how long it takes to fill. This works it out. You pick the pool's shape, enter its size and depth, and it returns the volume in gallons, litres, and cubic feet.

It covers the four common pool shapes, each with its own formula. Below is how it works and the one measurement people most often get wrong.

How to use it

1. Pick the shape: rectangular, circular, oval, or oblong.
2. Enter the size for that shape, the length and width, or the diameter, each with its unit.
3. Enter the shallow depth, and the deep depth too if the floor slopes, then press Calculate, or Reset to clear it.

How the volume is worked out

Every shape works the same way underneath: find the area of the water's surface, multiply by how deep it is, then turn cubic feet into gallons:

Gallons = surface area × average depth × 7.48

The 7.48 is the number of US gallons in a cubic foot of water, so it is what converts the volume into the unit you actually fill and treat the pool in. What changes between shapes is how the surface area is worked out, which the shapes section covers.

Why average depth matters

This is the measurement people get wrong, and it throws the whole figure off. Most pools are not one depth; they slope from a shallow end to a deep end. If you use the deep end's depth for the whole pool, you can overstate the volume by a quarter to a half, which means badly over-dosing chemicals and oversizing equipment.

The fix is the average depth, which is why the calculator asks for both ends. Add the shallow and the deep depth and divide by two, and you have a depth that represents the whole sloping floor. Enter just the shallow depth if the pool is a single constant depth, and both if it slopes, and the volume comes out right.

The four shapes

The shapes differ only in how the surface area is found. A rectangular pool is simply length times width. A circular pool uses pi times the radius squared, the area of a circle. An oval pool uses the ellipse area, length times width times pi over four, since an oval is a squashed circle and does not fill its bounding rectangle.

An oblong pool, the kidney or freeform kind, has no clean formula, so it uses an industry-standard shortcut: the two widths added together, times the length, times 0.45. That 0.45 is the long-accepted factor for approximating the area of a curved, indented pool, and it gets you close without measuring every curve. Pick the shape that matches yours and the right area formula is used for you.

A worked example: a 32 by 16 pool

Say you have a rectangular pool 32 feet long and 16 feet wide, with a 3 foot shallow end and an 8 foot deep end.

The average depth is (3 + 8) ÷ 2 = 5.5 feet. So the volume is 32 × 16 × 5.5 = 2,816 cubic feet, and multiplying by 7.48 gives about 21,000 gallons, or roughly 80,000 litres.

Had you used the 8 foot deep end for the whole pool, you would have got over 30,000 gallons, half as much again, which is exactly the average-depth trap.

Questions people ask

How do I calculate pool volume in gallons?

Find the surface area, multiply by the average depth for cubic feet, then multiply by 7.48 for gallons. A 32 by 16 foot pool averaging 5.5 feet deep holds about 21,000 gallons.

How do I find the average depth?

Add the shallow end depth and the deep end depth and divide by two. A 3 foot and 8 foot pool averages 5.5 feet.

Why multiply by 7.48?

Because there are about 7.48 US gallons in a cubic foot of water, so it converts the volume in cubic feet into gallons.

How is an oblong or kidney pool worked out?

With an industry-standard shortcut: the two widths added, times the length, times 0.45, which approximates the curved area. The calculator uses it for the oblong shape.

References

A quick note on the numbers. The surface areas are geometry, with the ellipse and the 0.45 oblong factor being the standard methods used across pool practice, and the average-depth approach is the accepted way to handle a sloping floor. The figure of 7.48 US gallons per cubic foot of water, and the litre conversions, follow the US National Institute of Standards and Technology guide.

1. National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811