How to calculate pool volume in gallons

Short answer. For a rectangular pool, volume is length x width x average depth x 7.48. Average depth is (shallow + deep) / 2 when the floor slopes evenly. Circular pools use 3.14 x radius squared x average depth x 7.48, ovals use length x width x average depth x 5.9, and freeform pools are best measured from the plan area.

What is the formula for pool volume in gallons?

Volume is area times depth, then converted to gallons. One cubic foot holds about 7.48 US gallons.

Rectangle: length x width x average depth x 7.48
Circle: 3.14 x radius squared x average depth x 7.48
Oval: length x width x average depth x 5.9

For metric, work in metres: cubic metres x 1000 = litres.

How do you find the average depth?

If the floor slopes evenly from the shallow end to the deep end, average depth is (shallow depth + deep depth) / 2. A pool 3.5 ft at one end and 8 ft at the other averages 5.75 ft.

Many pools are not one even slope. A flat shallow end, a slope and a flat deep end is common. Split the pool into sections, work out the volume of each, then add them. Guessing the average depth is an easy way to get the number wrong.

Worked example: a 30 ft by 15 ft rectangle

Depths are 3.5 ft and 8 ft, so the average is (3.5 + 8) / 2 = 5.75 ft.

30 x 15 x 5.75 = 2,587.5 cubic feet.

2,587.5 x 7.48 = about 19,355 gallons.

If the pool has a large sun ledge or wide steps, subtract their volume and the number drops.

What about freeform and kidney pools?

There is no reliable formula for a freeform or kidney shape. Measure the plan area instead: break the shape into rectangles, triangles and circles, or use the area from a scaled drawing, then multiply by the average depth and 7.48.

Steps, benches, ledges and tanning shelves all take up space, so they reduce the volume. Subtract them for a closer number.

What are the steps?

Measure the shape. Write down the length and width in feet, or the radius for a circle. For a freeform pool, use the plan area instead of a formula.
Find the average depth. For an even slope, add the shallow and deep depths and divide by two. If the floor has a flat shallow end, a slope and a flat deep end, split the pool into sections and add them.
Apply the formula. Rectangle: length x width x average depth x 7.48. Circle: 3.14 x radius squared x average depth x 7.48. Oval: length x width x average depth x 5.9.
Subtract steps and ledges. Steps, benches and ledges take up space. Estimate their volume and subtract it from the total.
Convert to litres if you work in metric. Cubic metres x 1000 = litres. Convert lengths and depths to metres first.

What is pool volume used for?

Volume is used to size equipment and chemicals and to estimate the water needed to fill the pool. It is one line in a pool cost estimate. It also feeds the takeoff on a pool estimate template.

PoolSketch shows the volume in gallons or litres on the live specs and quantities sheet, taken from the drawn shape and the depths you enter, so you do not do this by hand for freeform pools. The sheet also lists surface area, perimeter, average depth, coping and tile, plaster, excavation and deck area, with CSV download. Quantities only, no prices. Plans and sections are concept sketches for discussion, not construction drawings.

Try it on your own yard, free for 14 days

Common questions

How many gallons are in a 30 by 15 foot pool?

With depths of 3.5 ft and 8 ft, the average is 5.75 ft. 30 x 15 x 5.75 = 2,587.5 cubic feet, and 2,587.5 x 7.48 = about 19,355 gallons.

Why do you multiply by 7.48?

One cubic foot holds about 7.48 US gallons. The formula gives cubic feet first, then converts that figure to gallons.

How do I convert pool volume to litres?

Work in metres, then multiply cubic metres by 1000. For example, 50 cubic metres is 50,000 litres.

Do steps and ledges change the volume?

Yes. Steps, benches, ledges and tanning shelves take up space, so they reduce the volume. Estimate their volume and subtract it from the total.