How Many Gallons Is My Pool?

Get this wrong and you're wrong forever

A 32 × 16 pool sloping from 3 ft to 8 ft holds about 21,065 gallons.

That number matters more than it looks, because every chemical instruction you will ever read is per volume. Get it 20% wrong and you're 20% wrong on every bag of shock, every jug of acid, every calculation of turnover — in the same direction, for as long as you own the pool.

And because the error is consistent, it never announces itself as a mistake. It just quietly makes everything slightly off.

The arithmetic

Surface area — rectangle is length × width:

32 × 16 = 512 sq ft

Average depth:

(3 + 8) ÷ 2 = 5.5 ft

Volume:

512 × 5.5 = 2,816 cubic feet

Into gallons:

2,816 × 7.480519 ≈ 21,065 gallons

That conversion factor is exact, not rounded. A US gallon is defined as 231 cubic inches and a cubic foot is 1,728 — and 1728 ÷ 231 = 7.480519.

Round and oval pools

Switch the shape dropdown and the same calculation runs with one different factor: multiply by π/4 (about 0.7854).

For a round pool, put the diameter in both the length and width boxes. A circle is just the case of an ellipse where both axes match, so one shape setting covers round and oval together — there's no separate round option to hunt for.

A 24 ft round pool, 4 ft deep: 0.7854 × 24 × 24 × 4 = 1,810 cubic feet = 13,536 gallons.

Where averaging the depths goes wrong

This is the one real limitation, and it catches a lot of older pools.

Averaging shallow and deep is exact when the floor slopes evenly from one end to the other. Many pools are built that way.

But the common older layout — a flat shallow area, then a sudden drop, then a flat deep well — holds less water than an even slope between the same two depths. The calculator will read high.

If your pool has a distinct hopper or diving well, measure the sections separately and add the gallons. The printable has a table for exactly that.

Measure the water, not the wall

Depth means the depth of the water, not the height of the pool wall.

There's usually 4–8 inches of freeboard above the waterline. Counting it inflates the volume by several percent — more on a shallow pool, where those inches are a bigger share of the total.

What the surface area is for

Covers and solar rings are sized by area, not volume. So is evaporation, which depends on surface rather than depth.

It's also a fast sanity check on your own measurements. If 512 sq ft doesn't look like the pool you're standing next to, one of the numbers you typed is wrong.

What you'll actually use this number for

Three things, and they're why it's worth being accurate:

Chemical dosing. Every label is written per volume — per 10,000 gallons is the common unit. This is where an error repeats itself indefinitely.

Turnover. Pump sizing and run time are usually expressed as turning the whole volume over in a set number of hours. Your gallons divided by your pump's flow rate in gallons per hour gives you the run time for one turnover.

Heating and water bills. Both scale with volume, so knowing it turns "the pool costs a lot to heat" into a number you can compare against a cover.

The calculator also reports the volume in litres, because plenty of pool chemicals and most equipment sold outside the US are dosed and rated in metric. 21,065 gallons is about 79,740 litres — worth having if the instructions on the tub don't match the units in your head.

US gallons, not imperial

Worth knowing if you're following instructions from elsewhere. An imperial gallon is a different, larger unit — about 1.2 US gallons. A product dosed in imperial gallons needs converting before it meets this number.

Do it once, write it down properly

Take the measurements twice, ideally with someone else holding the other end of the tape. Then put the volume somewhere permanent — taped inside the equipment cupboard, not on a scrap of paper that lasts until autumn.

Print the pool data sheet. It has a section table for pools with a separate deep end, and a line for the surface area while you're there.