Pool Volume Calculator

How many gallons is my pool?

Pool volume is surface area multiplied by average water depth, converted into gallons. One cubic foot of water is 7.48 US gallons, so the whole job is three measurements and one multiplication. Measure to the waterline rather than the top of the wall: the six inches of freeboard above the skimmer is not water, and counting it inflates every chemical dose you calculate afterwards.

Worked example: a 32 x 16 ft rectangle with a flat bottom

Surface area is 32 x 16 = 512 square feet. At a constant 5 ft depth the pool holds 512 x 5 = 2,560 cubic feet, and 2,560 x 7.48 = 19,150 gallons — about 72,500 litres or 72.5 cubic metres. That is the number every product label refers to when it says "per 10,000 gallons", so in practice you are dosing 1.9 units of whatever the label calls one dose.

The same pool with a 3 ft to 8 ft slope

When the floor drops evenly from the shallow end to the deep end, use the average of the two: (3 + 8) / 2 = 5.5 ft. Volume becomes 512 x 5.5 = 2,816 cubic feet, or 21,065 gallons. The averaging trick is exact for a straight slope because the wedge of extra water in the deep half exactly matches the wedge missing from the shallow half. It is not exact for hopper bottoms, where the deep section is a cone-shaped pit in the middle of an otherwise flat floor — there the true volume runs 5 to 15% below the average-depth figure.

Round and oval above-ground pools

A round pool is pi x radius squared x depth. A 24 ft round pool filled to 4 ft holds 452 square feet x 4 = 1,810 cubic feet, or 13,536 gallons — close to the 13,500 printed in most manufacturer charts. Ovals use the same idea with two different axes: an 18 x 33 ft oval at 4 ft is (pi / 4) x 18 x 33 x 4 = 1,866 cubic feet, about 13,960 gallons. Note that a 52-inch wall is usually filled to roughly 48 inches, so use the water depth, not the wall height.

What you actually do with the number

Every bottle and bag in the chemical cupboard is dosed per 10,000 gallons. Using the 19,150 gallon pool above, the common jobs work out like this:

Get the volume wrong by 30% and every one of those doses is wrong by 30%. Under-dose and the chlorine reading never holds overnight; over-dose stabiliser and you are stuck with it, because CYA only leaves the pool when water leaves the pool.

Top-ups, leaks and evaporation

The calculator also prints gallons per inch of depth, which is surface area in square feet times 0.623. For the 512 square foot pool that is 319 gallons per inch. A typical uncovered pool in warm weather loses a quarter inch a day, so about 80 gallons daily and 560 gallons a week — normal, not a leak. If the water drops faster than roughly half an inch a day, run a bucket test: float a bucket of pool water on the step, mark both levels, and compare after 24 hours. If the pool drops more than the bucket, the difference is a leak rather than evaporation, and gallons per inch tells you how expensive it is.

The same figure sizes a top-up. Filling three inches with a garden hose at 8 gallons per minute means roughly 960 gallons and two hours of running time, and it tells you how much fresh, unstabilised water you just added before you re-test.

Freeform pools and other traps

For kidney, lagoon and other freeform shapes, take the longest length, the widest width, treat it as a rectangle and subtract about 10%. Curved edges cut corners off the box, and 10% is the field estimate pool professionals use. Deduct more if the shape is genuinely narrow-waisted. Spas and tanning ledges add volume back, so include the ledge as its own shallow rectangle if it is more than a few feet across. Finally, remember that vinyl liners and fibreglass shells follow the manufacturer's profile rather than a clean slope, so treat any calculated figure as accurate to within about 5% — plenty for chemistry, but check the manufacturer's chart before sizing a heater.