Pool Volume Calculator

Volume is the input every other calculation on this site scales from. Get it wrong and you do not make one bad decision — you make the same proportional mistake in every dose, for as long as you own the pool.

A 17% overstatement turns a carefully worked 2 ppm chlorine dose into 2.3 ppm. That is survivable. The same error on an acid dose overshoots pH past where you wanted it and sends you looking for soda ash, and the same error on stabilizer is not recoverable at all, because CYA only comes back down by replacing water.

It is also the number most owners never measured. It came with the house, or off a listing, or from the builder rounding up. Ten minutes with a tape fixes it permanently.

Length and width are hard to get badly wrong. Depth is another matter, and it is where nearly all the error lives.

Shallow plus deep, divided by two, only works if the floor slopes evenly the whole way. Most inground pools do not. They have a long flat shallow section, a transition, and then a hopper at the deep end — and that profile holds far less water than a smooth slope between the same two numbers.

Worked properly: a 16 by 32 pool with an 18 foot flat shallow at 3.5 feet, an 8 foot transition, and a 6 foot hopper floor at 8 feet holds about 18,800 gallons. Averaging 3.5 and 8 over the whole thing gives 22,000 — 17% high.

The fix is to measure in sections and add them. This calculator averages, because two inputs is what most people will actually fill in, so if your pool has a flat shallow and a hopper, run it twice and add the results rather than trusting a single pass.

A rectangle fills its bounding box, so length times width is the area. Everything else needs a correction, and the standard figures carry assumptions worth knowing.

Oval, 0.89. That is right for a stadium shape — straight sides with semicircular ends — at the usual one-to-two proportions. A 15 by 30 works out at 0.893 and a 12 by 24 the same. Squarer ovals drift: a 15 by 20 is nearer 0.84.

If your pool is a true ellipse, 0.89 is too generous. An ellipse fills exactly π/4 of its bounding box, which is 0.785, so using the oval factor overstates it by about 13%. Most above-ground ovals are stadium-shaped and fine; a curved-sided inground may not be.

Freeform, 0.85 is an approximation by nature, and the only honest thing to say about it is that a kidney with a generous bite taken out of it holds less than one that is almost a rectangle. If the shape is unusual, verify the answer rather than trusting the factor.

Worth doing once, because a verified volume is a number you stop second-guessing.

The water meter. Turn off everything else in the house, read the meter, fill the pool from empty or through a known drop, and read it again. A meter that reads in cubic feet needs multiplying by 7.48 for gallons, and one in cubic meters by 1,000 for liters. This is the most accurate method available to a homeowner and it costs nothing but attention.

A known dose. On a pool with no chlorine demand — clear water, no algae, tested at dusk — add exactly one gallon of 12.5% liquid chlorine and measure free chlorine before and after a full turnover. Your volume in gallons is 125,000 divided by the ppm it moved. A rise of 6.25 ppm means 20,000 gallons; a rise of 10 means 12,500. In liters, add exactly 4 liters instead: your volume in liters is 500,000 divided by the ppm it moved, so a rise of 6.25 ppm means 80,000 liters.

That second method is only as good as your test kit, so use a drop-based kit rather than strips, and treat it as a sanity check on the tape measure rather than a replacement for it. If the two agree within 5%, stop worrying and write the number inside the pump lid.