Pool Volume Calculator

How to use the pool volume calculator

Choose the shape first, because it decides which measurements the page asks for. A rectangle takes a length and a width, a round pool takes one diameter, and an oval takes its long and short axes. Measure the water surface at the water line, not the deck, the coping or the outside of the shell.

Then measure the depth twice, once at the shallow end and once at the deep end, from the water line down to the floor. This is the step that decides whether the answer is right. Almost every in ground pool slopes, so there is no single depth to enter, and picking either end gives an answer that is wrong by a large margin.

If the floor genuinely is flat, and plenty of above ground and lap pools are, turn the sloping floor switch off and enter the one depth. The arithmetic collapses to the same thing: a flat pool at 5.5 ft holds exactly what the sloping 3 to 8 ft pool holds, because the average is what the volume was always built on.

The result is given in US gallons and in litres from the same cubic feet, using the legal definitions rather than a rounded factor. A US gallon is exactly 231 cubic inches, so a cubic foot is 1728 divided by 231, which is 7.480519 gallons, and an inch is exactly 25.4 mm, which makes a gallon exactly 3.785411784 litres.

The depth is the part everyone gets wrong

Ask someone how deep their pool is and they will tell you the deep end, because that is the number that felt impressive when they bought it. Put that number into a volume calculation and every figure downstream is inflated by the ratio of the deep end to the true average, which on a normal pool is a very large error.

The fix is one extra measurement. Drop a tape at the shallow end and again at the deep end, take the mean, and the geometry is honest. If your pool has a flat shelf, a slope and a hopper, measuring the shelf and the hopper still gets you far closer than either end alone does.

What this page deliberately will not tell you

It will not tell you how much of anything to put in the water. Volume is a geometry question with one right answer. Dosing is a chemistry question that depends on the current reading, the product concentration, the water temperature, the bather load and the local rules, and getting it wrong ranges from expensive to genuinely dangerous.

Some chemicals used around pools react violently with each other and several are harmful to breathe or to touch. The correct source for a dose is the label on the container you are holding, read against a current test of the water, and where the numbers are not obvious, a pool professional rather than a web page.

The other honest limit is the geometry itself. Free form pools, kidney shapes, spas joined to the main body and tanning ledges are not rectangles, circles or ellipses. Split an odd pool into shapes the page does support, run each one, and add the results, which is closer than forcing the whole thing into a single rectangle.

The formula, worked line by line

Pool volume is a surface area multiplied by a depth, and then a unit conversion. The surface is whichever of three areas matches the shape: a rectangle is length times width, a round pool is pi times the radius squared, and an oval is treated as an ellipse, pi times the two half axes multiplied together.

The depth is the average of the shallow and deep ends. That is not a simplification for the sake of an easy page: where the floor slopes evenly from one end to the other, a prism of the mean depth holds exactly the same water as the sloping pool, so the average is the answer rather than an approximation of it.

Both unit factors are exact and both are derived rather than remembered. A US liquid gallon is defined as 231 cubic inches, and an inch has been exactly 25.4 mm since the 1959 International Yard and Pound Agreement, which fixes the litre figure to the last digit as well.

rectangle surface = length ft × width ft
round surface = pi × (diameter ÷ 2)²
oval surface = pi × (length ÷ 2) × (width ÷ 2)
average depth = (shallow end + deep end) ÷ 2
cubic feet = surface × average depth
US gallons = cubic feet × 1728 ÷ 231 = cubic feet × 7.480519
litres = US gallons × 3.785411784

The round and oval formulas matter more than they look. A round pool is pi times the radius squared, so an 18 ft round pool has a surface of 254.5 square feet, not 324. Squaring the diameter instead of the radius and skipping the pi overstates the water by about 27 percent, and it is a common slip.

An oval is the ellipse, pi over 4 of the rectangle that would box it. The same 32 by 16 ft footprint that gives 512 square feet as a rectangle gives 402.1 square feet as an oval, so an oval pool holds a little under four fifths of what the rectangle would, at 16,545 gallons against 21,065.

Two things this figure is not. It is not the volume at the top of the wall, since the water line sits below it, so measure to the water rather than to the coping. And it is not a dosing instruction: this page computes the water and stops there, deliberately, because how much of anything to add is a separate question with safety consequences.

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