Pool Square Footage Calculator

A pool has two numbers that matter and people mix them up constantly. Surface area, in square feet, is the water you see from above. Volume, in gallons, is the water in it. Covers, solar rings, and safety fencing come off the first. Chemicals, pumps, filters, and heaters come off the second. Enter your dimensions and the calculator gives you both.

Surface area: the square footage

Surface area is a straight area calculation on the shape of the water.

Pool shape	Surface area formula
Rectangle	length x width
Round	pi x radius squared, so diameter x diameter x 0.7854
Oval	pi x (length / 2) x (width / 2)
Kidney or freeform	Split it into rectangles and part circles and add them

A 16 by 32 foot rectangular pool has 512 square feet of surface. An 18 foot round pool has 254 square feet. A 15 by 30 oval comes out at 353 square feet, noticeably less than the 450 you would get if you treated it as a rectangle, which is why oval above-ground pools need less cover than people expect.

For a freeform or kidney shape, the irregular shape calculator handles the decomposition, and the offset method on that page works well on a curved pool edge.

Volume: the gallons

Volume is the surface area multiplied by the average depth, converted to gallons. A cubic foot of water is 7.48 gallons, and the pool industry rounds it to 7.5.

Gallons = surface area x average depth x 7.5

Swim University gives the same arithmetic in its longer form, length x width x depth x 7.5 for a rectangle, and 3.14 x radius squared x depth x 7.5 for a round pool.

Constant depth. A 32 by 16 pool at a flat 4 feet: 32 x 16 x 4 x 7.5 = 15,360 gallons.

Sloped floor. Add the shallow end depth to the deep end depth and halve it. The same 32 by 16 pool with a 3 foot shallow end and a 9 foot deep end has an average depth of 6 feet, so 32 x 16 x 6 x 7.5 = 23,040 gallons.

That average works because most pools slope evenly. A pool with a long flat shallow section and a sudden deep-end hopper holds less than the simple average suggests, so if the floor has a distinct break, measure each section separately and add the volumes.

Common pool sizes

Pool	Surface area	Average depth	Volume
12 x 24 ft rectangle	288 sq ft	4.5 ft	9,720 gallons
14 x 28 ft rectangle	392 sq ft	5 ft	14,700 gallons
16 x 32 ft rectangle	512 sq ft	5 ft	19,200 gallons
18 x 36 ft rectangle	648 sq ft	5.5 ft	26,730 gallons
20 x 40 ft rectangle	800 sq ft	5.5 ft	33,000 gallons
18 ft round	254 sq ft	4 ft	7,634 gallons
24 ft round	452 sq ft	4 ft	13,572 gallons
15 x 30 ft oval	353 sq ft	4 ft	10,603 gallons

Which number you need for what

Surface area sizes anything that sits on top of or over the water: safety covers, solar covers and rings, winter covers, and pool blankets. It also drives heat loss, since evaporation happens at the surface, which is why heaters are matched to surface area and temperature rise rather than to gallons.

Volume in gallons sizes everything that goes in the water or moves it: chlorine and shock doses, algaecide, stabilizer, pump turnover rate, filter capacity, and salt for a saltwater system. Almost every chemical label is written per 10,000 gallons, so getting the volume wrong by 20 percent means dosing wrong by 20 percent every time.

Perimeter covers coping, tile bands, and fencing. A 16 by 32 pool has a 96 foot perimeter. Check your local code for the fence, since barrier requirements are set locally and usually apply to any pool over a certain depth.

The deck around the pool

Decking is the pool's footprint subtracted from the outer edge of the paving. Measure the whole rectangle, measure the pool, and take one from the other.

A 16 by 32 pool with a 4 foot deck all the way around sits inside a 24 by 40 rectangle. That's 960 square feet total, minus the 512 square foot pool, leaving 448 square feet of decking. The border mode on the homepage calculator does this in one step, and the concrete calculator turns the result into cubic yards if you're pouring.

Common mistakes

Using the wall dimensions instead of the water. For an above-ground pool, the printed size is usually the outside of the frame. The waterline is a few inches smaller.

Treating an oval as a rectangle. An oval holds about 78.5 percent of the rectangle that would contain it. Using the rectangle figure overdoses the chemicals substantially.

Averaging the depth on a pool with a hopper. If the floor has a flat shallow area and then drops, split it into two sections and add the volumes.

Dosing on the wrong number. Chemicals are per gallon, covers are per square foot. They are never interchangeable.

Forgetting the water isn't at the top. Water sits a few inches below the coping or top rail. On a big pool those few inches are hundreds of gallons.

Frequently asked questions

How do I calculate the square footage of a pool?

Multiply length by width for a rectangle. For a round pool, multiply the diameter by itself and then by 0.7854. For an oval, multiply length by width by 0.7854. A 16 by 32 pool is 512 square feet.

How many gallons is my pool?

Multiply the surface area in square feet by the average depth in feet, then by 7.5. A 16 by 32 pool with an average depth of 5 feet holds 512 x 5 x 7.5, which is 19,200 gallons.

How do I find the average depth of a pool?

Add the shallow end depth to the deep end depth and divide by two. A pool that runs from 3 feet to 9 feet has an average depth of 6 feet. If the floor has a sharp break rather than an even slope, measure the sections separately.

How many gallons is a 15x30 oval pool?

About 10,600 gallons at a 4 foot depth. The surface area is 353 square feet, not 450, because an oval covers about 78.5 percent of the rectangle around it.

Why is 7.5 the number for gallons?

A cubic foot holds 7.48 US gallons. The pool industry rounds it to 7.5, which is within half a percent and easier to do in your head.

Do I use square footage or gallons for pool chemicals?

Gallons. Chemical labels are almost always written per 10,000 gallons of water. Square footage is for covers, heating, and anything that sits over the surface.

How do I calculate the square footage of a kidney shaped pool?

Break it into a rectangle plus part circles, or run the offset method: measure the width across the pool at evenly spaced points along its length, average those widths, and multiply by the length. Both are covered on the irregular shape calculator.

Sources

Pool volume calculator and formulas, Swim University