Pool Calculator

Free Online Pool Calculator

This pool calculator works out how many gallons of water your swimming pool holds, based on its shape and dimensions. Type in your measurements and it instantly gives you the volume in US gallons, imperial gallons, liters, cubic meters, and cubic feet, so you don't have to convert anything by hand. It covers the four most common pool shapes — rectangular, circular, oval, and kidney-shaped (also called free-form) — so whether you have a straightforward backyard rectangle or a curvy, natural-looking pool, you can get a solid estimate in a few seconds.

Knowing your pool's exact water volume matters for more than just curiosity. It's the number behind almost every other pool decision: how much chlorine or salt to add, how big a pump or filter you need, how long it will take to fill, and how much a chemical treatment or heater will actually cost to run. This calculator also throws in a few of those extra numbers automatically, including a chlorine dosing estimate, a salt level estimate for saltwater pools, and a rough fill-time estimate using a garden hose.

How to Calculate Pool Volume

The basic idea behind every pool volume calculation is the same: multiply the pool's surface area by its average depth. The formula changes slightly depending on the shape of the pool:

• Rectangular pool: Volume = Length × Width × Average Depth
• Circular pool: Volume = π × (Diameter ÷ 2)² × Average Depth
• Oval pool: Volume = π × (Length ÷ 2) × (Width ÷ 2) × Average Depth
• Kidney or free-form pool: Volume = Length × Width × 0.85 × Average Depth

Why Average Depth Matters

Most in-ground pools aren't the same depth from one end to the other. They start shallow near the steps and slope down toward a deeper end for diving or just deeper swimming. If you only plug in the deepest number, you'll end up with a volume that's too high, sometimes by a lot. The fix is simple: take the shallow-end depth and the deep-end depth, add them together, and divide by two. That average depth is what actually represents how much water is sitting in the pool, since the shallow section takes up just as much floor space as the deep section in most standard pool designs.

Above-ground pools, lap pools, and plunge pools are usually a flat, constant depth all the way across, so for those you can just enter the single depth measurement and skip the averaging step. This calculator lets you pick either option depending on which one matches your pool.

How to Measure Your Pool Accurately

A good volume estimate starts with good measurements. Here's how to measure each input this calculator asks for:

• Length and width (rectangular, oval, and kidney pools): measure the longest and widest points of the water's surface, edge to edge, not the outer edge of the deck or coping.
• Diameter (round pools): measure straight across the widest point of the pool at the water line.
• Shallow-end and deep-end depth: measure straight down from the water surface to the pool floor at each end, using a pole or weighted tape rather than guessing.
• For a kidney or free-form pool, measure the overall length and width as if you drew a rectangle around the widest points of the pool — the 0.85 factor already accounts for the curved shape taking up less area than that full rectangle.

Worked Example

Say you have a rectangular in-ground pool that's 32 feet long and 16 feet wide, with a shallow end of 3 feet and a deep end of 8 feet. The surface area is 32 × 16 = 512 square feet. The average depth is (3 + 8) ÷ 2 = 5.5 feet. Multiplying those together gives 512 × 5.5 = 2,816 cubic feet, and converting to gallons (multiplying by 7.48052) comes out to roughly 21,067 US gallons — which is what this calculator will show if you enter those exact numbers.

That's a pretty typical size for a family backyard pool, and it's a good reference point if you're trying to sanity-check a rough guess before you go out and measure your own.

Common Pool Sizes and Their Approximate Volume

To give you a sense of scale, here are a few typical rectangular pool sizes and roughly how much water they hold, using an average depth of 5 feet as an example:

• 12 ft × 24 ft pool: approximately 10,772 gallons
• 15 ft × 30 ft pool: approximately 16,830 gallons
• 16 ft × 32 ft pool: approximately 19,148 gallons
• 18 ft × 36 ft pool: approximately 24,242 gallons
• 20 ft × 40 ft pool: approximately 29,922 gallons

Chlorine Dosing Basics

Once you know your pool's volume, working out how much chlorine to add gets a lot easier. A common starting rule of thumb for liquid chlorine (10% sodium hypochlorite) is about 1 fluid ounce per 2,500 gallons of water for every 1 ppm you want to raise the free chlorine level. This calculator uses that same rough guideline to estimate how much liquid chlorine you'd need for the target rise you enter, though the exact amount you need on any given day still depends on your current chlorine reading, sunlight exposure, bather load, and water temperature, so always confirm with a test kit before dosing.

Salt Levels for Saltwater Pools

Saltwater pools use a chlorinator that converts dissolved salt into chlorine, and most systems run best somewhere around 2,700 to 3,400 ppm of salt, though the exact target depends on your specific chlorinator model, so it's worth checking the manufacturer's recommended range. This calculator estimates how many pounds of salt (and roughly how many standard 40-pound bags) you'd need to reach a target ppm level, based on your pool's total gallons. As with chlorine, always test your current salt level first, since adding salt on top of an already-adequate reading can push the pool above the ideal range.

How Long Will It Take to Fill My Pool?

Filling a pool from empty (or topping it off after a big drop) takes longer than most people expect. A standard garden hose typically delivers somewhere around 9 to 10 gallons per minute, while a higher-flow fill hose or a wide-open spigot can push closer to 15 to 20 gallons per minute. This calculator gives you a rough fill-time range using both flow rates, so you can plan around it — for a mid-size backyard pool holding 20,000 gallons or more, that can easily mean filling overnight or across a couple of days, and it's worth checking with your water utility if you're filling a large pool all at once, since some municipalities have restrictions or offer a lower "pool fill" rate if you ask in advance.

Common Mistakes When Estimating Pool Volume

A handful of errors show up again and again when people try to work out pool volume by hand:

• Using the deep-end depth alone instead of averaging the shallow and deep ends, which significantly overstates the true volume.
• Measuring the outer edge of the deck or coping instead of the actual water surface dimensions.
• Mixing units — measuring in feet but entering inches, or meters but entering centimeters — without converting first.
• Treating a kidney or free-form pool as a full rectangle without applying any shape correction, which overestimates the real water volume.
• Forgetting that a pool with a shallow tanning ledge or built-in bench effectively has a slightly lower average depth than the shallow-to-deep number alone suggests.

Limitations of This Calculator

This calculator gives a practical, reliable estimate — not an exact engineering measurement. It assumes a fairly standard sloped or flat pool floor and a shape that reasonably matches one of the four available options. Pools with multiple depth zones, built-in tanning ledges, swim-up bars, attached spas, or unusually complex free-form outlines will come out a bit higher or lower than the true volume, so for those it's worth breaking the pool into simpler sections and adding the results together, or checking your builder's original spec sheet if you have one. The chlorine, salt, and fill-time figures are general planning guidelines based on widely used rules of thumb, not exact chemistry — always confirm actual dosing against a water test kit and your equipment manufacturer's instructions.