Pool Volume Calculator: How Many Gallons Is Your Pool?
Every pool job starts with one number: how much water the pool actually holds. Add too much chlorine and you waste chemicals and sting eyes; add too little and algae wins within days. Pump and filter sizing, heater output, salt levels and even how long a refill takes all come back to the same figure, so guessing it from the pool’s outline is a poor plan. This pool volume calculator does the maths properly: pick the shape, enter the measurements, and it returns the volume in US gallons, litres and cubic metres, with cubic feet shown as well in imperial mode.
Use it before you dose a freshly filled pool, before you buy a pump, and whenever a chemical label asks for a volume you have never actually known. If your pool slopes from a shallow end to a deep end, read the next section first, because the depth you enter matters more than any other number.
Use the average depth
Few in-ground pools have a flat floor. Most slope gently, or drop away sharply, from a shallow end where children can stand to a deep end built for diving, and that slope breaks the simple length × width × depth shortcut if you only measure one end. The fix used across the pool trade is the average depth: add the shallow-end depth to the deep-end depth and divide by two, so (shallow + deep) ÷ 2. A pool that runs from 3 feet at the steps to 6 feet at the deep wall is worked out at 4.5 feet, and that single averaged figure is what the calculator uses when you choose the shallow and deep ends option.
Measure each depth from the floor to the waterline, not to the top of the wall, because the water itself is what you are dosing and pumping. The few inches above the waterline hold air, not water, and counting them can inflate the result by several hundred gallons on a family-size pool.
The formulas for each shape
The calculator uses the standard geometry for each shape, with the average depth wherever the floor slopes:
| Pool shape | Formula |
|---|---|
| Rectangular | Length × Width × average depth |
| Round | π × radius² × average depth (radius = diameter ÷ 2) |
| Oval | π × (Length ÷ 2) × (Width ÷ 2) × average depth |
In imperial mode the volume in cubic feet is converted at 1 cubic foot = 7.48052 US gallons. In metric mode everything is worked out in cubic metres first, and 1 cubic metre = 1,000 litres, so the litre figure is the cubic metre figure multiplied by a thousand. Both routes describe the same water — only the units change.
Pool Volume Calculator
Choose your units, shape and depth, enter the measurements, and the results update as you type, or press Calculate.
Formulas: rectangle = length × width × average depth; round = π × radius² × depth; oval = π × (length ÷ 2) × (width ÷ 2) × depth. 1 cubic foot = 7.48052 US gallons; 1 m³ = 1,000 litres.
Common pool sizes
To sense-check your result, compare it with the pools owners ask about most. These are approximate US gallon figures at the average depths shown:
| Pool size | Approx. volume |
|---|---|
| 12 × 24 ft, avg 4.5 ft deep | ≈ 9,700 gal |
| 16 × 32 ft, avg 5 ft deep | ≈ 19,150 gal |
| 18 ft round, 4 ft deep | ≈ 7,600 gal |
| 24 ft round, 4 ft deep | ≈ 13,600 gal |
If your pool comes out far from the figure for a similar size, re-check the depth first. A half-foot error in depth moves the answer far more than an inch or two lost on length or width, because depth is multiplied across the whole surface area of the pool.
Kidney and freeform pools
Kidney, lagoon and other freeform pools follow no single formula, but you can still get close. The usual method is to split the pool on paper into rough rectangles and ovals that together cover the water, work out each part with this calculator, and add the parts together. If that sounds like too much drawing, run the pool as an oval using its longest and widest measurements instead. Either way, treat the answer as an estimate and expect it to land within about ±15% of the true volume. For chemical dosing that is normally close enough, provided you dose, wait, test and adjust rather than adding a full calculated dose in one go.
Which gallon?
Every gallon figure on this page is a US gallon. An imperial (UK) gallon is about 20% larger, so a volume quoted in imperial gallons will always look smaller for the same pool. If a chemical label or an older British guide talks in imperial gallons, convert before you dose, or let the litre figure settle it — litres mean the same thing everywhere.
What to calculate next
Volume is only the first number most pool and garden projects need. These tools cover the measurements that usually come next:
- Square Footage Calculator — the area of a pool surround, patio or lawn, before you price materials for it.
- Cubic Yard Calculator — soil, gravel and other bulk materials in cubic yards.
- Electricity Cost Calculator — for pump running costs, once you know how much water the pump has to move.
Frequently asked questions
How many gallons is a 20×10 pool?
At a constant 4 ft depth, a rectangular 20 × 10 ft pool holds 5,984 US gallons — that is 800 cubic feet, or about 22,653 litres (22.65 m³). Change the depth and the figure moves in proportion: at 5 ft deep the same pool holds about 7,480 gallons.
How do I calculate volume with a sloped floor?
Measure the depth at the shallow end and at the deep end, add the two together and divide by two to get the average depth, then use that average in the normal formula for your pool’s shape. A pool running from 3 ft to 6 ft is calculated at 4.5 ft deep.
How many litres is that in metric?
Work in metres and the calculator gives you cubic metres; 1 cubic metre = 1,000 litres, so the litre figure is the cubic metre figure multiplied by 1,000. The 20 × 10 ft example above works out at about 22.65 m³, which is 22,653 litres.
How accurate is this for a kidney-shaped pool?
Treat it as an estimate. Split a kidney or freeform pool into rough rectangles and ovals, calculate each part and add them together, or use the oval setting with the longest and widest measurements. Either method usually lands within about ±15% of the true volume.
Estimates only — check your pool or chemical manufacturer’s figures for dosing.