Math & Science · Guide
How to Calculate Pool Volume (Gallons and Litres)
Measure the length, width and average depth. One multiplication later, you know how much water you are treating.
Want the answer for your own numbers? Use the free Volume Calculator — it does every step below for you.
Why pool volume matters
Every chemical dose — chlorine, pH adjusters, shock, salt — is given per gallon or per litre. Guess the volume wrong by 30% and every dose is wrong by 30% too. Pump and heater sizing also depend on it.
Step 1: Find the average depth
If the floor slopes, add the shallow and deep depths and divide by 2. A pool that is 3 ft at one end and 8 ft at the other has an average depth of 5.5 ft. For a pool with a sharp drop into a deep section, split it into two rectangles and add them.
Step 2: Use the formula for your shape (US gallons)
| Shape | Formula (feet) |
|---|---|
| Rectangle or square | Length × width × average depth × 7.48 |
| Round | Diameter × diameter × average depth × 5.9 |
| Oval | Long diameter × short diameter × average depth × 5.9 |
| Kidney (estimate) | (Widest width + narrowest width) × length × average depth × 3.38 |
Why 7.48? One cubic foot holds 7.48 US gallons. The 5.9 factor is π ÷ 4 × 7.48, which turns a circle's diameter into area.
Worked examples
Rectangle: 32 × 16 ft, 3.5 ft to 8 ft deep
Average depth: (3.5 + 8) ÷ 2 = 5.75 ft.
32 × 16 × 5.75 × 7.48 = 22,021.
About 22,000 gallons
Round above-ground: 24 ft across, 4 ft of water
24 × 24 × 4 × 5.9 = 13,594.
About 13,600 gallons
Oval: 15 × 30 ft, 4.5 ft deep
15 × 30 × 4.5 × 5.9 ≈ 11,950 gallons
Common pool sizes at a glance
| Pool | Average depth | Approx. US gallons |
|---|---|---|
| Rectangle 12 × 24 ft | 4.5 ft | 9,700 |
| Rectangle 16 × 32 ft | 5.5 ft | 21,100 |
| Rectangle 20 × 40 ft | 5.5 ft | 32,900 |
| Round 15 ft | 4 ft | 5,300 |
| Round 18 ft | 4 ft | 7,600 |
| Round 24 ft | 4 ft | 13,600 |
Above-ground pools are rarely filled to the very top of the wall. Measure the actual water depth, not the wall height, or you will overestimate.
In metric (litres)
Measure in metres and multiply length × width × average depth to get cubic metres. Each cubic metre is 1,000 litres. An 8 × 4 m pool with an average depth of 1.5 m holds 48 m³, or 48,000 litres.
Ponds and irregular shapes
Ponds rarely have neat sides. For a rough estimate, measure the average length, average width and average depth, multiply, and then multiply by about 0.85 for a typical rounded shape (more for steep sides, less for shallow saucer shapes). In gallons, that is length × width × depth × 7.48 × 0.85.
For an exact figure, note the water meter reading before and after filling. That beats any formula.
Putting the number to work
How long it takes to fill
A garden hose often delivers around 10 gallons a minute, though pressure and hose size change that a lot. At that rate, a 22,000-gallon pool takes about 37 hours. Time how long your hose takes to fill a 5-gallon bucket to get your own rate.
How much salt a saltwater pool needs
Pounds of salt = gallons × 8.34 × target ppm ÷ 1,000,000
To bring a new 22,000-gallon pool up to 3,200 ppm: 22,000 × 8.34 × 3,200 ÷ 1,000,000 = about 587 lb. Check your chlorinator’s manual for its own target range, and test before adding more.
How heavy the water is
A US gallon of water weighs about 8.34 lb. That same 22,000 gallons weighs around 183,000 lb — about 83 tonnes. It is why above-ground pools need firm, level ground.
Quick conversions
- 1 cubic foot = 7.48 US gallons = 28.3 litres
- 1 US gallon = 3.785 litres
- 1 cubic metre = 1,000 litres = 264 US gallons
- 1 imperial (UK) gallon = 4.546 litres
For a quick check, choose "Rectangular box" or "Cylinder" in the volume calculator, which shows litres and gallons alongside. Tanks and aquariums work the same way — see the tank calculator.
Frequently asked questions
For a rectangular pool, multiply length × width × average depth in feet by 7.48. A 32 × 16 ft pool averaging 5.75 ft deep holds about 22,000 gallons. For round pools, use diameter × diameter × depth × 5.9.
Multiply length × width × average depth in metres to get cubic metres, then multiply by 1,000. An 8 × 4 m pool with a 1.5 m average depth holds 48,000 litres.
With 4 ft of water: 24 × 24 × 4 × 5.9 = about 13,600 US gallons. At 4.5 ft of water it is about 15,300 gallons. Always measure the real water depth.
At a typical 10 gallons a minute, about 1.7 hours per 1,000 gallons — roughly 37 hours for a 22,000-gallon pool. Your hose may be faster or slower, so time it with a bucket.