Pool Volume Calculator – How Many Gallons Is My Pool? | CalcLoft
Find out how many gallons of water your rectangular, round or oval pool holds — just enter the size and the shallow-end and deep-end depths.
How to calculate pool volume in gallons
Most pools slope from a shallow end to a deep end, so the first step is the average water depth:
Average depth = (shallow-end depth + deep-end depth) ÷ 2
Next, find the surface area of the pool in square feet, multiply by the average depth to get the volume in cubic feet, then multiply by 7.48 (the gallons in one cubic foot):
- Rectangular: length × width × average depth × 7.48
- Round: π × (diameter ÷ 2)² × average depth × 7.48
- Oval: length × width × 0.9 × average depth × 7.48 (the × 0.9 allows for the rounded ends)
Gallons for common pool sizes (avg depth 4.5 ft)
An average depth of 4.5 ft is typical for a 3 ft shallow end and 6 ft deep end. Exact totals are rounded to the nearest 100 gallons below.
| Pool shape & size | Avg depth | Volume (cu ft) | Approx. gallons |
|---|---|---|---|
| Rectangular 16 × 32 ft | 4.5 ft | 2,304 | ~17,200 |
| Rectangular 18 × 36 ft | 4.5 ft | 2,916 | ~21,800 |
| Rectangular 20 × 40 ft | 4.5 ft | 3,600 | ~26,900 |
| Round, 18 ft diameter | 4.5 ft | 1,145 | ~8,600 |
| Oval 16 × 32 ft | 4.5 ft | 2,074 | ~15,500 |
Unrounded, these work out to about 17,234 / 21,812 / 26,928 / 8,565 / 15,511 gallons respectively — close enough to plan chemicals, fill times and water bills.
Tips for a more accurate pool volume
- Dose chemicals per gallon: chlorine, algaecide and salt products usually give a dose per 10,000 gallons — divide your total by 10,000 and multiply the label dose.
- Measure both ends, not just one: the estimate is only as accurate as your average depth, so measure the actual water depth at the shallow end and the deep end (a constant-depth pool gets the same number in both fields).
- Split irregular pools into sections: treat a free-form pool as rectangles, circles and ovals, calculate each section, then add the gallons together.
Related guides
Pool volume calculator FAQ
How many gallons is a 20×40 pool?
A 20 × 40 ft rectangular pool with a 4.5 ft average depth holds about 26,900 gallons (20 × 40 × 4.5 × 7.48 ≈ 26,928). Enter your own shallow-end and deep-end depths above for your exact pool.
How do I calculate gallons for a round pool?
Treat the diameter as the "length": the volume in cubic feet is π × (diameter ÷ 2)² × average depth, then multiply by 7.48 for gallons. An 18 ft round pool with a 4.5 ft average depth holds about 8,600 gallons.
Why multiply by 7.48?
Because one cubic foot of water holds exactly 7.48 US gallons. Converting your pool's volume from cubic feet to gallons is simply a matter of multiplying by 7.48.
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