The pool volume formula
gallons = length × width × average depth × shape factor
rectangular: 7.5 · round: 5.9 · oval: 6.7
The shape factor converts cubic feet to gallons (1 cu ft = 7.48 gal) while correcting for the shape's area. Round pools use diameter for both length and width; ovals use the long and short axes.
A worked example
A 32 × 16 ft inground pool sloping from 3 ft to 7 ft: average depth is (3 + 7) ÷ 2 = 5 ft, so volume is 32 × 16 × 5 × 7.5 = 19,200 gallons. Round it to 19,000 and write it on the inside of your equipment-pad door — every dose calculation for the life of the pool uses it.
Common pool sizes in gallons
A quick reference for popular sizes — but your depth is the swing factor, so run your own numbers above for anything you're dosing. Round figures assume about 4 ft of water; inground rectangles assume a 5 ft average depth.
| Above-ground round | |
| 15 ft round | ~5,300 gallons |
| 18 ft round | ~7,600 gallons |
| 21 ft round | ~10,400 gallons |
| 24 ft round | ~13,600 gallons |
| 27 ft round | ~17,200 gallons |
| 30 ft round | ~21,200 gallons |
| Inground rectangle | |
| 14 × 28 ft | ~14,700 gallons |
| 16 × 32 ft | ~19,200 gallons |
| 18 × 36 ft | ~24,300 gallons |
| 20 × 40 ft | ~30,000 gallons |
When to distrust the estimate
Freeform pools, tanning ledges, and swim-outs all displace water the formula can't see. Got a kidney or freeform shape? Use the kidney pool volume calculator instead — it averages the widths and applies a freeform shape factor. And if your chemical doses consistently overshoot or undershoot the predicted change, correct your working volume: add a known amount of chlorine, measure the actual FC rise, and scale — our chlorine calculator makes that comparison easy.