How this calculator works
A pool holds its water-surface area times its average depth. The surface area comes from the shape, and the average depth is either a single figure you enter or the mean of a shallow end and a deep end:
surface area = length × width (rectangle)
= π × (diameter ÷ 2)² (round)
= π ÷ 4 × long × short (oval)
= 0.45 × (wide + narrow) × length (kidney / free-form)
average depth = (shallow + deep) ÷ 2 (for an evenly sloping floor)
volume (m³) = surface area × average depth
US gallons = volume m³ × 264.17
litres = volume m³ × 1000
Pick the shape that matches your pool - rectangle, round, oval or kidney / free-form. For an above-ground pool, which is almost always a round or oval with the same depth all over, choose that shape and use the average depth (the water line is usually about 6 inches below the top rail, not the full wall height).
For a pool whose floor slopes evenly between the two ends, the average of the shallow and deep depths is exact. If the deep end is a localised diving hollow rather than an even slope, the real average is slightly less, so the sloping-floor figure errs on the safe side.
If your pool has a full-length step, a sun ledge or a bench, tick “Add a step, bench or ledge” and enter its length, width and height off the floor. The calculator removes that solid box of water from the volume - a full-length side step can be 5 to 8% of a pool, which is worth taking out before you dose chemicals.
Worked example
This example follows the unit system you pick in the calculator above.
A rectangular 26 × 13 ft pool that slopes from a 3 ft shallow end to a 8 ft deep end:
- Surface area: 26 × 13 = 338 sq ft.
- Average depth: (3 + 8) ÷ 2 = 5.5 ft for an evenly sloping floor.
- Volume: 338 × 5.5 = 1,859 cu ft.
- In US gallons: 1,859 × 7.48 = about 13,905 US gallons.
- That is roughly 11,580 imperial gallons or 52,640 litres.
Common pool sizes
These are ballpark volumes for popular pool sizes at a typical average depth. Your own depth and exact dimensions change the figure, so use them as a sanity check against the calculator:
| Pool | Average depth | US gallons | Litres |
|---|
| 12 × 24 ft rectangle | 5 ft | ~10,800 | ~40,800 |
| 14 × 28 ft rectangle | 5 ft | ~14,700 | ~55,600 |
| 16 × 32 ft rectangle | 5.5 ft | ~21,100 | ~79,700 |
| 18 ft round | 4 ft | ~7,600 | ~28,800 |
| 24 ft round | 4 ft | ~13,500 | ~51,200 |
Why the volume matters
The pool volume is the number nearly every other pool job keys off:
- Chemicals. Chlorine, acid, salt and algaecide are all dosed per 1,000 litres or per 10,000 gallons, so a wrong volume means a wrong dose.
- Filling. It sets how long the hose runs or how much a tanker delivery costs.
- Heating. Heater and heat-pump sizing is driven by the volume of water you are warming.
- Pump and filter turnover. Sizing the pump to turn the water over in a set number of hours starts from the volume.
Tips for measuring a pool
- Measure the water surface, not the coping or the deck - the volume is the water, not the shell.
- For an irregular or kidney shape, break it into a rectangle plus a round or oval end, work each out, and add them; the square footage calculator helps with the surface area.
- Take the average depth at a few points if the floor is dished or stepped, rather than just the shallow and deep ends.
- A vinyl-liner pool measures to the liner; a tiled pool measures to the tile face - the difference is small but real on a big pool.
- Round your final order of water and chemicals up, not down - it is easier to top up than to drain and re-balance.
Pool volume where you are
In the United States, pool volume is usually quoted in US gallons. Pools and almost every chemical label in the US are sized in US gallons (1 US gallon = 3.785 litres), so the US-gallon figure is the one to dose against.
The calculator shows US gallons, imperial gallons and litres together, so you can
read off whichever your test kit and chemicals use.
For the free-chlorine and water-balance figures to keep the pool to, follow the CDC's Model Aquatic Health Code.
