Pool volume calculator
Pool volume is the surface area times the average depth, and for a floor that slopes evenly the average of the two end depths is exact. An 8 × 4 m pool running from 1.2 m to 2 m averages 1.6 m and holds 51.2 m³: 51,200 litres, 13,526 US gallons or 11,262 imperial ones.
How to work out pool volume
Almost everything you do to a pool is dosed against its volume, so the number is worth getting right once and writing on the pump housing. Chlorine, acid, alkalinity increaser and algaecide are all quoted per cubic metre or per ten thousand litres, and a pool believed to be a fifth smaller than it is gets a fifth less of everything, permanently. The gallon is the trap in that sentence. An American gallon is 3.785 litres and an imperial one is 4.546, so the same pool reads 13,526 or 11,262 depending on the bottle in your hand, and a dosing instruction written for one and applied with the other is out by twenty per cent before you start.
The averaging shortcut is exact when the floor slopes evenly from one end to the other, because the volume of a wedge is the volume of a box at its mean depth. It is not exact for a hopper bottom, where a flat shallow section runs to a short slope and then a small deep dish. Averaging the two end depths there treats the deep end as though it ran the whole width of the pool, and over-reads, sometimes by ten per cent or more. If the deep end is a bowl rather than a slope, measure the depth at several points along the pool and average those instead of the two extremes.
The pump figure is a duty, not a product. Turning the whole volume over in eight hours means moving 106 litres a minute through this pool, and that flow has to survive the pipework, the filter and the fittings. A pump whose box says 200 litres a minute says it at zero head, and the same pump at ten metres of resistance may deliver little over half of that. Read the pump curve rather than the headline. Oversizing does not help either: a sand bed only filters properly up to a certain flow velocity, and water pushed through faster arrives clear and leaves untreated.
Heating is the one number people are most often surprised by, and it comes straight out of the volume. Raising water by one degree takes 1.163 kWh per cubic metre, so lifting this pool by a single degree is 59.5 kWh, and taking it from 18 to 28 °C is 595 kWh before any losses at all. Uncovered outdoors, most of that leaves again through evaporation within a day or two. A cover changes nothing about the volume and almost everything about the running cost.
What people use it for
- Dosing chlorine, acid or algaecide against the real volume
- Sizing a pump and filter to a turnover rather than to a box rating
- Estimating the cost of filling from mains water
- Working out what a heat pump has to deliver to reach a target temperature
- Converting between US and imperial gallons on a dosing label
- Checking a supplier’s quoted capacity against your own measurements
Questions
13,526 US gallons at an average depth of 1.6 m, or 11,262 imperial. The two differ by a fifth, which is enough to matter for every chemical you add.
Average the shallow and deep end depths. That is exact for a floor that slopes evenly end to end. For a hopper bottom it over-reads, so take depths at several points and average those.
Area times depth, with the area as π times the radius squared. A 4 m round pool at 1.1 m deep is 13,823 litres, about 3,652 US gallons.
Its area is π times half the length times half the width. A 6 × 3.5 m oval at 1.2 m holds 19,792 litres.
Enough to turn the whole volume over in six to eight hours, so 106 litres a minute here. Match that to the pump curve at your system’s head, not to the number on the box.
No, but the running cost does. A cover mostly stops evaporation, which is both the largest heat loss on an outdoor pool and the reason the level keeps dropping.
1.163 kWh per cubic metre per degree. For 51.2 m³ that is 59.5 kWh per degree, so a ten degree lift is 595 kWh before losses.
Multiply the cubic metres by your water rate and remember most utilities charge the wastewater volume too unless you tell them. A 50 m³ fill runs to well over a hundred euros at typical European rates.
More than people expect. Measure to the waterline you keep, usually 100 to 150 mm below the coping. On an 8 × 4 pool, 100 mm of depth is 3,200 litres, over six per cent of the volume.