Pool Volume Calculator
Every chemical dose on every label is written per 10,000 gallons, so this is the number the shelf wants from you. Pick the shape, give it the two depths, and it works out the volume.
Pool volume
21,065gallons
- Surface area
- 512.0 sq ft
- Average depth
- 5.50 ft
- Cubic feet
- 2,816.0
- Litres
- 79,740
- Imperial gallons
- 17,540
- Average depth is taken as the midpoint of the two ends, which assumes the floor slopes the whole way. A pool with a long flat shallow section holds less than this — measure the depth at a few points and average those instead.
- Water weighs about 8.34 pounds a gallon, so this pool holds roughly 87.8 short tons of water.
- Chemical doses are written per 10,000 gallons, so this is the number every label on the shelf wants from you.
About the pool volume calculator
Pool volume is surface area times average depth, and the only difficult part is the shape. A rectangle and a circle are straightforward. An oval is an ellipse, which is pi times half of each axis rather than the length times the width — treating an oval as a rectangle overstates it by more than a fifth.
A kidney has no exact formula at all. The approximation the trade uses is 45 per cent of the two widths added together, times the length, and it lands within a few per cent for the proportions kidney pools are actually built in. That is well inside the error in running a tape along a curved edge in the first place.
Average depth is where most of the real error lives. Taking the midpoint of the shallow and deep ends assumes the floor slopes all the way from one to the other. Most pools are not built that way — they hold the shallow depth over a flat section and then drop, so a pool with a long flat shallow end holds noticeably less than the midpoint suggests. If it matters, measure the depth at several points and average those.
Get the volume right and everything downstream follows: the chlorine dose, the acid demand, the salt for a chlorinator, the heater sizing and how long the pump has to run to turn the water over.
What it works out
- Rectangular, round, oval and kidney pools
- Sloping floors, from the shallow and deep ends
- US gallons, imperial gallons, litres and cubic feet
- The weight of water it holds
The formula
Gallons = Surface area × Average depth × 7.48
Surface area times average depth gives cubic feet, and a cubic foot of water is 7.48 US gallons.
The surface area depends on the shape. A rectangle is length times width. A circle is pi times the radius squared. An oval is an ellipse — pi times half the length times half the width — which is about 21 per cent less than the rectangle it sits inside, so treating an oval as a rectangle is a large error rather than a small one.
A kidney is approximated as 0.45 times the two widths added together, times the length. There is no exact formula for the shape, and this is the figure the pool trade uses.
Average depth is the shallow end plus the deep end, halved. That is exactly right for a floor that slopes evenly from one to the other and generous for the more common shape, where the shallow depth is held flat for some distance before the floor drops away.
- Surface area
- The water surface, worked out from the shape you pick.
- Average depth
- The midpoint of the shallow and deep ends.
- 7.48
- US gallons in a cubic foot. An imperial gallon is 6.23 to the cubic foot.
A worked example
A thirty-two by sixteen foot rectangular pool sloping from three feet at the shallow end to eight at the deep.
That works out to 21,065 gallons.
- Surface area
- 512.0 sq ft
- Average depth
- 5.50 ft
- Cubic feet
- 2,816.0
- Litres
- 79,740
- Imperial gallons
- 17,540
Questions
How many gallons is a 16 by 32 foot pool?
About 21,065 US gallons if it slopes from three feet to eight. The same pool with a flat five-foot floor holds 19,150, so the slope is worth entering properly rather than guessing an average.
How do I work out the volume of a round pool?
Enter the diameter as the length and pick the round shape — the width is ignored. A 24-foot round pool four feet deep holds about 13,536 gallons.
Why can I not treat an oval pool as a rectangle?
Because an ellipse is about 21 per cent smaller than the rectangle it fits inside. A 30 by 15 foot oval has a 353 square foot surface where the rectangle would be 450. Doing it the rectangular way overdoses every chemical you add.
How accurate is the kidney figure?
Within a few per cent for normal kidney proportions. There is no exact formula for the shape, and the 45 per cent rule is what the pool trade uses. The bigger source of error is measuring a curved edge with a tape.
How do I find the average depth?
The midpoint of the two ends works if the floor slopes the whole way. Most pools hold the shallow depth flat for a while and then drop, which means they hold less than the midpoint says. Measuring at four or five points along the pool and averaging those is more accurate.
Why do I need to know the volume?
Because every chemical dose is written per volume — usually per 10,000 gallons. Chlorine, acid, algaecide, stabiliser and salt all assume you know it. It also sets the pump run time, since the water has to turn over completely once or twice a day.
How much does the water weigh?
About 8.34 pounds a US gallon, so a 21,000-gallon pool holds roughly 88 short tons of water. It is worth knowing before building anything above ground or on a structure.
Does this work in litres?
Yes. Switch the units to metres and the result shows litres and imperial gallons alongside US gallons. A cubic metre is a thousand litres.