The number every chemical dose depends on. Handles a sloping floor properly instead of pretending your pool is one depth throughout.
Measure the depth of the water, not the wall. Pools are normally filled to the middle of the skimmer, leaving 4–6 inches of freeboard.
Filling (optional)
A typical garden hose delivers 6–12 gallons a minute. Check a recent water bill for your real rate — many utilities also bill sewer volume on water used, which can double the cost.
Pool volume
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Surface area—
Average depth—
Litres—
Cubic metres—
Imperial gallons—
Water weight—
Fill time from a hose—
Cost to fill—
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The calculation
Surface area × average depth = volume in cubic feet, then × 7.48 for US gallons. A 32 × 16 ft pool sloping from 3 ft to 8 ft has an average depth of 5.5 ft, so 512 ft² × 5.5 = 2,816 ft³, or 21,065 gallons.
Average depth, and when the simple average is wrong
Taking the mean of the shallow and deep ends assumes the floor slopes evenly from one to the other. That is close enough for most residential pools. It is not right for the common hopper-bottom design, where a long flat shallow section runs most of the length before dropping steeply into a small deep well. Those pools hold noticeably less than the simple average suggests — often 10 to 15% less — so if yours has a distinct flat shallow area, measure it as two sections and add them.
Shape
Surface area
Rectangular
length × width
Round
π × diameter² ÷ 4
Oval
π × long axis × short axis ÷ 4
Conversions worth memorising
One cubic foot is 7.48 US gallons, 6.23 imperial gallons or 28.32 litres. Water weighs 8.34 lb per US gallon, which is why a modest 20,000 gallon pool is holding about 83 tons — and why a full pool must never be drained without understanding the hydrostatic pressure under the shell.
Every chemical decision you make depends on it. Chlorine, acid, alkalinity increaser, stabiliser and algaecide are all dosed per 10,000 gallons, so an estimate that is 20% out means every dose is 20% out. Over months that compounds into water that never quite balances and a sanitiser bill larger than it needs to be.
Measure the water, not the pool
The depth that counts is from the floor to the actual waterline, which sits at the middle of the skimmer opening — typically four to six inches below the coping. Using the wall height instead consistently overstates volume by several percent.
Filling from a hose takes longer than people expect
At 8 gallons a minute, a 21,000 gallon pool takes about 44 hours of continuous running. Two hoses halve it. Many owners find a bulk water delivery cheaper once sewer charges are counted, because utilities frequently bill wastewater volume on metered water even when that water is obviously going into a pool — some will credit it if you ask in advance.
Turnover
Once you know the volume you can size the pump. The usual target is turning the whole pool over once or twice in 24 hours, so a 21,000 gallon pool wants a pump moving roughly 15 gallons a minute to achieve a single daily turnover. Variable-speed pumps achieve this far more cheaply by running longer at lower speed than briefly at full power.
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Frequently asked questions
How many gallons is my pool?
Multiply the surface area in square feet by the average water depth in feet, then multiply by 7.48. A 32 × 16 ft pool averaging 5.5 ft deep holds about 21,000 US gallons.
How do I calculate the average depth of a pool?
For a floor that slopes evenly, add the shallow and deep depths and divide by two. If the pool has a long flat shallow section before dropping into a deep well, measure the two sections separately and add their volumes — the simple average overstates that shape by 10 to 15%.
How many gallons is a 24 foot round pool?
A 24 ft round pool holds about 13,500 gallons at 4 ft of water depth, or roughly 17,000 gallons at 5 ft. Round pools are usually a constant depth, so use the flat floor option.
How long does it take to fill a pool with a garden hose?
At a typical 8 gallons per minute, roughly 2,000 gallons an hour — so a 20,000 gallon pool takes about 42 hours of continuous running. Using two hoses on separate supply lines halves that.
How much does a pool of water weigh?
US water weighs 8.34 pounds per gallon, so a 20,000 gallon pool holds about 167,000 pounds — roughly 83 tons. This is why an in-ground pool should never be fully drained without professional advice about groundwater pressure.