Pool Volume Calculator
Pool volume in gallons or litres, including the sloping floor most pools have.
Updated
You need
21,065US gallons
79,740 litres — computed on an average depth of 5.5 ft
- Water surface
- 512 sq ft
- Average depth
- 5.5 ft
- Volume
- 2,816 cu ft
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In short
How many gallons is my pool?
A 32 by 16 ft rectangular pool sloping from 3 ft to 8 ft averages 5.5 ft deep, so it holds 2,816 cubic feet, which is 21,065 US gallons or 79,740 litres. Use the average of the two ends rather than the deep end: sizing this pool off its 8 ft end alone would overstate the water by 45 percent.
1 US gallon is exactly 231 cubic inches, so a cubic foot is 7.48052 gallons and a gallon is 3.785411784 litres.
How to use the pool volume calculator
Choose the shape first, because it decides which measurements the page asks for. A rectangle takes a length and a width, a round pool takes one diameter, and an oval takes its long and short axes. Measure the water surface at the water line, not the deck, the coping or the outside of the shell.
Then measure the depth twice, once at the shallow end and once at the deep end, from the water line down to the floor. This is the step that decides whether the answer is right. Almost every in ground pool slopes, so there is no single depth to enter, and picking either end gives an answer that is wrong by a large margin.
512 sq ft
Water surface
32 by 16 ft rectangle
5.5 ft
Average depth
the mean of a 3 ft and an 8 ft end
21,065 gal
Water it holds
2,816 cu ft, or 79,740 litres
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If the floor genuinely is flat, and plenty of above ground and lap pools are, turn the sloping floor switch off and enter the one depth. The arithmetic collapses to the same thing: a flat pool at 5.5 ft holds exactly what the sloping 3 to 8 ft pool holds, because the average is what the volume was always built on.
The result is given in US gallons and in litres from the same cubic feet, using the legal definitions rather than a rounded factor. A US gallon is exactly 231 cubic inches, so a cubic foot is 1728 divided by 231, which is 7.480519 gallons, and an inch is exactly 25.4 mm, which makes a gallon exactly 3.785411784 litres.
Do
- Measure at the water line, since the shell and the coping are wider than the water.
- Take the depth at both ends and let the page average them.
- Use the round shape for a circular pool, because area goes as the radius squared.
- Re measure after a refill if the water level sits noticeably lower than usual.
Don't
- Enter the deep end as if it were the whole pool; that overstates the default by 45 percent.
- Multiply a round pool by its diameter squared, which overstates the surface by about a quarter.
- Assume litres and gallons are interchangeable at 4; the factor is 3.785411784.
- Use this figure to work out how much of any chemical to add.
Volume for the pool sizes people actually have, in cubic feet, US gallons and litres, with the average depth each one is computed on. Rectangular pools use the mean of a shallow and a deep end; the round and oval rows are flat floors.
| Pool | Surface, sq ft | Average depth | Cubic feet | US gallons | Litres |
|---|---|---|---|---|---|
| 12 × 24 ft rectangle, 3 to 6 ft | 288 | 4.5 ft | 1,296 | 9,695 | 36,699 |
| 14 × 28 ft rectangle, 3 to 6 ft | 392 | 4.5 ft | 1,764 | 13,196 | 49,951 |
| 16 × 32 ft rectangle, 3 to 8 ft | 512 | 5.5 ft | 2,816 | 21,065 | 79,740 |
| 18 × 36 ft rectangle, 3 to 8 ft | 648 | 5.5 ft | 3,564 | 26,661 | 100,921 |
| 15 ft round, flat floor | 176.7 | 4 ft | 706.9 | 5,288 | 20,016 |
| 18 ft round, flat floor | 254.5 | 4 ft | 1,017.9 | 7,614 | 28,823 |
| 24 ft round, flat floor | 452.4 | 4.5 ft | 2,035.8 | 15,228 | 57,646 |
| 15 × 30 ft oval, flat floor | 353.4 | 4 ft | 1,413.7 | 10,575 | 40,032 |
The depth is the part everyone gets wrong
Ask someone how deep their pool is and they will tell you the deep end, because that is the number that felt impressive when they bought it. Put that number into a volume calculation and every figure downstream is inflated by the ratio of the deep end to the true average, which on a normal pool is a very large error.
- Deep end only, 8 ft
- 30,640 gal, 45% too high
- Average of the ends, 5.5 ft
- 21,065 gal, correct
- Shallow end only, 3 ft
- 11,490 gal, 45% too low
- Flat pool at 5.5 ft
- 21,065 gal, identical
The ratio is exactly the ratio of the depths: 8 ÷ 5.5 is 1.4545, so the deep end reading is 45.45 percent high.
The fix is one extra measurement. Drop a tape at the shallow end and again at the deep end, take the mean, and the geometry is honest. If your pool has a flat shelf, a slope and a hopper, measuring the shelf and the hopper still gets you far closer than either end alone does.
Need the same water in litres?
The gallons to litres converter carries the exact 3.785411784 factor and a quick table, if you want to check a figure without re measuring the pool.
Open gallons to litres →What this page deliberately will not tell you
It will not tell you how much of anything to put in the water. Volume is a geometry question with one right answer. Dosing is a chemistry question that depends on the current reading, the product concentration, the water temperature, the bather load and the local rules, and getting it wrong ranges from expensive to genuinely dangerous.
Some chemicals used around pools react violently with each other and several are harmful to breathe or to touch. The correct source for a dose is the label on the container you are holding, read against a current test of the water, and where the numbers are not obvious, a pool professional rather than a web page.
The other honest limit is the geometry itself. Free form pools, kidney shapes, spas joined to the main body and tanning ledges are not rectangles, circles or ellipses. Split an odd pool into shapes the page does support, run each one, and add the results, which is closer than forcing the whole thing into a single rectangle.
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Full guide
How Many Gallons Is My Pool? Average Depth and the Three-Part Floor
The mean of the shallow and deep ends is exact for one shape of floor, and most pools are not that shape.
Read the full guide →The formula, worked line by line
Pool volume is a surface area multiplied by a depth, and then a unit conversion. The surface is whichever of three areas matches the shape: a rectangle is length times width, a round pool is pi times the radius squared, and an oval is treated as an ellipse, pi times the two half axes multiplied together.
The depth is the average of the shallow and deep ends. That is not a simplification for the sake of an easy page: where the floor slopes evenly from one end to the other, a prism of the mean depth holds exactly the same water as the sloping pool, so the average is the answer rather than an approximation of it.
Both unit factors are exact and both are derived rather than remembered. A US liquid gallon is defined as 231 cubic inches, and an inch has been exactly 25.4 mm since the 1959 International Yard and Pound Agreement, which fixes the litre figure to the last digit as well.
rectangle surface = length ft × width ft
round surface = pi × (diameter ÷ 2)²
oval surface = pi × (length ÷ 2) × (width ÷ 2)
average depth = (shallow end + deep end) ÷ 2
cubic feet = surface × average depth
US gallons = cubic feet × 1728 ÷ 231 = cubic feet × 7.480519
litres = US gallons × 3.785411784- Water surface
- 32 × 16 = 512 sq ft
- Average depth
- (3 + 8) ÷ 2 = 5.5 ft
- Volume
- 512 × 5.5 = 2,816 cu ft
- Gallons
- 2,816 × 7.480519 = 21,065
- Litres
- 79,740
Read the deep end as the whole pool and you would get 30,640 gallons, which is 45 percent too much. Read the shallow end and you would get 11,490, which is 45 percent too little.
The round and oval formulas matter more than they look. A round pool is pi times the radius squared, so an 18 ft round pool has a surface of 254.5 square feet, not 324. Squaring the diameter instead of the radius and skipping the pi overstates the water by about 27 percent, and it is a common slip.
An oval is the ellipse, pi over 4 of the rectangle that would box it. The same 32 by 16 ft footprint that gives 512 square feet as a rectangle gives 402.1 square feet as an oval, so an oval pool holds a little under four fifths of what the rectangle would, at 16,545 gallons against 21,065.
Two things this figure is not. It is not the volume at the top of the wall, since the water line sits below it, so measure to the water rather than to the coping. And it is not a dosing instruction: this page computes the water and stops there, deliberately, because how much of anything to add is a separate question with safety consequences.
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Questions people ask
How do I calculate the volume of a pool with a shallow and a deep end?
Average the two depths and use that. Take the depth at the shallow end and at the deep end, add them, divide by two, then multiply the water surface area by the result. A 32 by 16 ft pool going from 3 ft to 8 ft averages 5.5 ft, which is 512 times 5.5, or 2,816 cubic feet, and that is 21,065 US gallons. Where the floor slopes evenly the average is exact, not an estimate.
Covered in depth in How Many Gallons Is My Pool? Average Depth and the Three-Part Floor →
How many gallons are in a cubic foot of water?
A cubic foot is 7.480519 US gallons, and the number is derived rather than looked up. A US liquid gallon is defined as exactly 231 cubic inches, and a cubic foot is 12 cubed, or 1,728 cubic inches, so dividing 1,728 by 231 gives the factor exactly. That is also why a gallon is 3.785411784 litres: 231 cubic inches at 2.54 cm to the inch works out to 3,785.411784 cubic centimetres.
Why is my round pool smaller than I expected?
Because area goes as the radius squared and there is a pi in it. An 18 ft round pool has a water surface of 254.5 square feet, while the 18 by 18 ft square it sits inside is 324, so the circle is about 21 percent smaller than the square. At a flat 4 ft that pool holds 7,614 gallons. Squaring the diameter instead of the radius, or dropping the pi, is the usual reason a round pool comes out too large.
How much chlorine or shock does my pool need?
That is not a question this page answers, on purpose. Volume is geometry with one right answer, while dosing depends on your current water test, the concentration of the specific product, temperature, bather load and local rules, and errors range from wasted money to a genuine safety risk. Take the gallon figure from here to the label on the container you are holding, read against a current test, or to a pool professional who tests your water.
Does the volume change when the water level drops?
Yes, and by more than people expect, because the surface area is large. On the 32 by 16 ft example the surface is 512 square feet, so every inch of water level is 512 divided by 12, about 42.7 cubic feet, which is roughly 319 gallons. Measure the depth from the actual water line rather than the top of the wall or the skimmer, and re measure after a refill if the level has moved.
Sources
Where the constants and formulas on this page come from. Each line names the figure it backs.
The US liquid gallon defined as exactly 231 cubic inches, from which the 7.480519 gallons per cubic foot factor is derived.
NIST Guide to the SI, Appendix B.8 — Factors for units listed alphabetically — NIST
That the inch has been exactly 25.4 mm since 1959, which fixes the gallon at exactly 3.785411784 litres.
SI Units — Length — NIST Office of Weights and Measures
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