Why the volume number outlives every other measurement
Pool volume is used constantly and measured once. Every chlorine dose, every acid addition, every algaecide and every stabiliser adjustment is scaled by it, and so is the flow the pump must move and the output the heater must produce. Get it wrong at the start and the error is repeated in every bucket of chemical for the life of the pool.
The error is also invisible. A pool dosed on a volume 25% too high simply runs at higher chlorine than intended, which looks like nothing until the cyanuric acid climbs faster than expected or the vinyl liner fades. There is no feedback loop that corrects a bad volume figure; you have to get it right deliberately.
The calculation itself is trivial: a surface area, an average depth, and the conversion 7.4805 gallons per cubic foot. All of the difficulty is in the average depth, because almost no pool has a floor that slopes evenly from end to end. Most have a flat shallow section, then a slope, then a flat deep section - and averaging only the two end depths on such a pool overstates the volume, sometimes badly.
Once you have the figure, write it on the equipment pad in permanent marker. Everyone who ever services the pool will need it, and it is the input the chlorine dosage calculator and the turnover rate calculator both start from.
The four area formulas and the average depth trap
Rectangular pools are length times width. Round pools are π/4 × diameter², which is 0.7854 × D². Oval pools use the ellipse area π/4 × L × W, the same 0.7854 factor applied to both axes - this treats the pool as a true ellipse, which is close for most oval above-ground pools and slightly under for a stadium-shaped pool with straight sides and semicircular ends.
Kidney and freeform pools use the pool-industry approximation 0.45 × (A + B) × L, where A and B are the widths of the two lobes and L is the overall length. It is an empirical fit rather than a geometric identity, and on a genuinely irregular shape it is worth sanity-checking against your water meter the next time you fill.
Now the depth. On a floor that slopes evenly from the shallow end to the deep end, the average depth is exactly the mean of the two, because the cross-section is a trapezoid and a trapezoid's average height is the mean of its parallel sides. A 3 ft to 8 ft pool averages 5.5 ft, and that is right.
Almost no pool is built that way. A typical vinyl-liner pool has a flat shallow area over perhaps half its length, then a slope down to a hopper, then a small flat bottom. Averaging 3 and 8 on such a pool gives 5.5 ft when the true average may be nearer 4.3 ft - a 28% overstatement of volume, and therefore a 28% overdose on every chemical.
The advanced fields handle this properly. Enter the percentage of the pool's length that is flat at the shallow depth and the percentage flat at the deep depth; the remainder is treated as an even slope between them. The average becomes a weighted one: flat shallow fraction at the shallow depth, flat deep fraction at the deep depth, and the sloping fraction at the mean of the two.
Worked example: a 16 by 32 ft pool, both ways
Take a rectangular pool 16 ft wide and 32 ft long, 3 ft at the shallow end and 8 ft at the deep end.
- Surface area. 16 × 32 = 512 ft².
- Average depth, even slope. (3 + 8) ÷ 2 = 5.5 ft.
- Volume. 512 × 5.5 = 2,816 ft³.
- Gallons. 2,816 × 7.4805 = 21,065 gallons, which is 79,742 litres or 79.74 m³.
Now suppose the floor is really flat at 3 ft over the first half of the length and slopes over the second half.
- Weighted average depth. 0.50 × 3 ft + 0.50 × (3 + 8)÷2 = 1.50 + 2.75 = 4.25 ft.
- Volume. 512 × 4.25 = 2,176 ft³ = 16,278 gallons.
The difference is 21,065 − 16,278 = 4,787 gallons. Measured against the real volume that is 4,787 ÷ 16,278 = 29.4% too high. Dosing the second pool as though it were the first multiplies every chemical addition by 21,065 ÷ 16,278 = 1.294, so a dose intended to add 2.0 ppm of chlorine actually adds 2.59 ppm - every single time.
Checking the answer against reality
The most reliable check is the water meter. Note the reading before a fill, note it after, and compare the difference against the calculation. Utilities in the United States usually meter in cubic feet or in hundreds of cubic feet, so multiply by 7.4805 or 748.05 respectively. On a new pool this is the definitive measurement and it is worth taking.
The second check is chemical. Add a known weight of a stabiliser or a salt that neither evaporates nor decomposes, circulate for a full turnover, and test. If 8 lb of cyanuric acid raises the reading by exactly 30 ppm, the pool holds 8 × 1,000,000 ÷ (30 × 8.345) = 31,955 gallons. Any conserved chemical you can measure accurately works; chlorine does not, because it is consumed.
Third, sanity-check against the surface area. A residential pool is rarely deeper than 6 ft on average, so gallons above about 45 times the surface area in square feet suggests a depth error. Our 512 ft² example at 21,065 gallons is 41 gallons per square foot, which corresponds to the 5.5 ft average - on the high side, exactly as you would expect for a pool with a proper diving depth.
For a spa, a swim spa or a cylindrical storage vessel where the geometry is different again, the tank volume calculator and the horizontal cylindrical tank calculator cover shapes this page does not.
When "oval" is really a stadium shape
The oval option here is a true ellipse - area π/4 × L × W - and plenty of pools sold as oval are not ellipses at all. A stadium or racetrack pool has two straight parallel sides and two semicircular ends, which is a different shape with more area for the same length and width, because a true ellipse pinches in toward its ends while straight sides do not.
The gap is real enough to matter. Take the 30 × 15 ft example this calculator's own test suite uses: the ellipse formula gives 353.43 ft². A stadium shape with the same 30 ft overall length and 15 ft width - a 15 × 15 ft rectangle in the middle plus a full 15 ft diameter circle split across the two ends - has an area of 15 × (30 − 15) + π/4 × 15² = 225 + 176.7 = 401.7 ft². The ellipse formula understates that shape by (401.7 − 353.43) ÷ 401.7, about 12%, and the volume is short by the same fraction because depth is unaffected by which shape you pick.
Look at the waterline before choosing. If the two long sides run straight for a visible stretch before the ends curve away, it is closer to a stadium than an ellipse, and this calculator's oval option will read low. If the curve starts almost immediately at the widest point with no straight run, a true ellipse is the better model. When you cannot tell by eye, the water-meter check described above settles it directly, and settles it better than guessing which formula to trust.
Gallons for common rectangular pool sizes
| Size (ft) | Surface area (ft2) | Avg depth 4.0 ft | Avg depth 4.5 ft | Avg depth 5.5 ft |
|---|---|---|---|---|
| 12 x 24 | 288 | 8,618 | 9,695 | 11,849 |
| 14 x 28 | 392 | 11,729 | 13,196 | 16,128 |
| 15 x 30 | 450 | 13,465 | 15,148 | 18,514 |
| 16 x 32 | 512 | 15,320 | 17,235 | 21,065 |
| 18 x 36 | 648 | 19,390 | 21,813 | 26,661 |
| 20 x 40 | 800 | 23,938 | 26,930 | 32,914 |
| 16 x 40 | 640 | 19,150 | 21,544 | 26,331 |
| 20 x 44 | 880 | 26,331 | 29,623 | 36,206 |
Computed as length x width x average depth x 7.480519. Find your dimensions, then read the column that matches your real average depth rather than assuming the deeper one.
Where pool volume figures go wrong
- Averaging only the two end depths on a pool with a flat shallow section. The most common and largest error, and it always overstates the volume.
- Measuring depth from the coping rather than the waterline. The water usually sits 4 to 6 inches below the top of the wall, and on a 512 ft² pool six inches is 1,900 gallons.
- Using outside dimensions. Measure the water, not the shell. On a vinyl pool the difference is small; on a thick-walled gunite pool with a wide bench it is not.
- Forgetting attached spas and tanning ledges. A spillover spa can add 500 gallons and a tanning ledge several hundred more, and both are chlorinated by the same system.
- Rounding 7.4805 to 7.5. Harmless, at 0.26%, but only if you do it knowingly. Rounding a dimension to the nearest foot matters far more.
- Confusing US gallons with imperial gallons. An imperial gallon is 1.201 US gallons, so a volume quoted in imperial gallons is about 17% lower for the same water. This calculator uses US gallons throughout.
- Trusting the builder's brochure figure. Advertised capacities are for the standard model, and yours may have been built shallower, shorter, or with a different hopper.
Key terms
- Average depth
- Volume divided by surface area. On an evenly sloping floor it is the mean of the shallow and deep depths; on a floor with flat sections it must be weighted by length.
- Hopper
- The deep bowl at the diving end of a pool, with sloped sides meeting a small flat bottom. It holds much less water than a full-width deep end of the same depth.
- Turnover
- The time for the circulation system to pass a volume of water equal to the whole pool through the filter. Volume divided by flow rate.
- Surface area
- The water plan area. It drives evaporation, heat loss and cover sizing as well as the volume calculation.
- US gallon
- 231 cubic inches, or 3.785412 litres. A cubic foot holds 7.480519 of them. Not the same as an imperial gallon, which is 4.546 litres.