Measure any rectangle, round or oval pool, then carry the gallons into the salt dose.
Measure a pool at its deep end and it looks bigger than it is. The salt dose grows with the error. Take the 32 × 16 ft example pool at its 6.5 ft deep end instead of the 5 ft average, and it reads 30% too big. That adds 48 lb of salt to a 1,000 ppm rise.
Pick the shape, then type the length and width (or the diameter) and the shallow and deep ends, in feet or feet and inches. You get gallons, liters and cubic feet from the average depth.
Example result
Example: 32 × 16 ft, 3.5 to 6.5 ft deep. Type your own measurements.
This rectangular pool holds 19,150 gallons (72,491 liters, 2,560 cubic feet) at an average depth of 5 ft.
Math32 × 16 × 5 × 7.48052 ≈ 19,150 gallons
ft³One cubic foot is 7.48052 US gallons. Round and curved-sided oval pools use π/4 × 7.48052 ≈ 5.875.
Drag the ends to match your pool
Gallons to salt
Because a pool needs about 83 pounds of salt per 10,000 gallons for each 1,000 ppm. Any error in gallons becomes the same percentage error in pounds of salt.
Take the gallons from this page into the salt water pool calculator, or read pounds by pool size in the table on how much salt to add to a pool. The same gallons price a first fill; see the cost of pool salt by the bag for each salt system. If you overshoot, salt only comes out by draining and refilling. The gallons to drain for the same pool size are on how to lower pool salt.
Use the deep end (6.5 ft) as the depth of the worked-example pool instead of the 5 ft average, and you overstate it by 30%. That’s 5,745 gallons the pool doesn’t hold. At a 1,000 ppm rise, those gallons add 48 pounds of salt it doesn’t need.
Quick reference
Common pool sizes hold from 5,288 to 29,922 gallons. The table gives gallons and liters for 5 in-ground rectangles at 3.5 to 6.5 ft and 4 above-ground rounds at 4 ft all over.
| Pool size | Depth | Gallons | Liters |
|---|---|---|---|
| 12 × 24 ft rectangle | 3.5 to 6.5 ft | 10,772 | 40,776 |
| 14 × 28 ft rectangle | 3.5 to 6.5 ft | 14,662 | 55,501 |
| 16 × 32 ft rectangle | 3.5 to 6.5 ft | 19,150 | 72,491 |
| 18 × 36 ft rectangle | 3.5 to 6.5 ft | 24,237 | 91,747 |
| 20 × 40 ft rectangle | 3.5 to 6.5 ft | 29,922 | 113,267 |
| 15 ft round | 4 ft all over | 5,288 | 20,016 |
| 18 ft round | 4 ft all over | 7,614 | 28,823 |
| 21 ft round | 4 ft all over | 10,364 | 39,231 |
| 24 ft round | 4 ft all over | 13,536 | 51,241 |
For other sizes or depths, use the calculator at the top of this page.
The formula
Multiply the surface area by the average depth and by 7.48052. That last number is how many US gallons fit in one cubic foot of water. The table gives the formula and constant for 4 pool shapes.
| Shape | Formula | Constant |
|---|---|---|
| Rectangle | length × width × average depth × 7.48052 | 7.48052 gallons per cubic foot |
| Round | diameter² × average depth × 5.875 | 5.875 π/4 × 7.48052 |
| Oval (curved sides) | length × width × average depth × 5.875 | 5.875 π/4 × 7.48052 |
| Oval (straight sides) | ((length − width) × width + π/4 × width²) × average depth × 7.48052 | 7.48052 gallons per cubic foot |
Round pools and curved-sided ovals share one factor, 5.875 (π/4 × 7.48052, shown to three decimals). The π/4 turns diameter² into the area of a circle, and length × width into the area of an ellipse. A straight-sided oval is a rectangle with a half circle at each end. Its surface area is (length − width) × width plus π/4 × width², where width is the shorter side.
Measuring a pool’s average depth takes 3 steps, and the volume formula uses the result.
The two-end average assumes the floor slopes in a straight line from the shallow end to the deep end. A diving well that drops off in a short stretch isn’t a straight slope, so the two-end average overstates that pool’s volume.
Worked example
A 32 × 16 ft rectangular pool, 3.5 ft at the shallow end and 6.5 ft at the deep end, holds 19,150 gallons (72,491 liters, or 2,560 cubic feet).
The calculator at the top of this page opens on the same pool, so the figures there match the steps here.
Owner questions
The calculator and these answers share one set of unit constants.
It’s the surface area in square feet times the average depth in feet times 7.48052. That’s the number of US gallons in one cubic foot of water.
A round pool holds diameter² × average depth × 5.875 gallons, where 5.875 is π/4 × 7.48052. A rectangle holds length × width × average depth × 7.48052 gallons.
An oval pool with straight sides holds ((length − width) × width + π/4 × width²) × average depth × 7.48052 gallons. The shape is a rectangle with a half circle at each end. For the same length and width, the curved-sided ellipse formula (length × width × average depth × 5.875) gives a smaller number.
No. Averaging the shallow and deep ends assumes the floor slopes in a straight line. A diving well that drops off in a short stretch isn’t a straight slope, so the two-end average overstates that pool’s volume.
One US gallon is 3.785411784 liters. The calculator multiplies the gallons by that figure for the liters it shows.