Pool volume formulas
Average depth = (shallow depth + deep depth) / 2 only when the floor slope and plan shape make that average representative. An ellipse is different from an oval with long straight sides.
| Shape | Plan area | Volume |
|---|---|---|
| Rectangle | Length x width | Area x average water depth |
| Round | Pi x diameter squared / 4 | Area x water depth |
| Oval (ellipse) | Pi x length x width / 4 | Area x average water depth |
Measure the water, not the outside shell
Use inside dimensions at the normal waterline. A nominal 18-foot pool or a 52-inch wall does not necessarily contain 18 feet across or 52 inches of water. Measure actual fill depth before estimating chemical quantities.
For changing slopes, a diving well, steps or a freeform outline, split the water into non-overlapping sections and add their volumes. Account for large submerged benches separately. A water meter during a controlled fill can give a better baseline than a plan drawing.
Example: a 30 by 15 foot pool
With a linear slope from 3 to 5 feet of water, the mean depth is 4 feet. The volume is 30 x 15 x 4 = 1,800 cubic feet, or about 13,465 US gallons and 50,970 liters.
One US liquid gallon is 3.785411784 liters. Imperial gallons are a different unit and are not used here. Keep the unrounded estimate for follow-on calculations, but allow for measurement uncertainty when dosing.
Use volume with a measured water test
Pool volume establishes the amount of water to treat; it does not identify what treatment is needed. Measure salt, free chlorine, total alkalinity or cyanuric acid before choosing a chemical. For pH, obtain an acid/base demand test and its matching treatment table.
Common questions
Can I use the wall height as the pool depth?
Use actual water depth. Wall height includes the portion above the normal waterline and can overstate the water volume.
How do I calculate a kidney-shaped pool?
Divide it into measured non-overlapping sections or use a metered fill. An enclosing rectangle includes water that is not there, and a single ellipse may also be a poor approximation.
Does a deep end mean I can always average two depths?
No. That average assumes a representative linear slope. A long shallow shelf and a short deep well should be calculated separately.
Sources & calculation notes
Check the original source for product-specific instructions and current details.
- NIST: SI units of volumeChecked 2026-10-27