How the Pool Calculator Works
Pool capacity is surface area multiplied by average depth. The calculator handles the three shapes that cover almost every residential pool, and shows the dimension you are editing on the diagram so you can check you are measuring the right edge.
Enter the deep end only if your pool has one — leave the field blank for a pool with a flat floor and the shallow reading is used for both ends.
Formulas
Rectangular pool
volume = length × width × (shallow + deep) ÷ 2Round pool
volume = π ÷ 4 × diameter × diameter × (shallow + deep) ÷ 2Oval pool
volume = π ÷ 4 × length × width × (shallow + deep) ÷ 2The π ÷ 4 factor is the area of an ellipse written in terms of its full width and length rather than its two radii. A circle is simply the case where those are equal, which is why the round and oval formulas are the same expression.
To convert the result: one cubic foot is 7.48 US gallons (a US gallon is defined as 231 cubic inches, and a cubic foot is 1,728), or 28.32 liters.
Why Average Depth Works
A floor that slopes evenly from the shallow to the deep end holds exactly as much water as a flat floor sitting at the midpoint of the two. The wedge of water gained toward the deep end is precisely the wedge lost toward the shallow end.
That equivalence only holds for an even slope. A pool with a sudden drop-off, a diving well, or a long flat shallow section followed by a short steep ramp will not average out. Measure those pools in sections and add the volumes together.
Worked Example
A rectangular pool, 32 ft long and 16 ft wide, sloping from 3 ft at the steps to 8 ft at the deep end:
- Surface area: 32 × 16 = 512 ft²
- Average depth: (3 + 8) ÷ 2 = 5.5 ft
- Volume: 512 × 5.5 = 2,816 ft³
- In gallons: 2,816 × 7.48 = 21,065 gal
That is about 79,750 liters, or 176,000 lb of water.
Common Pool Sizes
| Shape | Dimensions | Depth | Capacity |
|---|---|---|---|
| Rectangle | 12 × 24 ft | uniform 4 ft | 8,618 gal |
| Rectangle | 16 × 32 ft | 3 ft → 8 ft | 21,065 gal |
| Rectangle | 20 × 40 ft | 3 ft → 8 ft | 32,914 gal |
| Round | 18 ft dia. | uniform 4 ft | 7,614 gal |
| Round | 24 ft dia. | uniform 4 ft | 13,536 gal |
| Round | 27 ft dia. | uniform 4 ft | 17,132 gal |
| Oval | 15 × 30 ft | uniform 4 ft | 10,575 gal |
| Oval | 18 × 33 ft | uniform 4 ft | 13,959 gal |
These are calculated with the formulas above, at the stated water depth.
Why Your Pool May Hold Less Than the Chart Says
Published capacities are optimistic more often than not:
- Water sits below the wall. A pool advertised as 52 inches deep normally runs 46 to 48 inches of actual water, because the skimmer opening sets the level.
- Steps and benches displace water. A set of corner steps can easily account for a few hundred gallons.
- Corners are rarely square. Many rectangular pools have radiused corners that trim a little off the surface area.
For dosing chemicals, a few percent either way is not a problem — and under-estimating is the safer error, since adding more is easier than diluting.
What You Need the Volume For
- Chemical dosing. Every label is written per 10,000 gallons. Salt, chlorine, acid, and stabilizer all scale directly with volume — see the pool salt calculator for the salt case.
- Pump and filter sizing. Turnover is usually specified as the whole volume passing through the filter in 8 to 12 hours, which sets the flow rate you need.
- Heater sizing and running costs. Energy needed to raise the temperature is proportional to the mass of water.
- Structural load. Water weighs roughly 8.34 lb per gallon, so even a modest pool puts tens of tonnes onto whatever is underneath it.