How Many Gallons Are in a Pool? Formulas for Every Shape
Knowing how much water your pool holds is the starting point for almost everything else: how long it takes to fill, how much of a product to add, and how large a pump or heater you need. The volume comes from a short formula once you know the shape and the average depth.
The basic method
Multiply the surface area by the average depth to get the volume in cubic feet, then multiply by 7.48 to convert to US gallons. In metric, volume in cubic metres times 1,000 gives litres.
Formulas by shape
- Rectangle: length × width × average depth × 7.48
- Round: 3.14 × (diameter ÷ 2)² × average depth × 7.48
- Oval: 3.14 × (length ÷ 2) × (width ÷ 2) × average depth × 7.48
- Kidney (approximate): (A + B) × length × 0.45 × average depth × 7.48, where A and B are the widths of the two ends
Finding the average depth
Most pools slope from a shallow end to a deep end. Add the two depths and divide by two. A pool that is 3 ft deep at one end and 7 ft at the other has an average depth of 5 ft. If the floor has a long flat section and then drops sharply, a simple average over- or under-states the volume, so estimate each section separately or use the plan from the builder.
Worked example
A rectangular pool is 32 ft long and 16 ft wide with an average depth of 5 ft. The volume is 32 × 16 × 5 = 2,560 cubic feet, and 2,560 × 7.48 is about 19,150 gallons, or roughly 72,500 litres.
Typical pool sizes
| Pool | Depth | Approx. US gallons |
|---|---|---|
| 12 × 24 ft rectangle | average 4 ft | 8,618 |
| 16 × 32 ft rectangle | average 5 ft | 19,150 |
| 18 ft round | average 4 ft | 7,614 |
| 24 ft round | average 4.5 ft | 15,228 |
| 15 × 30 ft oval | average 4.5 ft | 11,897 |
How long will it take to fill?
Divide the volume by the flow rate. A garden hose delivering about 9 gallons per minute fills a 19,150-gallon pool in around 35 hours. Time how long your own hose takes to fill a 5-gallon bucket to get a more accurate figure.
Why the number matters
Product labels for pool treatments are written for a stated volume of water, and filtration is usually sized to turn the whole pool over in a set time. An error of a few thousand gallons changes both. Water is also heavy, about 8.35 pounds per gallon, so a full pool carries a very large load that the structure and the ground must support.
Use the Pool Volume Calculator to get gallons, litres, water weight and fill time in one step.
Frequently asked questions
How many gallons are in an 18 ft round pool? With an average depth of 4 feet, an 18 ft round pool holds about 7,600 US gallons.
How do I measure the average depth? Add the shallow-end depth and the deep-end depth and divide by two, as long as the floor slopes steadily. Measure to the normal water line.
How many litres are in a pool? Multiply the volume in cubic metres by 1,000, or multiply US gallons by 3.785.
How accurate is the kidney pool formula? It is an approximation. For a free-form pool the builder's volume figure is usually more reliable.