How to measure a pool
Measure at the water line, across the inside of the pool. For a round pool, you only need the distance straight across at the widest point. For an oval, measure the length and the width at their widest. For a kidney, measure the length, plus the width across each of the two wide ends.
Depth is where most mistakes happen. You want the depth of the water and not the wall height, so a 52 inch above-ground wall generally holds about 4 feet of water. On a sloped floor, measure at the shallow and deep ends and take the average. A pole with a tape on it works well. If your floor drops suddenly, treat the shallow and deep parts as two pools and add them.
A kidney is a free shape, and no formula will get it exactly. If you need to be sure, read your water meter before and after your next fill, since that measures the real thing.
A worked example
Doheny's guide prints this pool: 12 by 36 feet, 3 feet at the shallow end and 9 feet at the deep end. These lines come from the same formula the calculator above runs.
12 by 36 ft pool, 3 to 9 ft deep
- Average depth: (3 + 9) ÷ 2 = 6 ft
- Volume: 36 × 12 × 6 = 2,592 cu ft
- Gallons: 2,592 × 7.48 = 19,389 gal
- Litres: 73,385 L
Doheny prints 19,440 gallons because it rounds to 7.5. The difference is about 52 gallons out of nearly 20,000, and it will not change a chemical dose in any way you can measure. For what the pool costs through the year, use the pool maintenance cost calculator, and to size a heater bill, the pool heating cost calculator.
Gallons in common pools
These are for typical water depths: about 3.5 feet in a 48 inch wall, and about 4 feet in a 52 inch wall, as in Akron's examples. A rectangle uses the average of a 3 to 8 foot floor.
| Pool | Average depth | Gallons | Litres |
|---|---|---|---|
| Round, 15 ft, 48 in wall | 3.50 ft | 4,627 | 17,511 |
| Round, 18 ft, 52 in wall | 4.00 ft | 7,614 | 28,818 |
| Round, 24 ft, 52 in wall | 4.00 ft | 13,536 | 51,232 |
| Oval, 15 × 30 ft, 52 in wall | 4.00 ft | 10,575 | 40,025 |
| Rectangle, 16 × 32 ft, 3 to 8 ft | 5.50 ft | 21,064 | 79,727 |
| Rectangle, 20 × 40 ft, 3 to 8 ft | 5.50 ft | 32,912 | 124,572 |
Mistakes that throw off your dosing
- Diameter for radius. A formula with radius squared needs half the distance across. Using the whole diameter makes the pool four times too big.
- Wall height for depth. A 52 inch wall does not hold 52 inches of water. Use what is really there.
- Deepest point for depth. Use the average on a sloped floor, or you will overstate it by a third or more.
- A kidney with no factor. One retailer's formula leaves out the 0.45, and so treats a kidney as far larger than it is.
- Overdosing from a wrong volume. A chemical amount follows the product label and your own test. An error here flows straight into the dose.
Questions
How many gallons in a round above-ground pool?
A 24 foot round pool at 4 feet of water holds about 13,500 gallons, and an 18 foot round one about 7,600. Multiply the area of the circle by the water depth, then by 7.48. Use the water depth, not the height of the wall.
How many gallons in an 18x52 pool?
An 18 foot round pool with a 52 inch wall holds about 7,600 gallons, at about 4 feet of water. The 52 is the wall height in inches, which catches people out. Akron's printed table has this row off by one wall size, so check any chart.
How do I measure a sloped pool?
Measure the water depth at the shallow and the deep ends, then use the average. A pool from 3 to 9 feet averages 6 feet, as in PHTA's formula. That assumes an even slope. Treat a sudden drop as two sections and add them.
How do you calculate a kidney pool's gallons?
Add the two widest widths, multiply by the length and by 0.45, then by the average depth and 7.48. It is PHTA's formula and an approximation. A kidney 20 feet long with widths adding to 18 and 8 feet deep comes to about 9,700 gallons.
Why does my oval pool's manual give a different number?
Because makers use two different ovals. PHTA and Akron treat an oval as a true ellipse. Hayward's manual prints a larger multiplier, which fits straight sides with round ends. On a 16 by 32 foot pool they differ by about 14 percent, so choose the shape that matches yours.
Why does 7.48 differ from the 7.5 on my chart?
The 7.5 is the rounded form that pool charts use. USGS prints 7.48 gallons per cubic foot, and the true figure is 7.48052. The difference is about 0.3 percent, which does not matter next to the error in measuring a depth.
This pool volume is a planning estimate from your measurements, and a kidney or a free-form shape is only ever an approximation. It assumes an even slope and a fair reading of water depth. Chlorine, salt and other doses follow the product label, and a wrong volume carries straight into them, so check with your supplier if yours matters closely.
Sources and review
- Pool operator handbook, Appendix B, Pool and Hot Tub Alliance, 2021.
- Estimated use of water in the United States, conversion factors, U.S. Geological Survey, 2004.
- AquaRite owner's manual, Hayward, 2019.
- How to calculate the gallons in a swimming pool, City of Akron, no date shown, read November 2026.
- Calculate pool volume, Doheny's, no date shown, read November 2026.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Average depth
Average depth = (Shallow + Deep) ÷ 2
- Shallow, Deep
- Water depth at each end, in feet
Worked through: (3 + 9) ÷ 2 = 6 ft
Gallons, rectangle
Gallons = Length × Width × Average depth × 7.48
- 7.48
- US gallons in one cubic foot
Worked through: 36 × 12 × 6 = 2592, and 2592 × 7.48 = 19388 gallons
Gallons, kidney
Gallons = (Width A + Width B) × Length × 0.45 × Average depth × 7.48
- 0.45
- PHTA's factor for a kidney shape
Worked through: 18 × 20 × 0.45 × 8 = 1296, and 1296 × 7.48 = 9694 gallons
The constants, and where they come from
GALLONS_PER_CU_FT
DefinitionalUS gallons in one cubic foot
Applies to: Definitional, everywhere
Source: Estimated use of water in the United States, conversion factors, U.S. Geological Survey, published 2004, read 2026-11. This source is about 22 years old, and it is the one the figure rests on.
Also from: Pool operator handbook, Appendix B, Pool and Hot Tub Alliance, published 2021, read 2026-11.
USGS prints 7.48. The exact figure is 7.48052, and pool industry charts round to 7.5, so your chart may be about 0.3 percent higher.
LITRES_PER_GALLON
DefinitionalLitres in one US gallon
Applies to: Definitional, everywhere
Source: Pool operator handbook, Appendix B, Pool and Hot Tub Alliance, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
PHTA prints gallons × 3.785 = litres. The exact figure is 3.78541.
KIDNEY_FACTOR
Published range, narrowedFactor for the area of a kidney pool from its two widths and its length
Applies to: A kidney shape measured at its two widest points
Source: Pool operator handbook, Appendix B, Pool and Hot Tub Alliance, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
PHTA's formula. It is an approximation of a free shape, and some retailers' formulas leave the factor out, which overstates the volume.
STRAIGHT_SIDED_OVAL_FACTOR
Published range, narrowedArea of a straight-sided oval, as a share of its bounding rectangle
Applies to: Roughly 2 to 1 ovals with straight sides and round ends
Source: AquaRite owner's manual, Hayward, published 2019-10, read 2026-11. This source is about 7 years old, and it is the one the figure rests on.
Hayward's manual prints a multiplier of 6.7, or 893 in metric, for an oval, where PHTA and Akron use a true ellipse of about 5.9. That this means straight sides is our inference, which matches the geometry. The two differ by about 14 percent.
What the labels mean
- Definitional:
- true by definition, like the number of inches in a foot, so there is nothing to cite.
- Published range, narrowed:
- a source publishes a range and this calculator uses a representative value from it.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the pool volume calculator
- 1
Choose the shape of your pool
- 2
Measure its length and width at the water line
- 3
Measure the water depth at both ends
- 4
Read the gallons
- 5
Use that figure on chemical labels and equipment sizing
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- The floor slopes evenly, so the average of shallow and deep is fair. Split an abrupt drop into sections
- You entered the water depth, not the wall height
- A kidney uses PHTA's formula, which is an approximation
- Gallons use 7.48 per cubic foot. Pool charts often round to 7.5, about 0.3 percent higher
Pro tips
- Measure the water, at the level you keep it, not the pool shell
- Keep the figure with your chemical log
- Check your volume against the water meter next time you fill it, if you can