Pool Volume Calculator
These estimates assume vertical walls. Measure to the actual water line, not the top of the wall, for the closest figure.
Knowing your pool volume is what lets you dose chlorine, shock, and pH balancers correctly. A 20 by 40 foot pool at 5 feet deep holds about 29,922 gallons. Enter your pool shape, dimensions, and depth below for the capacity in gallons and liters.
Standard Pool Volume Estimates
| Shape | Dimensions | Cubic feet | Gallons | Liters |
|---|---|---|---|---|
| Rectangle | 20' x 40' | 4,000 cu ft | 29,922 gallons | 113,267 liters |
| Round | 24' Diameter | 2,262 cu ft | 16,920 gallons | 64,050 liters |
| Oval | 16' x 32' | 2,010 cu ft | 15,041 gallons | 56,928 liters |
| Kidney | 32' Long (16'A + 10'B) | 1,872 cu ft | 14,003 gallons | 53,009 liters |
How pool volume is calculated
Pool volume drives your chemical dosing and tells you how hard your filter has to work. The approach is the same for every shape: find the cubic feet, then convert to gallons.
The cubic-foot to gallon conversion
Once you have the cubic feet, convert to gallons using the standard that 1 cubic foot holds 7.48 U.S. gallons of water.
- Rectangular pools: length times width times average depth. No extra factor needed.
- Round and oval pools: the radius sets the area via Pi. A common shortcut is to use a 5.9 multiplier in place of 7.48 to fold in the Pi math for a quick estimate.
- Kidney and custom shapes: add the widths of the shallow and deep ends, multiply by the length, then by about 0.45 to account for the curved sides.
Measuring depth
Measure depth to the filled water line, usually around the middle of the skimmer mouth. Measuring to the top of the wall instead overstates your volume and throws off your chemical dosing.
Volume by pool shape
Every shape uses the same surface-area × average-depth × 7.48 idea; what changes is how you find the surface area. Pick the closest shape above and the calculator applies the right factor.
Rectangular pools
The straightforward case: length × width × average depth, then × 7.48 for gallons. No shape factor, so a rectangular pool is the most accurate to estimate and the easiest to check by hand.
Round and circular pools
Area comes from the radius (π × radius²), common on above-ground pools sold by diameter. Measure the diameter across the widest point at the water line. A frequent shortcut folds the π math into a single 5.9 multiplier applied to diameter × diameter × average depth.
Oval pools
An oval (or stadium) pool sits between a rectangle and a circle, so it holds less than the rectangle its length and width would suggest. The calculator uses length × width × average depth × 5.9 to account for the rounded ends — the same formula whether the pool is above ground or in ground.
Kidney-shaped pools
A kidney pool has two different widths and a curved waist, so it takes the widths of both lobes (Width A and Width B), the length, and a 0.45 factor for the curve: (Width A + Width B) × Length × 0.45 × average depth. It's the go-to formula for most free-form pools that are wider at one end than the other.
Grecian and Roman pools
A Grecian pool is a rectangle with the corners cut at 45 degrees; a Roman pool rounds one or both ends. Both hold a little less than the full rectangle they're based on. Estimate them as a rectangle and trim 5 to 10 percent for the cut or rounded corners, or split the pool into a center rectangle plus the corner pieces and add the parts — the section method below.
Irregular shapes and above-ground vs. in-ground
This calculator covers the four standard shapes. For a free-form, lagoon, L-shaped, or figure-8 pool, split it into simple sections you can measure — a rectangle plus a half-circle, or two overlapping ovals — run each section above, and add the gallons together. The same trick handles a pool with an attached spa or a tanning ledge: figure the main pool and the add-on separately. You're estimating for chemical dosing, so within a few percent is close enough; you don't need it to the gallon.
Above-ground and in-ground pools use the identical formulas — a shape is a shape regardless of whether it sits in a hole or on the lawn. The one thing that trips people up on above-ground pools is depth: measure to the actual water line, which sits below the rim, not to the top of the wall. A 52-inch-wall pool filled to about 48 inches is calculated at 4 feet, not 4 feet 4 inches, and that difference alone can swing a dosing calculation.
About these figures: the surface-area × average-depth × 7.48 method and the per-shape multipliers follow the pool-industry and NIST references cited below. Shape factors (5.9 for round and oval, 0.45 for kidney) are standard field estimates that fold the geometry into a single number; they trade a little precision for speed, which is why measuring depth to the true water line matters more than chasing decimals on the factor.
Frequently asked questions
How do you calculate the volume of a kidney-shaped pool?
Kidney pools need four measurements: the length, the width of the shallow end (Width A), the width of the deep end (Width B), and the average depth. The formula is (Width A + Width B) × Length × 0.45 × Average Depth for cubic feet, then multiply by 7.48 to get U.S. gallons.
How do I calculate the average depth of a sloped pool?
Measure the water depth at the shallow end and at the deep end, add them together, and divide by two. Always measure to the actual water line, not the top of the pool wall.
Why do I need to know my pool's volume?
Volume drives your chemical dosing. Chlorine shock, algaecide, and pH balancers are all dosed per a set number of gallons, usually per 10,000. Using the wrong volume means over- or under-treating the water.
How many gallons are in a 20x40 rectangular pool?
At an average depth of 5 feet, a 20-by-40-foot rectangular pool holds 4,000 cubic feet of water. That works out to about 29,922 U.S. gallons, or 113,267 liters.
What multiplier converts cubic feet to gallons?
One cubic foot equals 7.48052 U.S. gallons. Many pool pros round it to 7.5 for quick estimates on site.
Does this formula work for above-ground oval pools?
Yes. The oval formula (Length × Width × Average Depth × 5.9) works the same for in-ground and above-ground pools. Just measure to the filled water level, not the wall height.
How do I calculate the volume of an irregular or free-form pool?
Split it into simple sections, calculate each, and add the gallons together. A free-form or lagoon pool breaks down into a rectangle plus a half-circle at one end, or two overlapping shapes; run each part above and total the results. It's an estimate, not an exact figure, but for chemical dosing you're aiming to be within a few percent, not to the gallon.
How do you calculate the volume of a Grecian or Roman pool?
A Grecian pool is a rectangle with the corners cut off at 45 degrees, and a Roman pool is a rectangle with one or both ends rounded, so both hold slightly less than the full rectangle they're based on. The quickest method is to run the rectangle for the overall length and width, then trim 5 to 10 percent for the cut corners. For a closer figure, split the pool into a center rectangle plus the corner or end pieces and add them.
How do I convert pool volume from gallons to liters?
Multiply U.S. gallons by 3.785 to get liters, which the calculator does automatically. So a 20,000-gallon pool holds about 75,700 liters. If your test kit or dosing chart is metric, work in liters throughout to avoid mixing units, since a U.K. imperial gallon is larger than a U.S. gallon and will throw the dose off.
How do I handle a pool that gradually deepens?
Use the average of the shallow and deep depths, which the sloped-depth option above calculates for you. Add the shallow-end depth to the deep-end depth and divide by two. This works for a pool with a steady slope from one end to the other; a pool with a flat shallow section, a slope, and a flat deep end (a hopper bottom) is closer to the average of all three readings, so measure at each break in the floor.
Related calculators
Sources
- How to Calculate the Gallons in a PoolPremier Pool ServiceStandard method: surface area x average depth x 7.48 gallons per cubic foot, with average depth found from shallow- and deep-end readings.
- NIST Handbook 44 — Appendix C, General Tables of Units of MeasurementNational Institute of Standards and Technology (NIST)Basis for the volume conversion constants (231 cubic inches per U.S. gallon; a cubic foot holds about 7.48 gallons).