Free pool volume calculator
Find your swimming pool's water capacity in US gallons, liters, and cubic feet — pick rectangular, round, or oval, enter your dimensions and depth, and the gallon count you need for dosing chemicals and sizing a pump is updated live, as you type.
On this page15 sections
Estimates only, based on the dimensions you enter. Measure water depth, not wall height.
Results are estimates. Consult a professional.
How the pool volume calculator works
A pool holds a fixed amount of water, and that number drives almost every maintenance decision you make. The calculator finds it in two steps: it works out the water-surface area from your pool's shape, multiplies that area by the average depth to get cubic feet, then converts cubic feet into US gallons and liters. Tell it the shape, the dimensions and whether the floor is flat or sloped, and it returns the capacity as you type.
What goes into a pool volume estimate
Three inputs decide the gallon count: the shape, the surface dimensions and the depth. Get all three right and the number is exact geometry. Miss one — most often the depth — and the estimate drifts enough to throw off your chemical doses.
Shape — sets the area formula
A rectangle uses length times width. A round pool uses π times the radius squared, where the radius is half the diameter. An oval uses π times half-length times half-width, which is the same as fitting an ellipse inside its bounding rectangle. The calculator picks the right formula once you choose the shape, so you never multiply by the wrong constant.
Average depth — the lever that moves the total
Depth is the input most people get wrong. Measure the water depth, not the wall height — pools are rarely filled to the rim, and a few inches of freeboard at the top removes hundreds of gallons. If the floor slopes from a shallow end to a deep end, the calculator averages the two for you with (shallow + deep) ÷ 2.
Surface area — the bridge to volume
Surface area is the footprint of water at the top of the pool. Multiplying it by the average depth turns a flat measurement into a three-dimensional volume. The same footprint holds far fewer gallons as an oval than as a rectangle, because the curved ends trim roughly 21% of the area.
A worked example using the pool volume calculator
Maria has a backyard rectangular pool, 32 ft long and 16 ft wide, with a floor that slopes from a 3-foot shallow end to an 8-foot deep end. She needs the gallon count to dose her chlorine and size a new pump.
Step 1 — Find the average depth
The floor slopes, so average the two ends: (3 + 8) ÷ 2 = 5.5 ft. That single number stands in for the whole sloped floor.
Step 2 — Find the surface area
A rectangle is length × width: 32 × 16 = 512 ft² of water surface.
Step 3 — Multiply to get cubic feet
Area times average depth: 512 × 5.5 = 2,816 ft³ of water.
Step 4 — Convert to gallons and liters
2,816 × 7.480519 = 21,065 gallons; the same volume is about 79,740 liters. A rounded 7.5 shortcut would read 21,120 — this calculator does not round, so the figure stays exact.
How many gallons is my pool?
If you want a ballpark before measuring, this table gives the capacity of common rectangular pool sizes at a flat 5-foot depth. For a sloped floor, find your average depth first, then scale: a pool half as deep holds half the gallons.
| Pool size (5 ft deep) | Cubic feet | US gallons | Liters |
|---|---|---|---|
| 12 ft × 24 ft | 1,440 | 10,772 | 40,776 |
| 16 ft × 32 ft | 2,560 | 19,150 | 72,491 |
| 16 ft × 40 ft | 3,200 | 23,938 | 90,614 |
| 18 ft × 36 ft | 3,240 | 24,237 | 91,747 |
| 20 ft × 40 ft | 4,000 | 29,922 | 113,267 |
Rectangular pools at a constant 5 ft water depth, using 7.480519 gallons per cubic foot. A round or oval pool of the same footprint holds less, because its area is smaller.
Round and oval pools: measuring and formulas
Above-ground pools are usually round or oval, and they trip people up because the area formula is not length times width. Here is what to measure and what the calculator does with it.
Round pools
Measure the diameter — the distance straight across the middle, widest point to widest point. The calculator halves it to get the radius, then uses π × radius² for the area. A 24-foot round pool at 4 feet deep holds about 13,536 gallons; an 18-foot one holds about 7,614.
Oval pools
Measure the full length and the full width at their widest points. An oval's area is π × (length/2) × (width/2), which works out to about 79% of the rectangle that would enclose it. A 15-by-30-foot oval at 4 feet deep holds roughly 10,575 gallons.
| Above-ground pool | Depth | US gallons | Liters |
|---|---|---|---|
| 15 ft round | 4 ft | 5,288 | 20,016 |
| 24 ft round | 4 ft | 13,536 | 51,241 |
| 27 ft round | 4 ft | 17,132 | 64,852 |
| 15 ft × 30 ft oval | 4 ft | 10,575 | 40,032 |
| 18 ft × 33 ft oval | 4 ft | 13,959 | 52,842 |
Round pools use π × radius²; ovals use π × (length/2) × (width/2). Figures assume a constant 4 ft water depth.
Swim University's pool-volume guide uses the same shape formulas and the (shallow + deep) ÷ 2 average-depth rule.Sloped floors and average depth
Most in-ground pools are not one depth. They step or slope from a shallow end where you stand to a deep end where you dive. For volume, only the average depth matters — and the simple average of the two ends is the standard way to find it.
The (shallow + deep) ÷ 2 rule
Add the shallow-end depth to the deep-end depth and divide by two. A pool that runs from 3 feet to 9 feet has a 6-foot average depth, and the calculator multiplies the whole surface by that single figure. This works because a flat slope removes exactly as much water near the shallow end as it adds near the deep end.
When the simple average runs slightly high
The (shallow + deep) ÷ 2 rule assumes the floor slopes in one straight line for the full length. Real pools often have a long flat shallow shelf, then a short steep drop into a deep well — a hopper bottom. When most of the floor sits at the shallow depth, the true average is lower than the two-end mean, so the simple rule reads a little high. For dosing chemicals that small overage is safe; for a tight equipment spec, split the pool into its flat and sloped sections and add them.
Why your pool's gallons matter
The gallon count is not trivia — it is the denominator for nearly every pool task. Pool-chemical labels and equipment specs are written per gallon or per thousand gallons, so an estimate that is off by 10% throws off everything downstream by 10%.
- Chemical dosing — chlorine, shock, algaecide, pH adjusters and stabilizer are all dosed per 10,000 gallons. Manufacturers such as Pentair and Hayward print their dose charts this way, so the gallon count sets every scoop.
- Pump and filter sizing — equipment is sized to turn the whole volume over in roughly 8 hours, so total gallons divided by turnover hours gives the flow rate you need.
- Heater sizing — the BTUs needed to raise water temperature scale directly with gallons; an undersized heater on an oversized volume never reaches setpoint.
- Salt and stabilizer — a saltwater system needs a target parts-per-million across the full volume, which only the gallon count can give you.
For pure capacity in other units, the cubic feet calculator handles any box, cylinder or sphere, and the rainwater collection calculator uses the same 7.48 gallons-per-cubic-foot factor for roof runoff.
Pentair and Hayward publish their homeowner chemical-dose and equipment-sizing charts per gallon (or per 10,000 gallons), which is why the volume figure drives the rest of pool care.How to measure your pool for an accurate count
Good measurements are the whole game; the math behind them is exact. Five habits keep your gallon count honest.
- Measure at the waterline — record length, width or diameter at the surface, where the water sits, not at the coping or the deck edge.
- Use water depth, not wall height — drop a weighted tape to the floor and read where the water surface crosses it.
- Take two depths on a slope — the shallowest and the deepest point, and let the calculator average them.
- Measure round pools across the center — the true diameter passes through the middle; a chord measured off-center reads short.
- Measure ovals at the widest points — full length down the long axis, full width across the short axis.
For odd shapes — kidney, L-shaped, freeform — split the pool into rectangles and circles you can measure, run each through the calculator, and add the gallons. It is more reliable than guessing one "average" length and width for a curved outline.
Pool volume definitions
How accurate is this pool volume calculator?
The geometry is exact. Surface area times average depth is the precise volume of your pool, and the gallon and liter conversions use the exact 7.480519 and 28.3168 factors rather than the rounded 7.5 most guides quote. If your measurements are right, the gallon count is right to within the precision you measured.
Two things limit real-world accuracy, and both are about the pool, not the math. First, the simple (shallow + deep) ÷ 2 average assumes a straight slope; a hopper-bottom pool with a long flat shelf holds slightly less than the two-end mean suggests. Second, fill level changes the volume — every inch of water below the rim trims gallons, so measure the depth the water reaches at the waterline. Treat the result as a precise figure for the dimensions you enter, re-measure the depth at the real waterline before you dose chemicals, and split complex shapes into simple sections for the closest count.
Frequently asked questions about the free pool volume calculator
About this pool volume calculator
This pool volume calculator runs entirely in your browser — nothing you enter is sent anywhere. It converts your pool's shape, dimensions, and water depth into US gallons, liters, and cubic feet using the exact 7.480519 gallons-per-cubic-foot factor, so the figure stays precise instead of drifting on a rounded multiplier.
It's part of our home & garden calculators collection. Browse the full set of free calculators for more measuring, material, and capacity tools.