Above Ground Pool Calculator
An above ground pool calculator finds gallons as π × radius² × water depth × 7.48. An 18 ft round pool filled 4 ft deep holds about 7,614 gallons.
How many gallons is my above ground pool?
This above ground pool calculator treats a round pool as a cylinder: gallons = π × (diameter in inches ÷ 24)² × water depth in feet × 7.48. Dividing the diameter in inches by 24 turns it into the radius in feet in one step. An 18 ft (216 in) pool filled 4 ft deep holds about 7,614 gallons.
Pool chemicals are dosed per 10,000 gallons. Pumps and filters are rated by the largest pool they can turn over. The first fill comes off your water bill by the same number, so the calculator also multiplies gallons by a price per gallon.
The formula in plain words
A round pool is a short, wide cylinder of water. Its volume is the area of the circle on the ground times the height of the water.
- Radius in feet = diameter in inches ÷ 24 (÷ 2 for radius, ÷ 12 for feet).
- Cubic feet = π × radius² × water depth in feet.
- Gallons = cubic feet × 7.48 (one cubic foot holds about 7.48 US gallons).
- Fill cost = gallons × your water price per gallon.
The calculator asks for the diameter in inches and the depth in feet. Enter 216 for an 18 ft pool, not 18.
Worked examples
- 18 ft round pool filled 4 ft deep (calculator default)
- Radius: 216 ÷ 24 = 9 ft
- Cubic feet: π × 9² × 4 = 1,017.88 ft³
- Gallons: 1,017.88 × 7.48 = 7,613.71 gallons
- At $0.01 per gallon: $76.14 to fill
- 24 ft round pool with a 54 in wall, filled to 4.5 ft
- Diameter: 24 × 12 = 288 in, so radius = 288 ÷ 24 = 12 ft
- Cubic feet: π × 12² × 4.5 = 2,035.75 ft³
- Gallons: 2,035.75 × 7.48 = 15,227.43 gallons
- At $0.01 per gallon: $152.27
Going from 18 ft to 24 ft looks like a small step, but the radius is squared. The bigger pool’s water surface is 1.78 times larger, and the extra half foot of depth pushes the total to double the smaller pool.
Gallons for common above-ground pool sizes
| Pool diameter | Diameter to enter | Water depth | Gallons |
|---|---|---|---|
| 12 ft | 144 in | 2.5 ft | 2,115 |
| 15 ft | 180 in | 4 ft | 5,287 |
| 18 ft | 216 in | 4 ft | 7,614 |
| 20 ft | 240 in | 4 ft | 9,400 |
| 24 ft | 288 in | 4 ft | 13,535 |
| 24 ft | 288 in | 4.5 ft | 15,227 |
| 30 ft | 360 in | 4.5 ft | 23,793 |
Pool kits are sold by nominal diameter, and the actual inside diameter can be a few inches smaller once the top rails and liner are in. The difference is small next to the uncertainty in water depth, which is the measurement that matters most.
How to measure an above-ground pool
- Diameter: stretch a tape across the inside of the wall, passing through the center. Take two readings at right angles; if they differ, the pool is out of round, so average them.
- Water depth: measure from the floor to the waterline, not the wall height. Most pools are filled to the middle of the skimmer mouth.
- Sloped or dished floors: some kits have a cove at the wall or a dug-out center. Measure depth at a few points and use the average.
- Oval pools: the cylinder formula does not fit. Treat it as length × width × depth × 7.48 and take about 89% of the result.
Common mistakes
- Typing the diameter in feet. The calculator reads that field in inches. Entering 18 instead of 216 gives an answer 144 times too small.
- Using the wall height as the water depth. A 52 in wall usually holds about 46 in of water. Using the wall height overstates gallons, and every chemical dose built on it will be too strong.
- Dosing from the box label. “18 ft pool” on the box says nothing about depth. Two 18 ft pools with 48 in and 54 in walls hold different volumes.
- Forgetting the top-up. Splash-out, backwashing, and evaporation take water out all season. The first-fill cost is only the start.
What to do with the gallon figure
Once you have gallons, dose sanitizer with the pool chlorine calculator. If you run a salt system, the pool salt calculator uses the same number to work out bags of salt. To check that your pump can turn the water over in about 8 hours, see the pool pump size calculator. For in-ground or rectangular pools, the pool gallons calculator handles length × width × average depth instead of a circle.
Frequently asked questions
How many gallons does an above-ground pool hold?
Gallons = π × (diameter in inches ÷ 24)² × water depth in feet × 7.48. A 15 ft round pool filled 4 ft deep holds about 5,287 gallons, an 18 ft pool about 7,614 gallons, and a 24 ft pool filled 4.5 ft deep about 15,227 gallons.
Why are almost all above-ground pools round rather than rectangular?
A round wall carries water pressure evenly as hoop tension, so thin steel, resin, or aluminum wall panels hold the water without extra bracing. Oval and rectangular kits need buttresses or straps to resist the flat sides bowing out, which is why they cost more for the same footprint.
My pool is oval, not round — can I still use this calculator?
Not directly, because the formula here is for a cylinder. For an oval, multiply length × width × water depth in feet × 7.48, then multiply by about 0.89 to account for the rounded ends. A 15 × 30 ft oval at 4 ft comes to roughly 11,980 gallons.
Should I measure the pool wall's height or my actual water level?
Use the actual water depth. Most above-ground pools are filled to about the middle of the skimmer opening, several inches below the top rail. On a 52 in wall filled to 46 in, enter 3.83 ft, not 4.33 ft, or the gallon count will run about 13% high.
What size pump and filter do I need based on this gallon count?
Divide the gallons by the hours you want for one full turnover, usually 8. A 7,614-gallon pool needs about 950 gallons per hour (about 16 GPM) to turn over in 8 hours. Match that to the maximum pool size listed on the pump and filter, not to the pool diameter.
Estimates only. Verify quantities with your supplier before purchasing.