How Many Gallons in an 18-Foot Round Pool? (2026 Guide)
How many gallons in an 18-foot round pool? About 7,600 at a 4 ft average depth and about 6,700 at 3.5 ft.
The formula that sizes any round pool
Pool gallons = surface area × average depth × 7.48 (gallons per cubic foot): Gallons = π × (diameter ÷ 2)² × average depth × 7.48
An 18-ft pool has a 9-ft radius, so its area is π × 9² ≈ 254.5 sq ft — fixed no matter the fill. Depth drives the total: about 7,600 gallons at a 4 ft average, about 6,700 at 3.5 ft. Sizing another pool? The pool volume calculator takes any diameter and depth.
18-foot round pool gallons by water depth
The wall is fixed; your fill level decides the volume. An 18-ft round pool (≈254.5 sq ft) at common depths:
| Average water depth | Gallons (18-ft round pool) |
|---|---|
| 3.0 ft | 5,710 |
| 3.5 ft | 6,660 |
| 4.0 ft | 7,610 |
| 4.5 ft | 8,570 |
Check the 4.0 ft row: 254.5 × 4 × 7.48 ≈ 7,610 gallons. Each 0.5 ft of depth adds about 950 gallons, so the same pool swings nearly 3,000 gallons with the fill level.
Gallons for common round pool sizes
Figures below are π × (D ÷ 2)² × 4 × 7.48 at a 4 ft average fill:
| Pool diameter | Radius | Surface area | Gallons (≈4 ft deep) |
|---|---|---|---|
| 15 ft | 7.5 ft | ≈177 sq ft | 5,290 |
| 18 ft | 9 ft | ≈254.5 sq ft | 7,610 |
| 21 ft | 10.5 ft | ≈346 sq ft | 10,360 |
| 24 ft | 12 ft | ≈452 sq ft | 13,540 |
Spot-check the 21-ft row: 346 × 4 × 7.48 ≈ 10,360 gallons. Because area grows with the square of the radius, the 24-ft pool holds nearly twice the 18-ft (13,540 vs 7,610) despite being only a third wider.
A 52-inch pool holds less than 52 inches of water
A "52-in" pool has a 4 ft 4 in wall, but you never fill to the rim: the top ring stays dry and the skimmer sits a few inches below. Real water depth usually lands at 46–48 in — roughly a 3.75 ft average. At that depth: 254.5 × 3.75 × 7.48 ≈ 7,150 gallons, so 52-in 18-ft pools are quoted in the 7,000–7,600 range, not the 8,000+ the wall height implies.
Three tips for an honest gallon total
- Dose chemicals by actual gallons. Chlorine, shock and algaecide labels dose per 10,000 gallons. Treat a pool that's really at 3.5 ft (≈6,700 gallons) as a full 7,600 and every dose overshoots by more than 10% — enough to push free chlorine high and irritate eyes.
- Measure water depth at the middle of the pool. The liner curves up near the wall, so read the center on a flat bottom; on a sloped bottom, average the shallow and deep sides. Recheck after a heavy rain tops the water up.
- Estimate before you order water or a pump. At 4 ft the pool needs about 7,600 gallons — know that before water is trucked in. And the filter pump has to circulate that whole volume every few hours, so size it and its electrical load against your real gallon figure.
Running the pump off solar? See how many solar panels do I need to cover the pump and house load. Sizing a rectangular or oval pool instead? The same math is in how many gallons of water in a pool.
FAQ
How many gallons are in an 18-foot round pool?
About 7,000–7,600 gallons, depending on water depth. An 18-ft pool's ≈254.5 sq ft surface holds roughly 5,710 gallons at a 3.0 ft average, 6,660 at 3.5 ft, 7,610 at 4.0 ft and 8,570 at 4.5 ft.
What depth should I use for an accurate pool gallon estimate?
Read the water depth at the middle of the pool, not the edge where the liner curves up. On a flat-bottom pool that center reading is your average; on a sloped bottom, average a shallow-side and a deep-side reading, and recheck after a heavy rain tops it up.
How much chlorine do I add to an 18-foot round pool?
Dose by your actual gallon total, not the pool's advertised size. Chlorine and shock labels dose per 10,000 gallons, so a 7,600-gallon 18-ft pool takes about three-quarters of a 10,000-gallon dose. If your water level is lower, use the real depth-based total from the chart — dosing for 7,600 when you hold 6,700 overshoots by more than 10%.
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