How this instrument works
A round above-ground pool is, geometrically, a simple cylinder standing on its side — a circular base of a given diameter, extended straight up (or in this case, down) by its depth. Its volume follows the standard cylinder formula: pi times the radius squared, times the height, where the radius is simply half the diameter and the 'height' here is the pool's average depth.
This instrument computes that cylinder volume in cubic feet, then multiplies by 7.480519 — the precise number of US gallons in one cubic foot — to return the pool's true capacity in gallons, the unit pool chemicals, fill times, and pump ratings are actually specified in. Because round above-ground pools typically have a flat or nearly flat bottom, average depth for this shape is usually close to the pool's simple measured water depth, unlike an in-ground pool with a sloped bottom between a shallow and deep end.
Wall height printed on a pool's packaging isn't quite the same as water depth — pools are normally filled to a level somewhat below the very top of the wall, both to leave splash-out margin and because most skimmers and returns are positioned a bit below the rim. Measuring the actual water level, or using the pool manufacturer's stated capacity depth rather than its full wall height, gives a more accurate figure for this instrument's Average depth field.
- Enter Pool diameter (ft) — the pool's inside diameter.
- Enter Average depth (ft) — ideally the actual water depth, which sits somewhat below the pool wall's full height.
- Read Volume (cu ft) and Volume (US gallons) beneath the inputs.
Worked example — a common 24 ft above-ground pool
Enter 24 into Pool diameter (ft) and 4.5 into Average depth (ft) — a common size for a large round above-ground pool. Radius is 24 ÷ 2 = 12 ft, so the base area is π × 12² ≈ 452.3893 ft², and volume is that area times the 4.5 ft depth: about 2,035.75 ft³.
Volume (US gallons) reads 15,228.5 — 2,035.75 cubic feet multiplied by 7.480519, the exact gallons-per-cubic-foot conversion. That's a realistic capacity figure for this common pool size, useful as a starting point for a chlorine dosing chart or estimating how long a garden hose will take to fill it.
Questions
Why does this calculator ask for diameter instead of length and width?
Because a round pool is a circle, and a circle needs only one dimension — its diameter — to fully describe its size, unlike a rectangle which needs both a length and a width. The diameter is then halved internally to get the radius used in the standard cylinder-volume formula.
What counts as 'average depth' for an above-ground round pool?
Usually just the actual water depth, since most round above-ground pools have a flat or nearly flat bottom rather than a sloped shallow-to-deep-end profile like many in-ground pools. Measure from the water's surface down to the pool floor, or use the depth figure the pool's manufacturer lists for its stated capacity, rather than the full height of the pool wall.
Does wall height equal water depth?
Not quite — pools are normally filled to a level somewhat below the top of the wall, both to leave a margin for splash-out and because skimmer and return fittings typically sit a bit below the rim. Using the full printed wall height as your depth figure will slightly overstate the pool's actual filled volume; the manufacturer's stated capacity depth, or a direct water-level measurement, gives a more accurate number.
How is this different from the site's general Pool calculator?
This instrument is scoped specifically to round pools only, with diameter and average depth as its two inputs — a simpler, single-purpose version of the math. The site's general Pool calculator handles both rectangular and round pools behind one shape selector, useful if you're not sure yet which shape calculation you need or want both options in one place.
Why 7.480519 instead of the more commonly seen 7.48052?
Both figures represent the same exact US gallons-per-cubic-foot conversion, which follows from the gallon's legal definition as exactly 231 cubic inches — they differ only in how many decimal places are carried, a rounding choice rather than two different constants. Either figure gives essentially identical results for any real pool measurement.