How to Calculate Your Pool’s Volume in Gallons (and Why Guessing Costs You)

Pool chemicals are dosed per volume, so getting the gallon figure wrong means every dose is wrong. Overestimate and you waste chlorine and shock. Underestimate and algae wins. Unlike a rectangular above-ground pool, an in-ground pool often has a varying depth, a sloped floor, or a curved shape that makes the “length × width × depth” shortcut unreliable.

The fix is to break the pool into simple geometric sections and sum them. A volume calculator handles the arithmetic, but you still need the right dimensions, and those are the part people get wrong.

Rectangular Pools: The Baseline

Volume (cubic feet) = length × width × average depth. Average depth is the arithmetic mean of the shallow and deep ends, not the deeper number. A pool 3 ft at the shallow end and 8 ft at the deep end averages 5.5 ft, not 8.

Then convert: 1 cubic foot = 7.48 US gallons.

Example: 30 ft × 15 ft × 5.5 ft = 2,475 cubic feet × 7.48 = 18,513 gallons.

Round and Oval Pools

A 24 ft round pool with an average depth of 4 ft: π × 12² × 4 = 1,810 cubic feet × 7.48 = 13,539 gallons.

Why Average Depth Trips People Up

Most in-ground pools slope from a 3 ft shallow end to an 8 ft deep end, but the slope usually starts a third of the way along the length, not at the midpoint. That means the true average depth is closer to 4.7 ft than 5.5 ft for a typical 32 ft pool. When in doubt, measure the depth at three or four points and average those readings — it takes two minutes and saves a season of misdosing.

A Worked Example

A 32 ft × 16 ft in-ground pool, 3 ft shallow, 8 ft deep. If you naively average to 5.5 ft you get 32 × 16 × 5.5 = 2,816 cu ft = 21,064 gallons. But measuring at four points gives 3, 4.2, 6.5, and 8 ft, averaging 5.43 ft — barely different in this case. The bigger error comes from assuming the deep end is deeper than the water actually is. Always measure water depth, not wall height.

FAQ

Do I need the exact figure? No. Within 5% is fine for dosing, but never guess the deep end by looking at it.

Does a pool’s volume change with a cover or solar blanket? Negligible for dosing purposes.

How do I convert to litres? US gallons × 3.785.

The Bottom Line

Measure water depth at several points, use the right shape formula, and convert with 7.48 gallons per cubic foot. Your chemical doses will finally match the pool you actually own.