How to use it

Pick the pool's shape and enter its length and width (or diameter for a round pool) in feet. Enter the water depth at the shallow end and the deep end. For an above-ground pool or any pool with a flat bottom, enter the same depth in both boxes.

Measure the water depth, not the wall height. Most pools are filled to about the middle of the skimmer opening, a few inches below the top.

How it's calculated

Surface area:
- Rectangle: length × width
- Round: 3.1416 × (diameter ÷ 2)²
- Oval: 3.1416 × (length ÷ 2) × (width ÷ 2)
Average depth = (shallow depth + deep depth) ÷ 2.
Volume = surface area × average depth, in cubic feet.
Gallons = cubic feet × 7.48.

Pool	Depth	About
15 ft round above-ground	4 ft	5,288 gallons
24 ft round above-ground	4.5 ft	15,228 gallons
16 × 32 ft in-ground	3.5 to 6 ft	18,193 gallons
20 × 40 ft in-ground	3.5 to 8 ft	34,410 gallons

Why the number matters

Chemicals: chlorine, shock, pH adjusters and algaecide are all dosed per gallon, usually per 10,000 gallons. An accurate volume stops you under- or over-treating.
Filling: a garden hose delivers roughly 5 to 10 gallons a minute, so a 15,000-gallon pool takes a day or more to fill. Many towns sell bulk water by truck.
Equipment: pumps and filters are sized to turn over the pool's full volume in a set number of hours.

Tips

Pools with a sloped "hopper" deep end hold a little less than the average-depth method suggests, so the result errs high. That's the safe side for most chemical dosing, but follow the product label.
Freeform pools: split the shape into rectangles and circles with the square footage calculator, then multiply the area by the average depth and 7.48.