How to Calculate Pool Volume: Gallons, Ovals and Sloped Floors

2026-10-15

To calculate pool volume, find the area of the water surface, multiply it by the average water depth, and convert the result into gallons. For a rectangle, that means length × width × average depth × 7.48 gallons per cubic foot (many guides round this to 7.5). A 32 × 16 ft pool that runs from 3 ft at the shallow end to 7.5 ft at the deep end has an average depth of 5.25 ft. The common 7.5 rule of thumb puts it at about 20,160 gallons. The exact gallon gives a figure about a quarter of a percent lower.

That's the whole method, and the US public-health manuals written for pool operators teach it the same way. People still end up with the wrong number for two reasons. They measure the wrong depth, or they pick the wrong formula for an oval. Both mistakes then carry straight into every chemical dose.

How to calculate pool volume in three steps

• Surface area. Rectangle: length × width. Round: π/4 × diameter², or 3.14 × radius². Oval: it depends which oval, see below.
• Average depth. Add the shallow-end and deep-end water depths and divide by two. A pool that goes from 2 ft to 10 ft averages 6 ft.
• Conversion. Area in square feet × depth in feet gives cubic feet. One cubic foot holds 7.48 US gallons. In metric, square metres × metres gives cubic metres, and 1 m³ is 1,000 litres.

Averaging the two ends only works when the floor slopes evenly between them. Many in-ground pools have a flat shallow area, a steep drop and a flat deep area. For those, the Oregon operator manual says to split the pool into sections of constant slope, work each one out and add them up. The same trick handles kidney, L-shaped and freeform pools: split them into rectangles and circles.

Measure the water, not the wall

Depth is the input people get wrong most often, and it has the biggest effect on the answer. Volume scales directly with depth. A 40 × 20 ft pool holds about 24,000 gallons at an average depth of 4 ft and about 30,000 gallons at 5 ft. One foot of error here is 6,000 gallons.

Above-ground pools are where this goes wrong most. They're sold by wall height ("18 x 48", "24 x 52"), but nobody fills them to the rim. The City of Akron's pool chart, for example, lists a 24 ft round pool with a 52-inch wall at about 13,594 gallons. That figure is worked out for 4 ft of water, not 52 inches. Manufacturers do the same thing. A retailer listing for an Intex 6.10 × 3.05 × 1.22 m frame oval gives its capacity as 18,202 litres "at 90% full."

So put a tape measure in the water and measure from the water line to the floor. If you work from the wall height on the box, you overstate the volume. Then every dose you work out from it will be too high by the same proportion.

Oval pools: why two calculators disagree

Search "how many gallons is my oval pool" and you'll find two camps. Some guides treat an oval as an ellipse and use area = π/4 × length × width, which works out to about 0.785 × L × W. In gallons that becomes the "× 5.9" multiplier. Others use a fixed factor of about 0.89 × L × W, or "× 6.7" in gallons. The two answers are 12–14% apart. Most guides never explain why.

They're describing different shapes. A true ellipse curves all the way round. Most above-ground and frame "ovals" have straight long sides and two semicircular ends. Geometry calls this shape a stadium, and its area is a rectangle plus one full circle whose diameter is the pool's width. The 0.89 factor isn't arbitrary. It's exactly what that shape gives when the pool is twice as long as it is wide. At other proportions it drifts. A pool 1.5 times as long as it is wide comes out at about 0.857 × L × W, and one three times as long at about 0.928.

Here's what that looks like with real numbers. A 24 × 12 ft pool:

• treated as an ellipse at 4 ft of water: about 6,797 gallons (Akron's chart);
• treated as a straight-sided oval at 4.33 ft: about 8,355 gallons (a pool-supply worksheet using × 6.7).

Look at your pool from above before you choose. If the long sides are straight, it's the straight-sided oval. If there's no straight edge anywhere, it's an ellipse.

Rules of thumb, and which gallon

The shortcut constants in pool guides (7.5 instead of 7.48, 3.14 instead of π, 5.9 for round pools) are rounded versions of exact geometry. Each one is off by well under one percent, which doesn't matter for dosing. The oval factor and the depth matter far more.

The gallon itself is another matter. A US gallon is 3.785 litres. A UK (imperial) gallon is about 4.546 litres. That same Intex frame pool is listed by a UK retailer as 4,003 gallons, and those are imperial gallons. In US gallons the pool would show about 17% more. If a box or listing comes from outside the US, compare litres. Chemical labels sold in the US that say "per 10,000 gallons" mean US gallons.

From pool volume to chemical dose

The main reason to know your volume is that every product label is written for some other volume. The Indiana State Department of Health gives operators a two-part formula:

• Dose = label amount × (your pool volume ÷ the volume on the label).
• If the label states the change it produces (for example "raises 10 ppm"), also multiply by (the change you need ÷ the change on the label).

The department's own worked examples show how it plays out. A label says 16 oz per 10,000 gallons, and the pool holds 95,000 gallons: the dose is 152 oz. Another label says 20 oz raises a reading by 10 ppm in 10,000 gallons. In a 60,000-gallon pool that needs a 9 ppm change, the dose is 108 oz. The Oregon manual works the same formula with 1.5 lb per 10 ppm per 10,000 gallons, a 14,000-gallon pool and a 20 ppm change, and gets 4.2 lb.

Notice what the formula doesn't contain: any opinion about what your chlorine or pH should be. The target depends on your pool, your product and your local rules. Product strengths vary a lot, and Indiana's own guide notes that calcium hypochlorite ranges from 47% to 78%. The formula only scales the manufacturer's instruction to your water. It's also linear, so a volume that's 20% too high gives a dose that's 20% too high. The manuals' advice applies: follow the label, add large doses in portions, let the water circulate and retest.

What to put in the calculator

The pool volume calculator does these steps with exact constants. It needs:

• the shape seen from above: rectangular, round, oval (ellipse) or oval with straight sides;
• length and width at the longest and widest points, or the diameter;
• the water depth at the shallow and deep ends (the same number twice for a flat floor);
• optionally, the dose from your product label, the volume it's for, and the ppm or pH change, if the label gives one.

It returns gallons and litres (or cubic metres in metric mode), the average depth and the surface area. It also returns the label dose scaled to your pool, in whatever unit the label uses. If you keep fish rather than a pool, the aquarium calculator works out a rectangular tank's volume from its length, width and height in centimetres.