Round Pool Volume Calculator

The formula: pi, radius squared, average depth

A round pool is a cylinder, and its volume is the area of the circle times the depth. Written out: volume in cubic feet = 3.1416 x r x r x average depth, where r is half the diameter. One more multiplication converts to gallons, because one cubic foot holds 7.48 gallons. That is the whole formula, and it works for above-ground and in-ground round pools alike.

The one measurement people get wrong is average depth. Most round pools slope from a shallow end to a deep end, so the depth varies across the floor. Measure both ends, then average them: (3 + 8) / 2 = 5.5 ft for a pool that runs 3 ft to 8 ft. Use the average, not the deep end, or you will overstate the water by 30% or more and over-buy every chemical all season.

The 7.48 figure is exact: a cubic foot of water is 7.4805 US gallons. Rounded to 7.48, it is accurate to within a hundredth of a percent, which is far tighter than any tape-measure error.

Worked example: a 24 ft round pool

Take the most common large above-ground size, a 24 ft pool sloping from a 3 ft shallow end to an 8 ft deep end. The steps:

• Radius: 24 / 2 = 12 ft
• Surface area: 3.1416 x (12 x 12) = 452.39 sq ft
• Average depth: (3 + 8) / 2 = 5.5 ft
• Volume: 452.39 x 5.5 = 2,488.14 cubic ft
• Gallons: 2,488.14 x 7.48 = 18,611.3 gallons

That is the brim number. In practice the water line sits below the skimmer mouth, so the working volume is closer to 15,800 to 16,800 gallons, roughly 10% to 15% less. Chemical dosing, salt math, and heating cost all belong to the working number, not the brim number. The calculator shows both, so you can dose with the real one and brag with the other.

Scale the same steps to smaller pools. A 15 ft pool with a 4 ft average depth gives a radius of 7.5 ft, an area of 3.1416 x (7.5 x 7.5) = 176.71 sq ft, and 176.71 x 4 x 7.48 = 5,287.3 gallons. One diameter step, from 15 ft to 24 ft, nearly triples the water, because the radius is squared in the formula.

Common above-ground round pool capacities:

Common round pool sizes in gallons

The table computes the formula for the standard above-ground and in-ground diameters, using typical shallow and deep end depths. Find your diameter, check the depth column, and read the gallons; the calculator above covers any combination in between.

Notice how fast the numbers climb. Going from a 21 ft pool to a 27 ft pool adds 6 ft of diameter, but the water grows from about 12,954 gallons to 23,555 gallons, an 82% increase. Radius squared is why the big pools cost proportionally more to heat and chemically maintain, and why measuring carefully pays off before you order chemicals by the bucket.

Getting average depth right

Average depth is the measurement that makes or breaks this calculation. The simple case is a pool with two distinct levels: measure the shallow end, measure the deep end, and average them. The table below shows the averages for common combinations.

Two wrinkles complicate the simple average. First, many round pools spend more floor area at the shallow level than the slope line suggests, so a straight average runs a little deep; if the flat shallow section is wide, use a depth closer to the shallow reading plus a third of the difference. Second, measure to the water line, not to the top rail. A pool that reads 48 inches on the wall holds 42 inches of water or less, and that 6 inch error moves the gallon count by more than 10%.

The most reliable method needs no geometry at all: check the water meter while filling, or time a known flow rate into an empty pool. A hose meter reading is the ground truth, and it doubles as a leak detector the second time you fill.

Why the gallon number matters: dosing chemicals

Every chemical on the pool shelf doses per gallon. Shock runs about 1 lb of cal-hypo per 10,000 gallons, chlorine stabilizer adds about 1.5 lb per 10,000 gallons to reach 30 ppm, algaecide doses at a quart per 10,000 to 20,000 gallons, and saltwater systems start near 3,200 ppm, which is about 2.7 lb of salt per 100 gallons. Miss the volume by 30% and every one of those doses misses by 30%, in the direction of green water or bleached liners.

A 24 ft pool at 18,611 gallons is a useful test case. Shock: 18,611 / 10,000 = about 1.86 lb. Salt on first fill: 18,611 x 2.7 / 100 = about 502 lb, over ten 50 lb bags. Daily chlorine demand in summer might be 1 to 2 ppm, which is 18,611 x 2 / 1,000,000 x 8.34 = about 0.31 lb of pure chlorine. None of these are guessable from the pool's size in feet; they all flow from the gallon number.

The CDC's healthy swimming guidance stresses keeping free chlorine and pH in range, which requires correct dosing, and correct dosing requires a real volume. The CDC disinfection and testing page covers the levels to aim for; this calculator supplies the denominator those levels multiply against.

Pump, heater, and fill-time math

Volume drives the equipment questions too. A pump sized for a 24 ft pool turns over 18,611 gallons; at the recommended one turnover per 8 hours that is 18,611 / 8 = about 2,326 gallons per hour, which points to a 2,400 to 2,500 gph pump. The same turnover on a 15 ft pool needs only 5,287 / 8 = about 661 gph, so a small pump runs it fine.

Heating cost scales the same way. Raising an 18,611 gallon pool 10 degrees F takes about 18,611 x 8.34 x 10 = 1,552,557 BTU, ignoring losses, so a 100,000 BTU heater runs roughly 15.5 hours at full burn for that rise, and a heat pump takes several times longer. Filling from empty is the other wait: at 10 gallons per minute, 18,611 gallons takes about 31 hours of hose time.

Water replacement rounds out the picture. Evaporation and splash-out in summer run a pool down 1 to 2 inches a week; on a 452 sq ft surface, one inch is (452.39 / 12) x 7.48 = 281.98 gallons. If you are topping up more than that, look for a leak. The Energy Department's pool covers page explains how a cover cuts both evaporation and heating cost.

How to use this calculator

Measure the diameter across the water at the widest point, measure the water depth at the shallow and deep ends with a pole or a marked broom handle, and enter all three. The results show the radius, the average depth, the surface area, the brim-full volume in gallons, and the working volume after typical skimmer loss, which is the number to dose against. Chemical line items, pump turnover, and fill time are shown where they help sanity-check a purchase.

The formula and the 7.48 gallons per cubic foot are exact; the depth measurements are where real error enters, so measure to the water line at both ends. For pools with a long flat shallow shelf, nudge the average depth slightly toward the shallow reading. All calculations run in your browser, and nothing you type is uploaded or stored.