A round above-ground pool that is 15 feet across with a 48 inch wall holds about 4,760 gallons at the 90 % fill line, and about 4,230 gallons if it is an inflatable-ring pool filled to 80 %. Intex prints 3,736 gallons for its 15 by 48 inch Easy Set, so the box figure is lower than the plain geometry. The calculator above starts from those above-ground sizes because that is where most people searching for pool gallons are standing.
How the gallons are calculated
Gallons are the plan area of the water times its depth times 7.4805 gallons per cubic foot. For an above-ground pool the depth is the wall height times the fill line, because nobody fills to the top.
Gallons = area (sq ft) × wall height (ft) × fill line × 7.4805
For the 15 ft round pool with a 48 in wall at 90 %: area is π × 7.5² = 176.7 sq ft, water depth is 4 × 0.9 = 3.6 ft, so 176.7 × 3.6 = 636.2 cu ft, which is 4,759 gal. Every inch of depth in that pool is 110 gallons.
Which fill line
Intex states 80 % on its inflatable-ring Easy Set pages (12 ft by 30 in: 1,485 gal, 15 ft by 42 in: 3,284 gal, 15 ft by 48 in: 3,736 gal, 18 ft by 48 in: 5,455 gal) and 90 % on its Ultra XTR frame rectangles (18 by 9 by 52 in: 4,545 gal, 24 by 12 by 52 in: 8,403 gal). The frame rectangle checks out: 24 × 12 ft × 52 in at 90 % is 8,402 gal by geometry. Round pools come out 6 to 12 % lower on the box than geometry, because the printed diameter is measured at the outside of the wall while the water is narrower. Use the maker's number if your pool is on the list, and the calculator otherwise.
Gallons by size for round pools
The table under the calculator lists 12, 15, 18, 21, 24, 27 and 30 ft rounds at 48 and 52 in walls, at both fill lines. A few rows: a 21 ft pool at 52 in and 90 % holds 10,100 gal, a 24 ft pool at 48 in holds 12,180 gal, a 30 ft pool at 52 in holds 20,620 gal. Rectangles and ovals use the same approach with their own areas: an 18 by 9 ft oval at 48 in is 3,430 gal.
In-ground pools: gallons from dimensions
Switch to In-ground for a fast answer from length, width and the depths at both ends. A 32 by 16 ft rectangle from 3.5 ft to 6.5 ft deep is 19,150 gal. In-ground pools have flat sections, curved ends and spas that change the answer, so for those use the pool volume calculator, which handles L-shapes, kidneys, grecian ends and breaks in the floor.
Above-ground pool gallons questions
How many gallons is a 24 ft round pool? 12,180 at 48 in and 13,200 at 52 in with a 90 % line. How many gallons is a 15 ft pool? 4,760 at 48 in and 5,160 at 52 in. How many litres? Multiply gallons by 3.785.
What to do with your gallons
Your result is saved as My pool on this device once you edit a field. Then go to the pool chemical calculator for dosing, the pool salt calculator if you run a salt system, or the pool filter size calculator to check that the pump on the box matches the water. The pool fill time calculator tells you how long the first fill will take, and the pool heater calculator how much energy to warm it. Every pool volume tool is on the pool volume and water hub.
Small pools and kiddie pools
The same formula works for any round or rectangular tank: a 10 ft by 30 in pool at an 80 % line is about 1,175 gallons by geometry, and a real product of that size will list less. When your pool is far below the sizes in the table, weigh a small error more heavily, because a 10 % error on 1,000 gallons is 100 gallons of chemicals mis-sized.