Volume in cubic feet, then times 7.48 for gallons
Length, width, and average depth in feet give cubic feet; one cubic foot holds 7.48 US gallons. Most calculators use 7.5 as a working shortcut, which is under 0.3 percent off. The number that matters for chemistry is average depth, not the deep end.
The multiplier depends on the shape
Round and oval pools are not just length times width. Use the shape factor below, then multiply the cubic feet by 7.48 for gallons.
| Shape | Cubic feet |
|---|---|
| Rectangular | L × W × avg depth |
| Round | π × radius² × avg depth |
| Oval | L × W × avg depth × 0.89 |
| Kidney | 0.45 × (width A + width B) × L × avg depth |
Treating an oval like a rectangle skips the 0.89 factor and overstates its volume by about 11 percent, which then leads to overdosing chemicals.
Common pool sizes
| Pool | Dimensions | Gallons |
|---|---|---|
| Small rectangle | 15 × 10, avg 3.5 ft | ~3,900 |
| Medium rectangle | 25 × 15, avg 5 ft | ~14,000 |
| Large rectangle | 40 × 20, avg 6 ft | ~36,000 |
| Round above-ground | 18 ft across, 4 ft deep | ~7,600 |
| Oval | 30 × 18, avg 5 ft | ~18,000 |
Measure to the actual water line, not the top of the wall. A pool filled a foot below the coping holds noticeably less than its full rating.
Common questions
How do I calculate pool volume in gallons?
For a rectangular pool, multiply length times width times average depth in feet to get cubic feet, then multiply by 7.48. A 30 by 15 pool averaging 5 feet deep is 2,250 cubic feet, or about 16,830 gallons.
What is average depth if my pool has a shallow and deep end?
Add the shallow-end depth and the deep-end depth and divide by two. A pool 3 feet at one end and 8 feet at the other has an average depth of 5.5 feet. Using only the deep end can overstate volume by a third or more.
How many gallons is a typical backyard pool?
Most home in-ground pools hold 15,000 to 20,000 gallons. A small above-ground round pool might be 3,000 to 5,000 gallons, while a large in-ground pool can pass 30,000.
