Getting a number you can trust
Pool chemistry forgives a volume that's off by a few hundred gallons — every dose scales linearly, so a 3% volume error means a 3% dose error. What it doesn't forgive is guessing. A homeowner who "thinks it's about 20,000" when it's 15,000 overdoses every chemical by a third, every time. Measure the pool once, save the number, and stop guessing.
How many gallons is my pool? The formulas
Every pool volume calculation is the same three steps: surface area, times average depth, times a conversion constant. The only thing that changes with shape is the area formula:
| Shape | Surface area | Then |
|---|---|---|
| Rectangle | length × width | × average depth × 7.48 = gallons |
| Round | diameter² × 0.785 | |
| Oval | length × width × 0.785 | |
| Kidney | (width A + width B) × length × 0.45 |
These are the same formulas pool builders and chemical labels use, so your result will match what a service tech would quote you. The calculator above just does the arithmetic — open show the math on any result to see each step.
One cubic foot of water holds 7.48 US gallons, so every US-unit formula here is just surface area × average depth × 7.48. Metric is tidier: one cubic meter is exactly 1,000 liters.
Freeform and odd-shaped pools
A true freeform pool doesn't have a formula — but it doesn't need one. Two approaches that get you within a few percent:
Break it into pieces. Nearly every freeform is a rectangle plus a circle-ish bump, or two overlapping ovals. Compute each section above, then add the results. An L-shaped pool is exactly two rectangles.
Or use the 0.85 shortcut. Measure the longest length and widest width as if the pool were a rectangle, then multiply the rectangular result by 0.85. Curved walls almost always trim 10–20% off the bounding rectangle, and 0.85 is the industry's standard middle estimate for lagoon-style shapes.
(shallow + deep) ÷ 2 assumes the floor slopes evenly from end to end. Most inground pools slope gently through the shallow half and drop faster into a hopper at the drain — which means the true average sits a little below the midpoint. If your deep end is a hopper, measure the deep depth at the start of the slope rather than the drain, or simply round your result down. For an above-ground pool, use the water depth, not the wall height — the water line sits about six inches below the top rail.
Common pool sizes, for a sanity check
Your result should land in the same neighborhood as the matching row. All values computed with the formulas on this page.
| Pool | Depth used | Volume |
|---|---|---|
| 15 ft round above-ground (48″ wall) | 3.5 ft water | ≈ 4,600 gal |
| 18 ft round above-ground (48″ wall) | 3.5 ft water | ≈ 6,700 gal |
| 24 ft round above-ground (52″ wall) | 4 ft water | ≈ 13,500 gal |
| 14 × 28 ft inground rectangle | 5.5 ft average | ≈ 16,100 gal |
| 16 × 32 ft inground rectangle | 5.5 ft average | ≈ 21,100 gal |
| 18 × 36 ft inground rectangle | 5.5 ft average | ≈ 26,700 gal |
| 20 × 40 ft inground rectangle | 6 ft average | ≈ 35,900 gal |
Above-ground volumes assume the water level rides ~6 inches below the wall height.
When you need the number exact
Two situations justify more precision than tape-measure math: sizing a heater (an undersized unit runs forever) and calculating a drain-and-refill for high CYA or calcium. The gold standard is your water meter: note the reading, fill the pool from empty (or refill a known fraction after a drain), and read it again — most US meters count in cubic feet, so multiply by 7.48. If the pool is already full and you have no fill records, a documented dose works too: add a precisely weighed amount of a chemical you can test sharply (CYA is the classic choice), measure the actual ppm rise, and back out the volume — a 10 ppm rise from a dose sized for 12,000 gallons means the pool is bigger than you thought, in exact proportion.
However you land on the number, hit Save as my pool above — the water test dashboard and every calculator here will use it automatically from then on.