Kidney & Freeform Pool Volume (Estimate)
Estimate the gallons in a kidney or freeform pool from its two widths, its length and the water depth. Curved pools are always an approximation — round up when dosing.
Calculator
A kidney / freeform pool (15.0 + 18.0 ft widths, 30.0 ft long) holds ≈ 13,329 gallons (estimate) — round up when dosing.
Kidney and freeform pools have no exact volume formula — the curves are unique to every pool. The industry-standard approach is to measure the pool at its two characteristic widths and its longest length, then apply a shape factor of about 0.45 to approximate the gallons. It is an estimate, and this calculator labels it as such, but it is close enough to buy salt and chemicals with confidence — as long as you round up rather than down.
As always, the volume depends on the average of the shallow and deep depths, which the tool computes for you.
Formula
The kidney / freeform estimate uses two widths (A and B) and the length:
gallons ≈ 0.45 × (width A + width B) × length × avg depth × 7.48 (estimate)
where avg depth = (shallow + deep) ÷ 2. The 0.45 factor accounts for the curved, pinched shape of a kidney, which covers less area than the two widths and length would suggest for a rectangle. Freeform pools that are closer to an oval will read a little low with this factor — err on the side of rounding up.
Worked example
A kidney pool with widths of 15 ft and 18 ft, a length of 30 ft, a 3 ft shallow end and a 5 ft deep end:
- Average depth = (3 + 5) ÷ 2 = 4 ft.
- Width sum = 15 + 18 = 33 ft.
- Volume ≈ 0.45 × 33 × 30 × 4 = 1,782 ft³.
- Gallons ≈ 1,782 × 7.48 ≈ 13,329 gallons (estimate).
Treat this as roughly 13,000–14,000 gallons for dosing, and lean toward the higher figure when buying salt or chemicals so you are never short.
Why curved pools are only an estimate
There is no way to measure a freeform pool exactly with a tape measure, so every method is an approximation. To get the most from this one:
- Width A and Width B are the two characteristic widths of the kidney — typically the widest lobe and the narrower waist. Measuring both captures the pinch that gives a kidney its shape.
- Length is the longest straight-line span of the pool.
- When precision matters — for example, before draining and refilling to lower cyanuric acid — verify the estimate with a flow meter on the fill hose, or ask your builder for the design volume.
Because the number is approximate, always add about ¾ of a chemical dose, re-test, then adjust. From here, feed the estimate into the salt calculator or the chlorine dose. For a regular shape, use the universal volume tool.