In this guide: The master formula · Average depth · By shape · Freeform pools · Calculators
Nearly everything in pool care — chlorine, shock, salt, heater sizing, fill cost — is dosed by gallons. Get the volume right and every downstream number is right. The good news: it's the same idea for every shape.
Volume is just surface area × average depth, converted to gallons. Because 1 cubic foot holds about 7.48 gallons, the full formula (with all dimensions in feet) is:
Gallons = Surface Area (ft²) × Average Depth (ft) × 7.48
So the whole job is (1) find the surface area for your shape and (2) find the true average depth. If a spec sheet already gives you the surface area, you can skip straight to step 2 — every calculator on this site accepts a known surface area as an input.
A flat-bottom pool's average depth is just the depth. But a pool with a shallow and deep end is not simply the two added and halved unless the floor slopes evenly. For a constant slope, average = (shallow + deep) ÷ 2. For a hopper bottom (flat shallow shelf, a slope, then a flat deep section), you weight each zone by how much of the floor it covers — using the flat-average can overstate volume by a fifth or more. Our shape calculators handle flat, sloped, and hopper floors for you.
Rectangle: length × width. Round: π × radius². Oval: π × ½length × ½width (an ellipse). Triangle: ½ × base × height. Kidney: the industry two-width method — (width A + width B) × length × 0.45. Roman: a rectangle plus two semicircular ends. Grecian: a rectangle with the corners cut. Each has a dedicated calculator with a diagram and worked example, and the full equations live on our formula reference.
For an irregular or lagoon pool, use the method the pros use: break it into simple shapes — rectangles, circles, ellipses — measure each, and add the areas. An L-shaped pool is just two rectangles. The freeform calculator lets you combine sections; the L-shaped calculator adds two rectangles automatically.
These calculators turn the steps above into your exact numbers:
This guide is built on published health codes, national standards, and government energy guidance. For the exact equations behind the linked calculators, see our formula reference.
This guide is general information, not a substitute for your local health code or a licensed professional. Always follow the dosing instructions on your chemical labels and confirm readings with a test kit.