Pool Volume Calculator (Rectangular)

The math. Volume in cubic feet equals length × width × average depth. Multiply by 7.48 (gallons per cubic foot) to get gallons.

Average depth, not max depth. This is where most online calculators get it wrong. If your shallow end is 3 feet and your deep end is 8 feet, you do not use 8. You average them: (3 + 8) / 2 = 5.5 feet. The constant-slope assumption holds for most rectangular pools.

When the bottom is not a constant slope. If your pool has a “hopper” bottom (deep end with a steep drop), or a separate diving well, split it into two volumes and add them. A 30 × 15 pool with a 15-foot shallow section at 3 feet and a 15-foot deep hopper averaging 6 feet is two slabs, not one.

Why the exact gallons matter. Every pool chemical is dosed per volume, so this one number sits underneath every other calculation you will ever do for the pool. Guess 25,000 gallons when it is really 18,000 and every dose lands 39% high.

The errors do not cancel out, either. They compound in one direction depending on which way you guessed. Over-estimate and you overdose everything, which is merely wasteful for chlorine and permanent for stabilizer, since cyanuric acid can only be removed by draining. Under-estimate and you chronically under-chlorinate, the free chlorine sits below where it should, and one warm week later you have algae and cannot work out why.

Measure it once, properly, and write the number down.

Worked example. A 32 × 16 pool, 3 feet at the shallow end and 7 feet at the deep end:

• Average depth = (3 + 7) / 2 = 5 feet
• Cubic feet = 32 × 16 × 5 = 2,560
• Gallons = 2,560 × 7.48 = 19,150

Round to 19,000 gallons for chemistry calcs. Round to the nearest 5,000 for general talk.

Liters versus gallons. Most pool products in the US are dosed in gallons. The calculator gives litres and cubic metres too, but check your product label: some are dosed per cubic metre, and 1 m³ = 1,000 L = 264 gallons.

Once and done. You only need to do this calculation once unless you change the pool’s structure. Save the number somewhere permanent. The inside of the equipment shed door is a good spot, because that is where you will be standing when you need it.