Pool Volume Calculator (Rectangular)
Calculate gallons in a rectangular swimming pool with sloped or flat bottom.
Enter length, width, and depths to get the exact volume for chemistry math.
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. Pool chemistry is dose-per-volume. If you guess 25,000 gallons but it is actually 18,000, you overdose chlorine by 39%. Over-stabilization is the #1 reason backyard pool owners burn through chlorine in summer — usually traced to a wildly wrong volume estimate at fill time.
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. If you are in a metric country, the calculator gives both — but check your product label, because some imported chemicals are dosed per cubic meter (1 m³ = 1,000 L = 264 gal).
Once and done. You only need to do this calculation once unless you change the pool’s structure. Save the number somewhere — write it on the inside of your equipment shed door if you have one.