Rectangular Pool Volume Calculator (Gallons)
Calculate the gallons in a rectangular pool from its length, width and the shallow and deep-end depths — the most common shape for in-ground pools.
Calculator
A rectangular pool 40.0 × 20.0 ft at 5.0 ft average depth holds ≈ 29,920 gallons.
The rectangle is the workhorse pool shape and the easiest to calculate: its footprint is just length times width, with no π involved. Multiply that footprint by the average water depth and by 7.48 gallons per cubic foot and you have the volume. Because a rectangle fills its footprint completely, it holds more water — and needs more salt, chlorine and heat — than a round or oval pool of the same length and width.
Most in-ground rectangles slope from a shallow end to a deep end, so the average of the two depths is what matters. Enter both and the tool averages them for you.
Formula
For a rectangular pool:
gallons = length × width × avg depth × 7.48
with avg depth = (shallow + deep) ÷ 2. Length × width is the surface area in square feet; times the average depth gives cubic feet; times 7.48 converts to US gallons. There is no shape factor to remember — the rectangle is the reference every other shape is scaled from.
Worked example
A 20 ft by 40 ft rectangular pool with a 3 ft shallow end and a 7 ft deep end:
- Average depth = (3 + 7) ÷ 2 = 5 ft.
- Surface area = 20 × 40 = 800 ft².
- Volume = 800 × 5 = 4,000 ft³.
- Gallons = 4,000 × 7.48 = 29,920 gallons.
A smaller 16 × 32 ft rectangle at the same 5 ft average depth holds 16 × 32 × 5 × 7.48 ≈ 19,149 gallons — the classic mid-size in-ground volume you will see quoted most often.
Handling odd bottoms and L-shapes
Pure rectangles are easy, but real pools have shaped bottoms. A couple of adjustments keep the estimate accurate:
- Hopper (spoon) bottoms: the deep end is often a small bowl, not a long slope. Averaging the shallow and deep depths slightly overstates the volume. If you want to be precise, measure the depth at several points across the pool and average those readings before entering them.
- L-shaped or T-shaped pools: split the pool into rectangles, compute each with this tool, and add the gallons together.
- Constant-depth lap or plunge pools: enter the same depth in both fields.
With the gallons known, continue to the salt calculator, the chlorine dose, the pump turnover and the heater size. Not a rectangle? Try the universal volume tool.