Rectangle Pool Volume Calculator

Already know your surface area?

If your pool plans or spec sheet already list the surface area, you can enter it directly instead of measuring. Switch to “I know my surface area”, type in the square footage, and set your average depth. For a simple rectangle it’s usually just as quick to enter length and width — but if the area’s on your plans, this saves a step.

The formula

Length × Width × Average Depth × 7.48 = US gallons

Worked example

A 32 ft × 16 ft pool that slopes from 3 ft to 8 ft has an average depth of 5.5 ft. Volume = 32 × 16 × 5.5 × 7.48 ≈ 21,065 gallons. If instead it has a flat 3 ft shallow shelf and a separate deep end, the true average depth is lower — use the Hopper floor option for an accurate figure.

This calculator uses the precise cubic-foot-to-gallon value (about 7.48 US gallons per cubic foot) for your rectangular pool and lets you switch between US gallons, imperial gallons, litres, and cubic metres.

How to calculate rectangular pool volume step by step

You will need these measurements:

Length (L) — the long side, in feet
Width (W) — the short side, in feet
Average depth (D) — add shallow and deep depths and divide by 2

Measure length, width, and average depth. Measure the pool's length and width in feet. For depth, average the shallow and deep ends: (shallow + deep) ÷ 2.
Multiply for cubic feet. Volume in cubic feet = Length × Width × Average Depth.
Convert to gallons. Multiply the cubic feet by 7.48 to get US gallons (1 cubic foot ≈ 7.48 US gallons).

Worked example

A 32 ft × 16 ft pool averaging 5.5 ft deep: 32 × 16 × 5.5 = 2,816 cubic feet, then × 7.48 ≈ 21,065 US gallons.

Common rectangular pool sizes

Approximate capacity at 5 ft average depth, computed with this calculator. Your exact number depends on your real depth.

Pool size	US gallons
12 × 24 ft	10,772
14 × 28 ft	14,662
16 × 32 ft	19,150
18 × 36 ft	24,237
20 × 40 ft	29,922

Measuring a rectangular pool accurately

The single biggest error with a rectangular pool is using one depth instead of the average. Most in-ground rectangles slope from a shallow end to a deep end, so measure both and use (shallow + deep) ÷ 2 — using only the deep depth can overstate your volume by thousands of gallons. Measure length and width at the waterline, not across the coping or decking, and keep your tape square to each wall. If the pool has a constant depth (many lap pools and above-ground rectangles do), the average simply equals that single depth. Because volume scales directly with every dimension, a small mismeasurement compounds: being a foot off on length, width, and average depth together can shift the result by 15–20%.

Sources & references

The method behind this calculator draws on established standards, peer-reviewed academic sources, and reference values:

Standard geometric area formula — area of a rectangle: length × width
U.S. customary unit conversions (NIST reference values) — 1 ft³ = 7.4805 US gallons; water weighs ≈ 8.34 lb per gallon
See the exact working on our formula reference and the reasoning on our methods page.

Results are planning estimates. Where a figure depends on your local rates, codes, or site conditions, confirm with the cited authority or a qualified professional.

Related calculators & guides

Round pool volumeCircular pools & spas Oval pool volumeEllipse footprint Triangle pool volumeBase × height ÷ 2 Guide: Measure volume for any shapeMaster formula & depth

Questions

Common answered

Yes. Switch to “I know my surface area”, enter the square footage, and set your average depth — volume is surface area × average depth × 7.48. For a rectangle, entering length and width is just as easy, but this is handy when your plans already list the area.

A 16 ft × 32 ft rectangular pool holds about 19,150 gallons at 5 ft average depth, or roughly 21,000 gallons at 5.5 ft. Enter your real average depth above for an exact figure — depth is what most changes the answer.

Use the average: add the shallow and deep depths and divide by two. A pool that runs 3 ft to 8 ft has a 5.5 ft average, not 8 ft. Using the deep figure alone significantly overstates the volume.

Measure at the waterline along each wall, keeping the tape square to the wall rather than diagonal. Don't include the coping, deck, or any attached spa — measure only the water surface.

Yes. The deep end is why you average the depths. If the floor steps down in stages rather than sloping evenly, treat each section separately and add the volumes — the freeform calculator can combine them.

Length × width × average depth is exact for a true rectangle with a flat or evenly sloped floor. The only error comes from mismeasurement, so taking careful average-depth and waterline measurements is what matters most.