Rectangular pools: when average depth works and when to use zones

By Kyle Bell · Reviewed November 9, 2026

For a rectangular pool with vertical sides, water volume equals the inside plan area multiplied by the average water depth. The important question is how that average is obtained. Taking the shallowest and deepest depths and dividing their sum by two is appropriate for a uniform linear slope, but it is not a universal rule for every pool floor.

A uniform slope has a simple average

Imagine an ideal pool that is 32 feet long and 16 feet wide. Its water depth increases evenly along its length from 3 feet to 8 feet. The average depth is (3 + 8) ÷ 2 = 5.5 feet.

The plan area is 32 × 16 = 512 square feet. Multiplying by 5.5 gives 2,816 cubic feet. Using the precise US liquid-gallon conversion gives about 21,065 US gallons, or 79,740 liters.

This is a geometric model: constant width, vertical sides and a depth changing at a constant rate. The Pentair basic training manual explains the area-times-average-depth approach. Checking whether the chosen average represents the actual floor is the next essential step.

A shallow platform changes the weighting

Now consider a different rectangular pool, 40 feet long and 20 feet wide. Its first 20 feet of length are flat at 3 feet deep. The next 10 feet form a uniform transition from 3 to 8 feet. The final 10 feet are flat at 8 feet.

Calculate each nonoverlapping section separately. The shallow section contains 20 × 20 × 3 = 1,200 cubic feet. The sloped section contains 10 × 20 × 5.5 = 1,100 cubic feet. The deep section contains 10 × 20 × 8 = 1,600 cubic feet. Together they hold 3,900 cubic feet, about 29,174 US gallons.

Using only the two endpoint depths would give 40 × 20 × 5.5 = 4,400 cubic feet. That is 500 cubic feet, or about 3,740 US gallons, too high for this model. The shallow region occupies more area than the deep flat region, so the area-weighted average is 3,900 ÷ 800 = 4.875 feet, not 5.5 feet.

Build zones that do not overlap

Sketch the floor from above and label each region's area and depth model. The regions must cover the footprint once. For a rectangular strip of constant width, multiply its length by that width. For a uniform ramp, use its two endpoint depths. For a flat section, use its single water depth.

Add the section volumes before converting units and rounding. If a bench has already been modeled as a shallow-water zone, do not subtract its displacement a second time.

Keep the zone sketch with the result so another person can reproduce it. Curved transitions, sloping side walls and unknown ledges make the answer an approximation even when the arithmetic is exact. Use verified pool drawings or professional measurements when a closer volume estimate is needed; this calculation does not determine construction safety, equipment selection or chemical quantities.

Sources & further reading

• Pentair Sta-Rite Basic Training Manual, Swimming Pool Gallons, printed page 7 / PDF page 9
• NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement

Source material checked November 9, 2026. This guide is an original explanation; source publishers do not endorse this website.