Account for pool steps, benches and tanning ledges once

By Kyle Bell · Reviewed November 9, 2026

Steps, benches and shallow ledges change the water space inside a pool. A simple length-times-width-times-depth calculation can count solid material as though it were water. Correcting that estimate is useful, but only when the base model and the adjustment describe the same space.

Choose a water-zone model or a displacement model

In a water-zone model, divide the footprint into nonoverlapping regions and calculate the actual water above each one. A ledge with six inches of water uses that shallow depth, while the main floor uses its own depth. Add the volumes. The solid material below the ledge never enters the water calculation.

In a displacement model, begin with a simpler full-depth water box and subtract the known underwater solid volume that occupies part of it. This can be convenient for a rectangular bench with well-established dimensions. It is not a reason to guess the full external size of a step and subtract it blindly.

Use one approach for each feature. If a ledge is already represented as shallow water, subtracting its solid volume again counts the correction twice and makes the answer too small.

Work the same ledge two ways

Consider a hypothetical rectangular interior measuring 32 feet by 16 feet. Most of it has 4 feet of water depth. A 10-by-2-foot ledge inside that footprint has 1 foot of water above it. Assume vertical sides, a flat main floor and a rectangular ledge for this example.

Using zones, the main area is 512 − 20 = 492 square feet. Its volume is 492 × 4 = 1,968 cubic feet. Water over the ledge adds 20 × 1 = 20 cubic feet. Total water volume is 1,988 cubic feet, about 14,871 US gallons.

Using displacement, the empty full-depth box would contain 32 × 16 × 4 = 2,048 cubic feet. The ledge replaces 20 square feet of water over a depth difference of 4 − 1 = 3 feet. Subtract 20 × 3 = 60 cubic feet. The result is again 1,988 cubic feet.

The two routes agree because they describe identical geometry. Subtracting another 60 cubic feet from the zoned result would be incorrect.

Treat steps as distinct regions when possible

For several rectangular steps, model each tread's footprint at its actual water depth, ensuring the regions do not overlap. Do not use each larger lower step's entire footprint if that footprint includes the smaller steps above it. A side-view drawing or verified dimensions can make the partition much clearer.

Curved steps, sloped benches and irregular features require a closer geometric model or a clearly labeled approximation. Keep a separate note for any adjustment whose dimensions are uncertain, and make sure total displacement cannot exceed the original modeled volume.

Convert the final cubic volume using NIST's unit relationships. The underlying area-times-average-depth method is also illustrated in Pentair's training reference. Retain the sketch with the estimate. This is a calculation of water space, not an assessment of whether a ledge or pool structure is safe or suitable for a particular use.

Sources & further reading

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

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