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Stairs Calc

Waist slab

The inclined structural slab beneath cast concrete stairs.

The waist slab is the inclined structural slab that runs beneath a flight of cast-in-place concrete stairs, with the triangular steps formed on top of it. Its thickness — the waist — is measured perpendicular to the slope and carries the load down to the supports, much as a stringer does in a timber stair. Example: a typical residential concrete flight uses a 100–150 mm (4–6 in) waist, and the concrete calculator estimates volume as the waist slab plus the step triangles, then adds a builder's waste allowance. A thicker waist spans farther between supports but uses more concrete. The step risers and runs still follow the same code limits as any other stair.

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Related terms

Stringer The notched board that carries the treads and risers; usually a 2×12. Riser height The vertical height of a single step; every riser must be equal within 3⁄8 in. Run (going) The horizontal depth of one step, measured nosing-to-nosing, not counting the overhang. Total run The horizontal distance the whole flight covers, floor to floor. Stair soffit The finished underside of a staircase or its landing.

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Concrete Stairs Calculator Get the concrete volume for your steps in cubic yards, cubic meters and bags — using the waist‑slab method (structural slab plus step noses) with a builder's 10% waste allowance.
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Written by the Stairs Calc editorial team. Methodology and code references: see our methodology.

Built and maintained by builders, drafters and engineers who plan stairs for a living — every code limit is transcribed from the published standard and cited to its exact section.

Last reviewed 2026-06-20 against IRC 2021/2024

Stairs Calc gives accurate geometry and checks it against published building-code limits, but results are estimates for planning. Codes are adopted and amended locally and change over time. Always confirm dimensions against your local adopted code and a licensed professional before you build.