How thick should the waist of a concrete stair be?

The waist (the thin slab beneath the steps) is typically 150-200mm for domestic stairs and 175-250mm for commercial. The waist thickness depends on the span and loading — a structural engineer must design it. The waist is the structural element; the steps above are essentially additional concrete that adds dead load.

How do I calculate concrete volume for stairs?

The concrete volume has two components: the waist slab (flight length × width × waist thickness) plus the step triangles (number of steps × width × going × rise / 2). The flight length is the diagonal: sqrt(total going² + height²). Add 10% for wastage. A typical domestic flight uses 0.8-1.5 m³ of concrete.

What reinforcement do concrete stairs need?

Concrete stairs are typically reinforced with main bars (T12 or T16) running along the waist in the direction of span, plus distribution bars (T10) running across the width. Typical reinforcement density is 60-100 kg/m³ of concrete. The main bars must extend into the landing at each end with adequate anchorage length.

What formwork is needed for concrete stairs?

Concrete stair formwork includes: soffit boards (supporting the underside), side strings (edge boards), and riser boards (the vertical face of each step). The soffit must be supported on adjustable props. Formwork is typically 18mm plywood. It's one of the most complex formwork tasks — experienced carpenters are essential.

What are the Building Regulations for stairs?

Part K of Building Regulations (England) requires: maximum rise 220mm, minimum going 220mm, 2R + G = 550-700mm, minimum headroom 2.0m, minimum width 800mm for domestic stairs, handrails on both sides for stairs over 1m wide. Maximum 16 risers in a straight flight. Concrete stairs must also comply with Part A (structural) requirements.

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Concrete Stair Calculator — Formwork, Rebar & Concrete Volume

Calculate concrete volume, formwork area and reinforcement for in-situ concrete stairs.

Floor-to-Floor Height

Stair Width

Min 800mm domestic

Rise

Max 220mm (Part K)

Going

Min 220mm (Part K)

Concrete Price

Formwork Price

How We Calculate This

This calculator determines concrete volume, formwork area and reinforcement weight for in-situ cast concrete stairs.

Concrete volume

Waist Volume = Flight Length × Width × Waist Thickness
Step Volume = Steps × Width × Going × Rise / 2
Total = Waist + Steps

Formwork

Formwork area = soffit (underside) + side strings + riser boards. Rebar estimated at 60-100 kg per m³ of concrete.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.