Concrete Stair Calculator — Formwork, Rebar & Concrete Volume

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.

The concrete volume has two components: the waist slab (flight length × width × waist thickness) plus the step triangles (number of treads × width × going × rise / 2). The number of treads is one fewer than the riser count, because the top riser lands on the upper floor with no tread above it. 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. Use our Concrete Volume Calculator for ordering quantities.

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. As an estimating rule of thumb, reinforcement works out around 60-100 kg/m³ of concrete (this calculator defaults to 80 kg/m³) — but the actual bar size, spacing and weight must come from a structural engineer’s design to BS EN 1992-1-1 (Eurocode 2), which depends on span, loading and cover. The main bars must extend into the landing at each end with adequate anchorage length.

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.

For private (domestic) stairs, Approved Document K (England) requires: maximum rise 220mm, minimum going 220mm, maximum pitch 42° and the 2R + G = 550-700mm relationship (all from Table 1.1), plus minimum headroom 2.0m (para 1.11, Diagram 1.3). A handrail is needed on both sides only where the stair is 1000mm or wider (para 1.34c). Part K sets NO minimum width for private domestic stairs — 800-900mm is trade convention (around 850mm is widely regarded as the comfortable minimum), and the 900mm figure in Part K applies only to entrance-storey flights on severely sloping plots (para 1.16). There is also NO 16-riser-per-flight limit for dwellings: a change of direction (intermediate landing) is required only where consecutive flights exceed 36 risers (para 1.17). The 16-riser (utility) and 12-riser (general access) per-flight limits apply only to buildings other than dwellings (para 1.18). Concrete stairs must also comply with Part A (structural) requirements.