About this calculator
This calculator sizes cast-in-place concrete steps — entrance steps up to a door, garden steps — and takes off the materials. Enter the total rise, a preferred riser height and the going; it works out how many risers you need (all the same height), then the concrete, formwork and reinforcement mesh, with an optional landing at the top.
Two constructions are covered:
- Sloping waist slab — step triangles on an inclined slab of a given thickness (the usual cast-in-place stair). The slab can be cast on a prepared sloping base or span between supports (then it needs soffit formwork).
- Solid on the ground — every step is a full block of concrete down to ground level (common for one to three low steps).
Formula
risers N = round(total rise / preferred riser height)
riser r = total rise / N (all equal)
treads = N − 1 (the top riser leads onto the landing or threshold)
comfort = 2 × r + going checked against 630 mm ± 30 mm
The concrete is the side profile area × width. The side profile is the polygon formed by the risers and treads on top and by the underside:
solid: underside = ground level (y = 0)
waist: underside = line parallel to the inner step corners, t / cos(pitch) below them
pitch = atan(r / going); t = waist thickness (perpendicular)
the landing slab (thickness tl) meets that line at
x = (total rise − tl + t / cos(pitch)) × going / r
riser formwork = formed risers × width × r
side formwork = side profile area × number of formed sides
soffit = underside length × width (spanning waist slab only)
mesh = underside length × width × (1 + lap %)
mesh sheets = ceil(mesh / sheet area)
Worked example
Total rise 600 mm, preferred riser 150 mm, going 330 mm, width 1.5 m, 150 mm waist on a prepared base, 1.2 m landing 150 mm thick, 5% waste.
N = round(600 / 150) = 4 risers of 150 mm, 3 treads; 2 × 150 + 330 = 630 mm ✓
pitch = atan(150 / 330) = 24.4°; waist depth = 150 / 0.9104 = 164.8 mm
landing underside meets the waist line at x = (450 + 164.8) × 2.2 = 1352.5 mm
side profile area = 0.4472 m² → concrete = 0.4472 × 1.5 = 0.671 m³ (+5% = 0.704 m³)
riser forms = 4 × 1.5 × 0.15 = 0.90 m²; side forms = 2 × 0.447 = 0.89 m²
mesh = 2.323 m × 1.5 = 3.48 m² (+15% = 4.01 m²) → 1 sheet of 2 × 3 m
FAQ
What riser height and going should outdoor steps have? Comfortable steps satisfy 2 × riser + going ≈ 630 mm (DIN 18065's ideal step; STR 2.02.01:2004 gives 650 mm for common stairs in apartment buildings). Outdoor steps usually have low risers (140–160 mm) with deep treads; the calculator warns when the pair falls outside 630 ± 30 mm.
Why is there one tread fewer than risers? The top riser always leads onto the landing (when you cast one) or onto the existing floor or threshold, so it has no tread of its own in the flight.
Should I choose a waist slab or solid steps? For one to three low steps solid concrete is simple. For more steps a waist slab saves a lot of concrete; it can be cast on a compacted sloping base or, when it spans, on soffit formwork — then it is a structural element and needs an engineer's design.
Which standard does this follow? The 2 × riser + going check follows DIN 18065 (6.1.2) and STR 2.02.01:2004 (Annex 11), which disagree by 20 mm; the calculator checks against 630 mm and names both. STR 2.02.01:2004 loses force on 1 January 2027, replaced by STR 2.02.12:2026 "Pastatai"; the stair requirements of the new regulation have not yet been checked here. LST EN 206 is cited for the concrete class designation. Reinforcement and frost protection of the base are a design matter this calculator does not perform.
Assumptions and limits
- A straight flight of equal risers and equal goings; no nosings, no tapered steps.
- Waist: the slab starts with a vertical cut at the foot of the first riser (it reaches below ground by the waist depth) and meets the landing's underside; a bottom nib or foundation under it is not included.
- Solid: steps and landing are cast full depth to ground level, with no fill inside.
- The mesh is one layer along the underside; laps are an estimated percentage, bar chairs and any bars along the edges are not counted.
- Not included: excavation, the compacted base, frost protection, insulation, finishes.
- The concrete class is recorded for ordering only; it changes no quantity.