Stair Calculator
Riser count and height, run, slope length and slab area, with comfort check.
Inputs
Result
Formula
| Risers | n = storey height (mm) ÷ target riser, rounded |
| Actual riser | h = storey height (mm) ÷ n |
| Goings | n = risers − 1 |
| Horizontal run | L = goings × going |
| Slope length | L = √(run² + height²) |
| Comfort | 2h + b = 600-630 mm reads comfortably |
A 3 m storey with a 280 mm going and 150 mm target: 20 risers at 150.0 mm, 19 goings, a 5.320 m run, 6.108 m slope length, 29.42° pitch and 2h + b = 580 mm — acceptable.
FAQ
Why can't the actual riser equal the target?
Because the storey height is fixed and the riser count must be a whole number. At 3 m, a 160 mm target needs 18.75 risers — so 19 at 157.9 mm or 20 at 150 mm. Never build 18 even steps plus one odd 190 mm riser to make up the height: it is the classic stair trip hazard and fails inspection immediately.
What riser height is appropriate?
150-175 mm for dwellings, 140-160 mm for public buildings, 120-150 mm for nurseries and primary schools. Code maxima are 175 mm domestic and 150 mm for schools; below about 100 mm the stride becomes unnaturally short. Shallower risers lengthen the stair and consume floor area — comfort traded against usable space.
How is the inclined area used for concrete and formwork?
Multiply by slab thickness for concrete volume, and take it as the soffit formwork area. Remember the triangular steps on top: add 0.5 × riser × going × stair width × number of goings, which is often 25-30% of the flight's concrete. Omitting it leaves the pour badly short.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.