Stair Calculator
Riser count and height, run, slope length and slab area, with comfort check.
Quick answer
With the default inputs: Storey height 3 · Going 280 · Target riser 150 etc.Number of risers 20 lifts
Edit the inputs below to recalculate.Inputs
Result
Number of risers
20lifts
| Actual riser height | 150.0mm |
| Number of goings | 19pcs |
| Horizontal run | 5.320m |
| Slope length | 6.108m |
| Inclined slab area | 7.329m² |
| Pitch angle | 29.42° |
| Comfort 2h + b | 580mm |
| Assessment | Acceptable |
| Going to riser ratio | 1.87 |
| Rise per flight | 3.00m |
The riser count is storey height divided by the target riser, rounded, then the height is divided back — actual risers must divide the storey exactly, or uneven steps trip people. The 600-630 mm figure for 2h + b comes from stride length; 580-600 suits dwellings, and schools need flatter stairs. Provide a landing beyond 18 risers in one flight.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
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.
How to work it out
- Compute the riser count n = storey height (mm) ÷ target riser, rounded to a whole number. A 3 m storey at 150 mm gives 20 risers.
- Actual riser h = storey height ÷ n. 3000 ÷ 20 = 150 mm exactly. Where it does not divide evenly the actual riser inevitably differs from the target.
- Tread count = n − 1, because the top riser lands on the floor and occupies no tread. Off-by-one here is the classic error.
- Horizontal going L = tread count × tread width. Twenty risers at 280 mm gives 19 × 280 = 5320 mm, which sets the stairwell depth required.
- Sloped length = √(L² + storey height²), needed for the waist slab concrete and tread formwork.
- Comfort check: 2h + b should fall between 600 and 630 mm. At h = 150 and b = 280 it is 580 mm, slightly steep; at b = 300 it is 600 mm, correct.
Where it goes wrong
- Using n treads instead of n − 1. Twenty risers counted as twenty treads adds 280 mm of going, the stairwell is short, and the tread has to be cut or the structure changed on site.
- Rounding the riser to a whole millimetre and carrying on. 3000 mm over 21 risers is 142.86 mm and must be built as such; setting out at 143 mm accumulates 3 mm over the flight and produces a step at the floor.
- Applying residential limits (150-175 mm riser) to every building. Kindergartens and schools are held to 150 mm or less, and public buildings should not exceed 150 mm.
- Forgetting the minimum flight width. Residential flights need at least 1.1 m clear, public buildings 1.4 m, and more in crowded spaces — undersized flights fail inspection.
Material and cost
Cast-in-place stairs run 350-550 CNY/m² of plan area including formwork and reinforcement; precast 400-600. Steel stairs 800-1500 CNY/m² with balustrade. Timber handrail 80-200 CNY/m, stainless 150-350. Stairs are among the highest-cost elements per square metre because both formwork and labour are demanding.
On site
- A flight should have no more than 18 risers and no fewer than 3. Beyond 18 needs a landing; below 3 is a trip hazard, and both are mandatory limits.
- A riser variance over 10 mm gets reported as a trip. Level every riser individually during construction — never trust accumulation.
- Treads need a 1-2% fall or anti-slip inserts, and external stairs must have them or they become treacherous in rain.
- Headroom from tread nosing to the flight soffit above must be at least 2.2 m — the dimension most often missed on drawings, and people hit their heads on site.
Basis
- GB 50352-2019 Uniform standard for design of civil buildings, stairs
- GB 50096-2011 Code for design of residential buildings, stairs
- GB 55031-2022 General code for civil buildings
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Stair Calculator. 2026-10-25. https://jianzhujisuan.com/en/stair-calc
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.