Stair Calculator
Work out how many steps a straight staircase needs, the exact riser height, the total run and the stair angle from your floor-to-floor height, and see how it compares with UK or US code limits. Straight single-flight stairs only; winders, landings and stringer sizing are out of scope.
Rise & steps
Reference code
Approved Document K (2013), Table 1.1, private stair. Reference only.
Stair profile drawn to scale. H is the total rise, the number under the stair is the run, the label at the top right is riser × tread, all in mm.
Reference checks
- Comfort, 2h + b: 626.7 mm (limit 600–640 mm) within limits
- Riser height: 173.3 mm (limit 150–220 mm) within limits
- Tread depth: 280.0 mm (limit 220–300 mm) within limits
- Pitch: 31.8 ° (limit ≤ 42 °) within limits
- 2R + G per Approved Document K: 626.7 mm (limit 550–700 mm) within limits
- Risers in one flight: 15 (limit ≤ 36) within limits
A reference comparison with headline numbers only, not a statement that the stair meets building regulations.
Before you build
- Measure the total rise from finished floor to finished floor, including flooring that isn't laid yet. A few millimetres of new tile or carpet at the top or bottom makes the first or last step different from the rest, and uneven steps are the classic trip hazard: codes allow at most 3/8 in (9.5 mm) of variation within a flight.
- Tread depth here is the going, measured nosing to nosing. The board you buy is deeper by the nosing overhang (US IRC requires ¾–1¼ in where the tread is under 11 in).
- Check headroom separately: at least 2.0 m above the pitch line under England's Approved Document K, and 6 ft 8 in under the US IRC. It depends on the floor thickness and where the opening is, which this calculator doesn't know.
- The stringer length shown is the mathematical pitch-line length. The actual board must be longer, and its size and supports are a structural question, not something this tool checks.
How this is calculated
The total rise is divided by your maximum acceptable riser height and rounded up to a whole number of risers, so every step comes out equal and no taller than your limit. The exact riser is then the total rise divided by that number. There is one tread fewer than risers, because the top "step" is the upper floor itself, so the total run is the number of treads times the tread depth. The angle comes from riser ÷ tread, and the pitch-line length from Pythagoras. Then 2 × riser + tread (Blondel's step formula) is compared with the comfortable range of 600–640 mm, roughly an adult's stride. Finally the stair is checked against the code you pick: England's Approved Document K (rise 150–220 mm, going 220–300 mm, pitch ≤ 42°, 2R+G 550–700 mm) or the US IRC (riser ≤ 7¾ in, tread ≥ 10 in).
Common questions
Why is the number of steps always rounded up?
Because the riser height you enter is a maximum. Rounding down would make each step taller than that, often above the code limit. A worked example from a professional deck-building magazine shows exactly this: 102.5 in ÷ 13 gives 7.88 in risers, over the US 7¾ in limit, while 14 risers give 7.32 in.
Why does it show one tread fewer than risers?
On a normal straight stair the last rise lands you on the upper floor, which acts as the final tread. So 15 risers means 14 treads to buy and lay out, and the floor space the stair needs is 14 treads deep. If your top step sits below the floor edge, add one tread to the run.
Does "within limits" here mean my stair meets building regulations?
No. It only compares the geometry with the headline numbers of the code you picked. Real compliance also covers headroom, width, handrails, guarding, landings, winders and local amendments. Treat it as a sanity check, and ask your building control body if you need sign-off.
Part of Home Improvement Calculators