Stair Calculator
Stair rise, run, stringer length and step count for a total height.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Risers = round(Total rise ÷ target riser); Riser height = Total rise ÷ risers
Treads = Risers − 1; Total run = Treads × tread depth
Stringer length = √(Total rise² + Total run²)
Run the calculation to see each step with your own numbers.
9 ft total rise, ~7 in target riser, 10.5 in treads.
Risers = round(108 ÷ 7) = 15; riser height = 108 ÷ 15 = 7.20 in
Treads = 14; total run = 14 × 10.5 = 147 in = 12.25 ft
Stringer = √(9² + 12.25²) = 15.2 ft
- A straight run with equal risers and treads (required by code — the largest and smallest riser may differ by no more than 3/8 in).
- Total rise is finished-floor to finished-floor, including the thickness of the upper floor finish.
- Nosing overhang (¾–1¼ in) is added on top of the tread depth entered here — it does not change the run.
- Riser and tread limits shown are from the IRC for reference; BuildFactor does not enforce code and your local amendments may differ.
- Why 'round' and not 'round up' for the riser count?
- You want the riser height as close to the comfortable ~7 in as possible. Rounding to the nearest whole number of risers gets you there; rounding up would make every stair unnecessarily shallow and long.
- How many steps will I have?
- Divide the total rise by about 7 in and round to the nearest whole number — a 9 ft rise is usually 15 risers and 14 treads. A steeper target riser gives fewer, taller steps; a gentler one gives more.
- What length stringer stock do I buy?
- At least the stringer length the calculator gives — the diagonal of the rise and run — plus a couple of feet for the top and bottom cuts and the offcut. A 15 ft diagonal usually means 16 ft stock.
Geometry
Standard right-triangle stair geometry.
Code limits (reference)
IRC R311.7 (riser ≤ 7¾ in, tread ≥ 10 in, width ≥ 36 in). Shown for reference only.
About the Stair Calculator
The Stair Calculator turns a floor-to-floor height into a full straight-run layout — the number of risers and treads, the exact riser height, the total run, and the minimum stringer length to buy. It rounds to the number of risers that puts the riser height closest to a comfortable 7 in. Riser and tread code limits are shown for reference; nosing overhang is added on top of the tread depth you enter.
What you enter
- Total rise — Finished floor to finished floor.
- Target riser height
- Tread depth (run) — Nose-to-nose, excluding any nosing overhang. IRC minimum 10 in.
- Stair width — For reference / material only. IRC minimum 36 in.
- Number of stringers
What you get back
- Number of risers — the exact result of the calculation.
- Riser height — IRC R311.7.5.1: 7¾ in maximum. If this is over, pick a lower target riser.
- Number of treads — the exact result of the calculation.
- Total run — the exact result of the calculation.
- Stringer length (min.) — The diagonal — buy stringer stock at least this long, plus cutting allowance.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Concrete Stairs Calculator — Concrete volume for a poured stair flight.
- Spindle Spacing Calculator — Evenly spaced baluster / spindle layout for a railing run.
- Spiral Staircase Calculator — Tread count, angle and dimensions for a spiral staircase.
- Ramp Calculator — Ramp length and landings for a rise at a given slope ratio.
Part of the Stairs calculators.