Stair Calculator
Measure the floor-to-floor rise, pick a target riser height, and get a full stair layout — actual riser height after rounding, tread count, total run, stringer length, angle, plus automatic checks against the common IRC limits.
How to calculate stairs
- Number of risers = round(total rise ÷ target riser). For a 106" rise at 7½": round(14.13) = 14 risers.
- Actual riser = total rise ÷ risers = 106 ÷ 14 = 7.57" (7 9/16").
- Treads = risers − 1 when the top tread is the finished landing = 13.
- Total run = treads × tread depth = 13 × 10" = 130".
- Stringer length = √(rise² + run²) = √(106² + 130²) ≈ 167.7". Angle = atan(rise ÷ run per step) ≈ 37°.
Code quick-reference (IRC residential)
| Dimension | Common limit |
|---|---|
| Riser height | max 7¾" (min 4") |
| Tread depth | min 10" |
| Headroom | min 80" (6'-8") vertical |
| Width | min 36" above handrail |
| Risers per flight | varies; landings required on long runs |
The comfort rule of thumb: 2 × riser + tread should land between 24" and 25". The calculator warns you when it doesn't.
Frequently asked questions
- Why is the actual riser different from my target?
- Risers must divide the rise evenly, so the calculator rounds to the nearest whole riser count first, then snaps the true riser height to tape precision. All risers in a flight must match within 3/8".
- Does this design my stairs for me?
- It produces the layout math only. Stair construction must follow your local building code — verify with your inspector or an engineer for permitted work.