This stair calculator works out the number of steps, riser height, total run, stringer length and angle from your floor-to-floor height. It checks the result against the US International Residential Code (IRC) or England’s Approved Document K, in inches or millimetres.
How the stair is calculated
- Risers = total rise ÷ maximum riser height, rounded up.
- Riser height = total rise ÷ number of risers, so every step is equal.
- Treads = risers − 1, because the top floor acts as the last step.
- Total run = treads × tread depth; stringer length = √(rise² + run²).
Building code limits
| Rule | US (IRC R311.7) | England (Approved Document K, private stair) |
|---|---|---|
| Riser height | 7¾ in maximum | 150–220 mm |
| Tread depth (going) | 10 in minimum | 220–300 mm |
| Comfort rule | 2R + T ≈ 24–25 in (guide) | 2R + G between 550 and 700 mm |
| Maximum pitch | Set by riser and tread limits | 42° |
| Headroom | 6 ft 8 in | 2.0 m |
Worked example
A total rise of 108 inches with risers of 7.5 inches or less needs 15 risers of 7.2 inches and 14 treads. At 10.5 inches per tread, the total run is 147 inches (12.25 ft), the stringer is about 182.4 inches long and the stair angle is about 34.4°.
Frequently asked questions
How many steps do I need for a 9 ft ceiling?
Floor-to-floor height is usually about 9 ft plus the floor structure, often 108 to 120 inches, which needs about 15 to 16 risers under the IRC limit.
Why is there one fewer tread than risers?
The upper floor serves as the top step, so a straight stair always has one tread fewer than risers.
What is the most comfortable stair angle?
Usually between about 30° and 37°. Twice the riser plus the tread at around 24 to 25 inches tends to feel natural.
Do all steps have to be the same height?
Yes, as closely as possible. The IRC allows no more than 3/8 inch difference between the largest and smallest riser in a flight.
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Sources
Last reviewed: October 2026. For information only; results are estimates; check important figures with a professional.