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Stair Calculator: Steps, Risers, Treads and Stringers
Enter the total height from floor to floor to find how many steps you need, the exact riser height, the total run and the stringer length, checked against the US residential code.
How to calculate stairs
- Measure the total rise from the top of the lower finished floor to the top of the upper finished floor, in inches.
- Divide by the maximum riser height (7¾ in under the IRC) and round up. That is the number of risers.
- Divide the total rise by the number of risers for the exact riser height. Every riser must be the same, within ⅜ in.
- Treads = risers − 1, because the top step is the upper floor.
- Total run = treads × tread depth.
Example: a total rise of 105 in (8 ft 9 in) ÷ 7.75 = 13.5, so 14 risers of 7.5 in. That gives 13 treads; at 10 in each the total run is 130 in (10 ft 10 in). The stringer must be at least √(105² + 130²) = 167 in, so buy 16 ft boards.
Stair code rules (IRC residential)
- Riser height: 7¾ in maximum.
- Tread depth: 10 in minimum, measured nose to nose.
- Headroom: 6 ft 8 in minimum along the whole stair.
- Width: 36 in minimum.
- Handrail: required with 4 or more risers, 34 to 38 in above the tread nosing.
Local codes can differ, and commercial stairs follow stricter rules (7 in maximum riser and 11 in minimum tread). Check with your building department.
The comfort rule
Stairs feel natural when 2 × riser + tread is about 24 to 25 in. A 7½ in riser with a 10 in tread scores 25 in. Shorter risers with deeper treads feel gentler but take up more floor space.
Frequently asked questions
How many steps for an 8 ft ceiling?
A typical 8 ft ceiling plus floor framing gives about 105 in of rise, which needs 14 risers of 7.5 in and 13 treads.
What is the maximum stair riser height?
7¾ inches under the International Residential Code, with a minimum tread depth of 10 inches.