How to use the stair calculator
- Measure the total rise: the vertical distance from the finished floor at the bottom to the finished floor at the top. Include flooring that isn't installed yet.
- Choose the tallest riser you're allowed or want. Most US homes follow the International Residential Code limit of 7 3/4 in.
- Enter the tread depth (the run of each step, not counting the nosing). The code minimum is 10 in; 10 1/2 to 11 in feels more comfortable.
Stair formulas
risers = total rise ÷ max riser height (rounded up)
riser height = total rise ÷ risers
treads = risers − 1
total run = treads × tread depth
stringer length = √(total rise² + total run²)
There's one less tread than risers because the top floor acts as the last step. Every riser must be the same height: the IRC allows no more than 3/8 in difference between the tallest and shortest riser.
Stairs for common floor heights
| Total rise | Risers | Riser height | Treads | Total run (10 in treads) |
|---|---|---|---|---|
| 30 in (2' 6") | 4 | 7.50 in | 3 | 30 in |
| 48 in (4' 0") | 7 | 6.86 in | 6 | 60 in |
| 96 in (8' 0") | 13 | 7.38 in | 12 | 120 in |
| 105 in (8' 9") | 14 | 7.50 in | 13 | 130 in |
| 108 in (9' 0") | 14 | 7.71 in | 13 | 130 in |
| 117 in (9' 9") | 16 | 7.31 in | 15 | 150 in |
| 120 in (10' 0") | 16 | 7.50 in | 15 | 150 in |
Common code requirements (US residential)
- Maximum riser height 7 3/4 in, minimum tread depth 10 in (IRC R311.7.5).
- Nosing of 3/4 to 1 1/4 in on treads with solid risers when the tread is less than 11 in deep.
- Minimum headroom of 6 ft 8 in, measured from the line of the nosings.
- Minimum width of 36 in.
- A handrail on at least one side when there are four or more risers.
Local codes can differ, so check with your building department before you build.
Comfort rule
Stairs feel natural when two risers plus one tread add up to 24–25 in. A 7.5 in riser with a 10 in tread gives 25 in. The calculator shows this number so you can adjust the tread depth.
Frequently asked questions
How many steps do I need for 9 feet?
A 108 in total rise needs 14 risers of about 7.71 in each, and 13 treads. With 10 in treads the stairs run 130 in (10 ft 10 in) horizontally.
How do I calculate stair stringer length?
Use the Pythagorean theorem: the square root of total rise squared plus total run squared. For 108 in of rise and 130 in of run, that is about 169 in, so buy 16 ft lumber.
What is the maximum riser height?
The International Residential Code allows up to 7 3/4 in for homes. Commercial buildings follow the IBC, which limits risers to 7 in.
Why is there one less tread than riser?
The top floor takes the place of the last tread, so a flight with 14 risers has 13 treads.