Stair Calculator
Enter the total rise from finished floor to finished floor. This free stair calculator works out how many steps you need, the exact riser height, the total run and the stringer length, and gives a mark for every step so you can lay out the stringer.
Layout marks for each step
| Step | Height from floor | Run from first riser |
|---|
How the stair calculator works
All the risers in a flight have to be the same height. So the calculator divides the total rise by your maximum riser height and rounds up to get the number of risers, then divides the total rise by that number to get the real riser height. A straight flight has one tread fewer than risers, because the top step is the upper floor. Total run is treads × tread depth, and the stringer is the diagonal: √(rise² + run²).
Worked example: 8' 10-1/2" floor to floor
106.5" ÷ 7.75" = 13.7, so you need 14 risers. 106.5" ÷ 14 = 7.607", about 7-5/8" each. 13 treads at 10" give a total run of 10' 10". The stringer runs about 14' 1/16" along the slope, so buy a 16-foot 2x12. The stair angle is 37.3°, and 2 risers + 1 tread is 25.2", right at the comfortable end.
US residential stair rules of thumb
- Risers no taller than 7-3/4" and treads at least 10" deep (IRC R311.7.5).
- Riser heights in one flight within 3/8" of each other, so cut every riser the same.
- Nosing of 3/4" to 1-1/4" when treads are under 11" deep.
- At least 6' 8" of headroom, measured straight up from the line of the nosings.
- Width of at least 36", and a handrail once there are four or more risers.
These are summaries of the US model code. Your city or county may differ, and decks and commercial stairs have their own rules, so check with your local building department before you build.
FAQ
How do I measure total rise?
Measure from the finished surface of the lower floor to the finished surface of the upper floor. If the flooring isn't down yet, add its thickness on top and take it off the bottom, or the first and last steps will come out a different height.
Why does the first riser on the stringer look short?
When you cut the stringer, take the tread thickness off the bottom riser so the first step comes out the same height as the rest once the treads are on. The marks table shows heights to the top of each finished tread.
Can I choose the number of steps myself?
Yes. Type a number in "Number of risers" and the calculator uses it instead, then warns you if the risers come out too tall.