Stair Stringer Calculator
Lay out stair stringers from the total rise. Get the riser count and exact riser height to the sixteenth, tread count, total run, stringer length and how many stringers to cut.
How it's calculated
The number of risers is the total rise divided by your maximum riser height, rounded up. The exact riser height is then the total rise divided by that number, so every step is equal.
There is always one fewer tread than risers, because the top step is the upper floor. Total run is treads times the unit run. The stringer length is the diagonal of the rise and the run, and the stock you buy must be longer to allow for the top and bottom cuts.
Worked example
A total rise of 108-1/2" with a 7-3/4" maximum gives 14 risers at exactly 7-3/4". With 13 treads at 10", the total run is 10'-10" and you need 16 ft 2×12s.
Field notes
- Measure total rise at the stair location after finished flooring is planned. A floor that drops or a new floor added later changes the first or last step.
- Drop the bottom of the stringer by the tread thickness so the first riser matches the rest.
- IRC limits riser variation to 3/8" within a flight, and requires a 6'-8" minimum headroom.
- Cut one stringer, test it in place, then use it as the pattern for the rest.
Common questions
- What is the maximum stair riser height?
- 7-3/4" under the IRC for residential stairs, with a 10" minimum tread depth. Commercial codes are stricter: 7" max riser, 11" min tread.
- How many stringers do I need?
- Divide the stair width by the maximum spacing your treads allow and add one. A 36" stair needs 3 stringers for 2× lumber treads at 18" spacing, or 4 for 5/4 deck boards at 16". Composite treads may require 12".
- What is the 2R + T rule?
- Two risers plus one tread should equal about 24–25". It is a long-standing check for a comfortable stride.