Building
Stair Stringer Calculator
Enter the total rise from finished floor to finished floor plus your target tread depth. The calculator finds the fewest risers that stay within the IRC 7 3/4 in maximum and computes the stringer geometry.
Formula
Risers = ceiling(total rise ÷ max riser). Actual riser = total rise ÷ risers. Treads = risers - 1 (the upper floor is the last step). Total run = treads × tread depth. Stringer length = √(rise² + run²). Angle = arctan(riser ÷ tread).
Worked example
A 108 in floor to floor rise: 108 ÷ 7.75 = 13.9, so 14 risers at 7.71 in. 13 treads at 10.5 in = 136.5 in total run. Stringer length = √(108² + 136.5²) = 174.1 in (14.5 ft). Angle = 36.3 degrees.
Assumptions and limits
- Straight run stairs without landings.
- Tread depth excludes nosing; IRC requires 3/4 in to 1 1/4 in nosing on solid risers when treads are less than 11 in.
- Lumber stringer stock must be longer than the hypotenuse to allow for cuts.
Frequently asked questions
What are the IRC stair dimension limits?
IRC Section R311.7.5: maximum riser height 7 3/4 in, minimum tread depth 10 in, with risers and treads in a flight varying by no more than 3/8 in.
How many stringers do I need?
Stair width and tread material govern. A common rule is stringers no more than about 16 in apart for 1 in thick treads, which usually means three stringers for a 36 in wide stair.
What is the minimum stair width?
IRC Section R311.7.1 requires a clear width of at least 36 in above the handrail height.