Stair Stringer Calculator
Building code-compliant stairs requires careful calculation of the riser height (rise) and tread depth (run) relationship. The IRC 7-11 guideline recommends a 7-inch rise and 11-inch run as a comfortable, safe combination, while code allows up to 7.75 inches of rise with a minimum 10-inch run. Enter the total vertical height to climb and your target riser height. The calculator divides total rise by riser height (rounding up to whole risers), then adjusts the actual riser to be uniform. The stringer length is the hypotenuse: sqrt(total run squared + total rise squared). This is the board length you need to purchase for each stringer.
Stair stringer formula
Number of risers = ceil(Total rise / Target riser height)
Actual riser = Total rise / Number of risers
Total run = (Number of risers - 1) x Tread depth
Stringer length = sqrt(Total run squared + Total rise squared)
The number of treads is always one less than the number of risers because the top landing serves as the final tread. The stringer length (the hypotenuse of the rise-run triangle) is the minimum board length needed for each stringer.
IRC stair code requirements
- Maximum riser height: 7.75 inches (IRC R311.7.5.1).
- Minimum tread depth: 10 inches, measured horizontally (IRC R311.7.5.2).
- Maximum variation in riser height within one flight: 0.375 inches (IRC R311.7.5.1).
- Minimum headroom: 6 feet 8 inches measured vertically (IRC R311.7.2).
- Handrails are required on stairs with 4 or more risers (IRC R311.7.8).
Stair stringer calculator: frequently asked questions
What is the IRC 7-11 rule for stairs?
The International Residential Code (IRC Section R311.7) requires residential stairs to have a maximum riser height of 7.75 inches and a minimum tread depth (run) of 10 inches. The common shorthand '7-11' refers to 7-inch rise and 11-inch run as practical targets.
How do I calculate the number of steps?
Divide the total rise (the vertical height to climb) by the desired riser height, then round up to the nearest whole number. For example, a 48-inch rise divided by 7.5 inches per riser = 6.4, rounded up to 7 risers (6 treads since there is no tread at the top landing).
What is stringer length?
The stringer length is the diagonal length of the stair stringer board, calculated as: sqrt((total run)squared + (total rise)squared). This is the minimum lumber length needed per stringer.
How many stringers do I need for a staircase?
Per IRC R311.7.1.2, the maximum stringer spacing is 36 inches on center. A 36-inch wide staircase needs at least 3 stringers (two outer and one center). Wider stairs need additional stringers.
What is the maximum riser height allowed by code?
The IRC allows a maximum riser height of 7.75 inches for residential stairs. The maximum variation in riser height within a flight of stairs is 0.375 inches. Always check local building codes, which may be more restrictive.
Official sources
- International Code Council: International Residential Code (IRC) Section R311.7 Stairways.
- USDA Forest Products Laboratory: Wood Handbook.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.