Stair Stringer Calculator
Calculate exact stair stringer cuts, individual step rise and run, stringer lumber stock length, and IRC building code compliance.
Recommended Tools & Equipment
Tested hardware and components for high reliability
How Stair Stringer Math Works
Laying out stair stringers requires calculating an equal riser height for every step to avoid tripping hazards. Even a 3/8" difference between steps trips people according to building safety statistics.
The Golden Rule of Stairs (IRC Code)
- Max Riser Height: 7-3/4 inches (196 mm)
- Min Tread Depth: 10 inches (254 mm)
- 7-11 Rule: Optimal comfort is roughly 7" rise and 11" run
- Blondel's Formula: $2 imes ext{Rise} + ext{Run} = 24 ext{ to }25 ext{ inches}$
Why the Bottom Stringer Must Be Trimmed
A common beginner mistake is forgetting to cut off the bottom of the stringer by the thickness of the tread. If your tread is 1" thick, you must cut 1" off the bottom of the stringer so that the first step ends up the exact same height as every other step once the tread board is attached.
Frequently Asked Questions
How many stringers do I need for a deck staircase?
For standard 5/4 wood decking, stringers should be spaced no more than 12 to 16 inches on center (e.g., a 36-inch wide stair requires 3 stringers; a 48-inch wide stair requires 4 stringers). Composite decking typically requires 12-inch maximum spacing.
Why should I use a 2x12 instead of a 2x10 for stringers?
When you notch out 7.5" risers and 10.5" runs on a 2x10, only about 3.5 inches of uncut meat remains in the throat. A 2x12 leaves over 5 inches of solid wood, providing essential structural rigidity.
What is the maximum angle for residential stairs?
Residential stairs typically slope between 30° and 37°. Angles steeper than 40° are uncomfortable to climb and violate building codes.