Home & Cooking
Stair Stringer Calculator
Calculate stair stringer cuts for a deck or porch. Solve for risers, treads, stringer length, and angle with IRC R311.7 code compliance flags.
Stair stringer layout
Top of upper floor to top of lower floor
Horizontal distance, nose of first tread to last
IRC max 7.75 in
IRC min 80 in
Stringer layout
16 risers, 15 treads
- Riser height
- 6.75 in
- Tread depth
- 8.8 in
- Total run
- 132 in
- Stair angle
- 39.29 deg
- Stringer length
- 170.55 in
- Notch depth
- 6.75 in
- Remaining web (2x12)
- 5.89 in
- 2R + T
- 22.3 in
IRC R311.7 compliance
Riser <= 7.75 in
Tread >= 10 in
Headroom >= 80 in
Code and comfort notes
- Tread depth 8.8 in is below the IRC R311.7.5.2 minimum of 10 in. Lengthen total run or remove a tread.
- Stair angle 39.29 deg is outside the 30-37 deg comfortable band.
- 2R + T = 22.3 in falls outside the 24-25 in stair-builder rule.
Frequently Asked Questions about the Stair Stringer Calculator
What is a stair stringer?
A stair stringer is the diagonal board that supports the treads and risers of a stair. Residential decks and porches usually use 2x12 lumber notched at each step. You need at least two stringers under any stair, and three once the stair gets wider than 36 in or the stringer length passes 120 in.
How do I figure out how many risers I need?
Divide your total rise by your preferred riser height and round up. For a 108 in rise with a 7 in preferred riser, ceil(108 / 7) is 16 risers, which gives an actual riser height of 6.75 in. The number of treads is always one less than the risers on an open stringer, so 16 risers means 15 treads.
What is the IRC limit on riser height and tread depth?
IRC 2021 R311.7.5.1 caps residential riser height at 7.75 in. R311.7.5.2 sets the minimum tread depth (nose to nose) at 10 in. The variance between the tallest and shortest riser on the same flight cannot exceed 3/8 in. The calculator flags any output that breaks these limits.
How long does my stringer board need to be?
The stringer is the hypotenuse of the rise/run right triangle, so its length is sqrt(rise squared plus run squared). For a 108 in rise and 132 in run, that comes out to about 170.55 in or just over 14 ft. Order at least one size up so you can scribe and trim the top and bottom cuts cleanly.
Why does the calculator show remaining stringer web?
After you notch a 2x12 for the risers and treads, what is left of the board between the deepest cut and the back edge is called the web. IRC R311.7.3 effectively requires at least 5 in of intact web for the stair to carry load safely. Steep stairs with deep treads can drop the web below that minimum, in which case you need wider stock or a shallower layout.