Stair stringer calculator
Enter one measurement, the total rise from finished floor to finished floor, and this gives you the riser count, riser height, tread depth, total run, and stringer length. It is the same math the CutOnce app runs, checked against the same IRC limits.
Updated
Type it the way your tape reads: 9 ft 1-1/2 in, 109.5, or 9′ 1½″
- Risers
- 15
- Riser height
- 7-5/16 in
- Treads
- 14
- Tread depth
- 10-3/8 in
- Total run
- 12 ft 1-5/8 in
- Stringer length
- 15 ft 2-3/16 in
- Stair angle
- 35.1°
Riser height is within the IRC R311.7.5.1 maximum of 7.75 in, and tread depth meets the R311.7.5.2 minimum of 10 in.
A worked example
Say your total rise is 9 ft 1-1/2 in, floor to floor. Here is the whole layout and where each number comes from.
| Total rise | 9 ft 1-1/2 in | given |
| Risers | 15 | round(109.5 / 7.5) = 15, cap check ceil(109.5 / 7.75) = 15 |
| Riser height | 7-5/16 in | 109.5 / 15 |
| Treads | 14 | 15 - 1 |
| Tread depth | 10-3/8 in | 25 - 2 x 7.30 |
| Total run | 12 ft 1-5/8 in | 14 treads x tread depth |
| Stringer | 15 ft 2-3/16 in | sqrt(rise squared + run squared) |
| Angle | 35.1° | atan(riser / tread) |
How the layout is derived
- 01
Pick the riser count
Divide the total rise by a comfortable 7.5 in riser and round to the nearest whole number. Then check that count against the code maximum: if it puts the risers over 7-3/4 in, add one. Both rules run, and the larger count wins.
- 02
Divide the rise
Total rise divided by riser count gives the exact riser height. Every riser in a flight has to be within 3/8 in of the others, which is why you divide rather than measure up from the bottom.
- 03
Count the treads
A standard stringer mount has one fewer tread than risers. The top riser lands on the finished floor above, so it needs no tread.
- 04
Set the tread depth
The rule of thumb carpenters use is two risers plus one tread equals 25 in. That keeps the stair walkable. If the result would fall under the 10 in code minimum, the minimum wins.
- 05
Multiply out the run
Tread depth times tread count is the total run, the horizontal distance the stair eats. The stringer itself is the diagonal: the square root of rise squared plus run squared.
What the code requires
The limits below are the 2021 IRC residential defaults the app ships. This page checks the three geometric ones: riser height, tread depth, and total rise between landings. Headroom and riser variance need the built stairwell, so the app checks those. Your jurisdiction can differ, so confirm locally before you cut.
| Maximum riser height | 7-3/4 in | IRC R311.7.5.1 |
| Minimum tread depth | 10 in | IRC R311.7.5.2 |
| Maximum rise between landings | 151 in | IRC R311.7.3 |
| Minimum headroom | 6 ft 8 in | IRC R311.7.2 |
| Maximum riser variation in a flight | 3/8 in | IRC R311.7.5.1 |
Common questions
- Why does the riser count jump by one sometimes?
- Because two rules run at once. The comfort rule aims at 7.5 in risers; the code rule caps them at 7-3/4 in. On a 63 in rise the comfort rule alone suggests 8 risers at 7-7/8 in, which is over the cap, so the count goes to 9 and the risers drop to 7 in exactly.
- Do I measure the rise before or after finish flooring?
- After. Total rise is finished floor to finished floor. If the upper floor is getting 3/4 in hardwood and the lower slab is getting tile, both of those change your riser height, and a 3/4 in surprise at the bottom step is the classic failed inspection.
- Why is my tread depth exactly 10 in?
- That is the code minimum doing its job. On steep rises the two-risers-plus-a-tread rule would produce a tread under 10 in, so the calculator clamps to the minimum instead of returning something you cannot build.
- Is the stringer length the board length I should buy?
- It is the diagonal of the flight, not the board. Add for the top and bottom cuts and whatever your stringer attachment needs, and buy the next stock length up.
Cut the whole stair, not just the layout
This page solves the layout. The app does the rest of the stair: stringer notching with throat depth, headroom and stairwell opening, landing splits, and a printable cut plan you can hand to whoever is cutting.
CutOnce runs this same math on the jobsite, then prints the cut plan with the notches marked.
See also