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Stair Stringer Calculator
Enter the total rise, tread depth, board size and stair width. You get the framing square numbers to mark the board, the shorter first riser, the throat depth left under the deepest notch, the stock board length and how many stringers to cut. A 105 in rise needs 3 stringers of 14 ft.
Updated · Sources re-checked by
7 1/2 in riser · 10 in tread
Set these two numbers on the framing square, 14 steps, 13 treads.
| Bottom riser cut to | 6 1/2 in |
| Throat depth (2×12) | 5.25 in (enough wood) |
| Slope length | 13.93 ft |
| Stock board | 14 ft |
| Stringers for 36 in | 3 |
| Step | Tread top above floor | From first riser |
|---|---|---|
| 1 | 7 1/2 in | 0 in |
| 2 | 15 in | 10 in |
| 3 | 22 1/2 in | 20 in |
| 4 | 30 in | 30 in |
| 5 | 37 1/2 in | 40 in |
| 6 | 45 in | 50 in |
| 7 | 52 1/2 in | 60 in |
| 8 | 60 in | 70 in |
| 9 | 67 1/2 in | 80 in |
| 10 | 75 in | 90 in |
| 11 | 82 1/2 in | 100 in |
| 12 | 90 in | 110 in |
| 13 | 97 1/2 in | 120 in |
| 14 | 105 in | 130 in |
Check the layout on a spare board first. The final stringer count and fastening depend on your local code. Nothing you type is saved or sent anywhere.
How the stringer layout works
Steps come from the total rise ÷ the riser you want, rounded, so riser = total rise ÷ steps. Mark the board with a framing square at the riser on one blade and the tread on the other. Throat depth = board width − (riser × tread ÷ √(riser² + tread²)); under about 3 1/2 in the stringer is weak at its deepest notch. The bottom riser is cut shorter by the tread thickness, because the tread adds that height back. Board length: the slope line √(rise² + run²), rounded up to the next stock length. Stringers = the stair width less one stringer thickness ÷ the most you allow between them, rounded up, plus 1.
| Riser and tread | On a 2×10 (9 1/4 in) | On a 2×12 (11 1/4 in) |
|---|---|---|
| 7 in and 11 in | 3.34 in | 5.34 in |
| 7.5 in and 10 in | 3.25 in | 5.25 in |
| 7.75 in and 10 in | 3.12 in | 5.12 in |
| Total rise | Steps | Slope length | Stock board |
|---|---|---|---|
| 60 in | 8 | 7.68 ft | 8 ft |
| 84 in | 11 | 10.88 ft | 12 ft |
| 105 in | 14 | 13.93 ft | 14 ft |
| 120 in | 16 | 16.01 ft | 18 ft |
Worked example: 105 in rise, 10 in tread, 2×12 board, 36 in wide stairs
- Steps: 105 ÷ 7.5 = 14 risers of exactly 7 1/2 in. Treads: 13, total run 130 in.
- Framing square: 7 1/2 in on one blade and 10 in on the other.
- Throat depth on a 2×12: 11.25 − (7.5 × 10 ÷ 12.5) = 5.25 in. Enough wood remains.
- Bottom cut: with 1 in treads, the first riser is cut to 6 1/2 in.
- Board length: slope line √(105² + 130²) = 167.1 in = 13.93 ft, so a 14 ft board. A 36 in stair with stringers no more than 18 in apart needs 3.
Common questions
What is a stair stringer?
The sloped board that carries the treads. For cut stringers, notches are cut into a 2×10 or 2×12 so each tread has a flat seat and each riser a vertical face.
How do I use a framing square to lay out a stringer?
Set the riser measurement on one blade and the tread on the other, so both numbers touch the top edge of the board. Mark the notch, slide the square along and repeat for each step. Stair gauges clamped at those two numbers keep it consistent.
Why is the bottom riser cut shorter?
Because the treads sit on top of the stringer. Cutting the bottom step short by the tread thickness makes the first step from the ground the same height as all the others once the tread is on.
What is the throat of a stringer?
The least wood left between the inside corner of a notch and the bottom edge of the board. A thin throat can split under load, so many builders want at least 3 1/2 in left.
How many stringers do I need?
It depends on the width, the tread material and your local rules. Treads that span a long way need more support. This page lets you enter the most you allow between stringers, and counts them for you.
Should I use a 2×10 or a 2×12?
A 2×12 leaves more throat, so it suits taller risers and shorter treads. A 2×10 can work with a lower riser and a longer tread. The throat figures above show the difference for your numbers.
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Sources
- International Residential Code, chapter 3 (R311.7 stairways): riser, tread and headroom limits. Local codes can differ (checked October 28, 2026)
- OSHA 1926.1052: stairways on construction sites (riser and tread uniformity) (checked October 28, 2026)
- Stairs: rise, run and the 2 x riser + tread comfort rule (checked October 28, 2026)
This calculator is an estimate for planning. Check the layout on a spare board before you cut, and confirm stringer spacing, lumber grade and fastening with your building department.