Stair Stringer Calculator
Lay out and cut stair stringers right the first time: riser and tread marks, the bottom cut for tread thickness, board length to buy, wood left under the cuts and how many stringers you need.
Set a framing square to the riser height and tread depth and step it along a 2×12 once per tread. Stringer length ≈ √(rise² + run²) plus one step. For a 36 in deck rise: 5 risers of 7 ³⁄₁₆ in, 4 treads of 11 in, a stringer about 5.8 ft long, and 3 stringers for a 36 in wide stair.
How to lay out and cut a stringer
- Clamp stair gauges to a framing square: one at the riser height on the tongue, one at the tread depth on the blade.
- Mark each step along the crown edge of a straight 2×12, sliding the square so the gauges touch the edge each time.
- Cut the bottom short by one tread thickness (the "bottom cut" above) so the first step isn't taller than the rest.
- Cut with a circular saw, finish with a handsaw so you don't over-cut the inside corners and weaken the throat.
- Use the first stringer as a template for the others, then check it on site before cutting the rest.
Stair dimensions for common heights
Using the 7.75 in maximum riser and 10.5 in treads. Treads are one fewer than risers because the top step is the floor or deck.
| Total rise | Risers | Riser height | Treads | Total run | Stringer length |
|---|---|---|---|---|---|
| 24 in (2 ft) | 4 | 6 in | 3 | 2.6 ft | 4.3 ft |
| 30 in (2.5 ft) | 4 | 7 1/2 in | 3 | 2.6 ft | 4.7 ft |
| 36 in (3 ft) | 5 | 7 3/16 in | 4 | 3.5 ft | 5.7 ft |
| 48 in (4 ft) | 7 | 6 7/8 in | 6 | 5.3 ft | 7.6 ft |
| 60 in (5 ft) | 8 | 7 1/2 in | 7 | 6.1 ft | 9.0 ft |
| 72 in (6 ft) | 10 | 7 3/16 in | 9 | 7.9 ft | 11.0 ft |
| 84 in (7 ft) | 11 | 7 5/8 in | 10 | 8.8 ft | 12.3 ft |
| 96 in (8 ft) | 13 | 7 3/8 in | 12 | 10.5 ft | 14.3 ft |
| 108 in (9 ft) | 14 | 7 11/16 in | 13 | 11.4 ft | 15.6 ft |
| 120 in (10 ft) | 16 | 7 1/2 in | 15 | 13.1 ft | 17.6 ft |
| 132 in (11 ft) | 18 | 7 5/16 in | 17 | 14.9 ft | 19.6 ft |
| 144 in (12 ft) | 19 | 7 9/16 in | 18 | 15.8 ft | 20.9 ft |
Frequently asked questions
How do you calculate a stair stringer?
Find the riser height (total rise ÷ number of risers) and pick a tread depth. The stringer length is the hypotenuse: √(total rise² + total run²), plus one more step diagonal so the bottom bears fully on the ground or pad. Lay out each step with a framing square set to the riser and tread.
What length board do I need for stair stringers?
Round the stringer length up to the next stock length (8, 10, 12, 14 or 16 ft). For a 108 in rise with 10 ½ in treads the stringer is about 15.6 ft, so buy 16 ft 2×12s.
Why do you cut the bottom of a stair stringer shorter?
So every step is the same height once treads are on. The treads add their thickness to each step, so the bottom riser on the stringer is cut shorter by one tread thickness. The calculator shows this "bottom cut" for you.
How much wood should remain under a stringer cut?
Keep at least 5 in of solid wood (the "throat") between the bottom edge and the inside corner of each cut, as the AWC deck guide requires for cut stringers. A 2×12 usually leaves 5 to 6 in; a 2×10 often does not.
Can I use 2×10 for stair stringers?
Only if the throat stays at least 5 in, which with typical risers and treads it does not. 2×12 is the standard choice for cut stringers.
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Code limits from the 2021 International Residential Code, section R311.7; stringer spacing (18 in) and minimum throat (5 in) from the American Wood Council DCA 6 deck guide. Board depths are actual sizes (2×12 = 11 ¼ in). Your local code and inspector have the final say.