3-Step Stair Stringer: Dimensions, Length & Cut Angle
Three steps is the classic front-porch and low-deck stair, covering surfaces about 16 to 23 inches up. Every row is computed by the same engine as our interactive calculator.
3-step stringer dimensions · 10.5" tread
| Total Rise | Rise per Step | Total Run | Stringer Length | Cut Angle |
|---|---|---|---|---|
| 16" (1' 4") | 5.33" (5 5/16") | 21" | 26.4" (26 3/8") | 26.9° |
| 17" (1' 5") | 5.67" (5 5/8") | 21" | 27.0" (27") | 28.4° |
| 18" (1' 6") | 6.00" (6") | 21" | 27.7" (27 11/16") | 29.7° |
| 19" (1' 7") | 6.33" (6 5/16") | 21" | 28.3" (28 5/16") | 31.1° |
| 20" (1' 8") | 6.67" (6 5/8") | 21" | 29.0" (29") | 32.4° |
| 21" (1' 9") | 7.00" (7") | 21" | 29.7" (29 11/16") | 33.7° |
| 22" (1' 10") | 7.33" (7 5/16") | 21" | 30.4" (30 7/16") | 34.9° |
| 23" (1' 11") | 7.67" (7 5/8") | 21" | 31.1" (31 1/8") | 36.1° |
Total run is fixed at 21" for 3 steps with a 10.5" tread: (steps − 1) × tread, because the upper floor or deck acts as the top step. For a different tread depth, total run = 2 × tread.
Rise per step is floored to the nearest 1/16" for framing-square layout. Different total rise? Use the interactive stringer calculator.
When do you need 3 steps?
Three steps is the classic front-porch and low-deck stair, covering surfaces about 16 to 23 inches up.
A 3-step stringer covers total rises from 16" (1' 4") to 23" (1' 11") while keeping every riser at or under the IRC maximum of 7¾" (R311.7.5.1). Below 16", use a 2-step stringer; above 23", move to 4 steps.
What board do you need?
The longest stringer in the table is 31.1", so a 8-foot 2×12 covers every case with layout margin. Use solid, straight-grained 2×12 stock — after notching, the remaining "throat" must keep at least 3.5" of material for structural integrity.
Code checkpoints for a 3-step stair
- No handrail required by the IRC at 3 risers (the threshold is 4, per R311.7.8) — but many builders add one anyway, and local amendments can differ.
- Guards generally not required — the whole 3-step range stays at or under the 30" trigger (IRC R312.1.1), though landings and local rules can change that.
- Riser uniformity: the largest and smallest riser may differ by no more than 3/8" (IRC R311.7.5.1) — divide total rise by 3 and mark every notch from the same reference edge.
- Drop the bottom riser by one tread thickness so the first step matches the rest after treads go on.