12-Step Stair Stringer: Dimensions, Length & Cut Angle
Twelve steps is a short full flight — common in older homes with 8-foot floor-to-floor heights. Every row is computed by the same engine as our interactive calculator.
12-step stringer dimensions · 10.5" tread
| Total Rise | Rise per Step | Total Run | Stringer Length | Cut Angle |
|---|---|---|---|---|
| 86" (7' 2") | 7.17" (7 1/8") | 115.5" | 144.0" (144") | 34.3° |
| 87" (7' 3") | 7.25" (7 1/4") | 115.5" | 144.6" (144 5/8") | 34.6° |
| 88" (7' 4") | 7.33" (7 5/16") | 115.5" | 145.2" (145 3/16") | 34.9° |
| 89" (7' 5") | 7.42" (7 3/8") | 115.5" | 145.8" (145 13/16") | 35.2° |
| 90" (7' 6") | 7.50" (7 1/2") | 115.5" | 146.4" (146 7/16") | 35.5° |
| 91" (7' 7") | 7.58" (7 9/16") | 115.5" | 147.0" (147 1/16") | 35.8° |
| 92" (7' 8") | 7.67" (7 5/8") | 115.5" | 147.7" (147 11/16") | 36.1° |
| 93" (7' 9") | 7.75" (7 3/4") | 115.5" | 148.3" (148 5/16") | 36.4° |
Total run is fixed at 115.5" for 12 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 = 11 × 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 12 steps?
Twelve steps is a short full flight — common in older homes with 8-foot floor-to-floor heights.
A 12-step stringer covers total rises from 86" (7' 2") to 93" (7' 9") while keeping every riser at or under the IRC maximum of 7¾" (R311.7.5.1). Below 86", use a 11-step stringer; above 93", move to 13 steps.
What board do you need?
The longest stringer in the table is 148.3", so a 14-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 12-step stair
- Handrail required. 12 risers is at or over the 4-riser threshold — a graspable handrail is required on at least one side (IRC R311.7.8).
- Guards required. Every total rise in the 12-step range exceeds 30", so open sides of the walking surface need 36" guards (IRC R312.1.1–.2) — see the baluster spacing calculator.
- Riser uniformity: the largest and smallest riser may differ by no more than 3/8" (IRC R311.7.5.1) — divide total rise by 12 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.