120 Inch Stair Stringer Layout
A 120 in floor-to-floor rise for a basement or a tall interior stair, divided into equal risers.
Formula SSC-2026.1
IRC check pass
Risers / treads
16 / 15
Riser inside 4 to 7.75 in
Exact riser
7 1/2 in
190.5 mm
Total run
13 ft 1 1/2 in
4,001 mm
Stringer length
16 ft 6 in
Order 18 ft 2x12
Incline
37.30°
Square 7 1/2 in rise · 10 1/2 in run
Stringers @ 16 in
3
Throat 5.15 in on a 2x12
Rise, run, and the incline are scaled from the live riser and tread. Framing-square gauges match the exact riser and the tread depth.
| Item | Net | Waste | Order | Unit price | Line total |
|---|---|---|---|---|---|
| 18 ft 2x12 | 17.13 ft cut | end cuts included | 3 boards | USD | $113.40 |
| Stringer delivery weight | 151 lb | — | 69 kg | 2.8 lb/ft planning weight | — |
| Total rise | Risers | Exact riser | Treads | Run | 2x12 stock | Check |
|---|---|---|---|---|---|---|
| 10 ft 0 in | 16 | 7 1/2 in | 15 | 13 ft 1 1/2 in | 18 ft | Pass |
| 11 ft 0 in | 18 | 7 5/16 in | 17 | 14 ft 10 1/2 in | 20 ft | Pass |
| 12 ft 0 in | 19 | 7 9/16 in | 18 | 15 ft 9 in | 24 ft | Pass |
Show the deterministic formula
Risers = round(total rise ÷ target rise). Exact riser = total rise ÷ risers, reported to the nearest 1/16 in and in millimeters. Treads = risers − 1. Total run = treads × tread depth.
Diagonal = hypotenuse of stringer rise and total run. A dropped header uses total rise minus landing thickness. Stock is the next 8, 10, 12, 14, 16, 18, 20, or 24 ft board.
Stringers = floor(width ÷ 16) + 1, with a minimum of 2. Throat ≈ 11.25 − (riser × tread) ÷ hypotenuse.
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Project: 120 Inch Stair Stringer Layout
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This sheet is an estimate for ordering and layout discussion. It is not a sealed design. Confirm the quantities with a licensed local professional and the authority having jurisdiction before you buy or build.
Engineering notes
The 120 in total rise is divided by a 7.5 in target and rounded to a whole riser count. Every riser is then the same height, reported to the nearest sixteenth and in millimetres. Treads are one fewer than risers. The diagonal is the hypotenuse of that rise and the total run, and the 2x12 stock is the next standard length. A rise over 7.75 in or under 4 in, or a tread under 10 in, is flagged.
Code reference
IRC R311.7.5.1 limits risers to 7.75 in and R311.7.5.2 requires at least a 10 in tread. Confirm local amendments, guards, and headroom.
Field questions
- How many risers does a 120 in rise need?
- The count is the total rise divided by 7.5 in, rounded to the nearest whole number, then each riser is total rise divided by that count.
- Why is the tread count smaller?
- The upper floor is the last riser. The stringer cuts one fewer tread than the number of risers.
- Does this include headroom?
- No. IRC headroom, guards, and landings still have to be checked on the building.
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