A straight stair between floors 9 ft apart — 108 in finished floor to finished floor — takes 15 risers of 7.2 in and 14 treads of 10½ in. It runs 12 ft 3 in, climbs at 34.4°, and its stringers are 15 ft 2⅜ in long, cut from 16 ft 2x12s. For 6 ft 8 in of headroom the floor opening above must be at least 10 ft 11¼ in long. The calculator above lays out straight, L-shaped and U-shaped stairs, checks them against the IRC and shows them in 3D.
How the calculator works
Choose the stair type and enter the total rise, the target riser and the stair width. Then either set the tread depth or let the calculator fit the treads to the run you have, and enter the tread thickness, the nosing, the stringer stock, the handrail height and the floor above. The result is the risers, treads, run, angle and comfort, the stringers and their throat, the floor opening for headroom, the railing, and the parts list.
Risers and treads
Risers are the total rise divided by the target riser, rounded up — never down, or the steps get taller than the target. A flight has one tread fewer than risers: the floor above is the top step. The IRC limits risers to 7¾ in and treads to at least 10 in, with a ¾–1¼ in nosing where treads are under 11 in (R311.7.5).
| Floor to floor | Risers | Riser | Treads at 10½ in | Run |
|---|---|---|---|---|
| 8 ft 1⅛ in (97⅛ in) | 13 | 7.47 in | 12 | 10 ft 6 in |
| 9 ft (108 in) | 15 | 7.20 in | 14 | 12 ft 3 in |
| 10 ft (120 in) | 16 | 7.50 in | 15 | 13 ft 1½ in |
Comfort
Two risers plus one tread should come to about 24–25 in (600–640 mm): 2 × 7.2 + 10.5 = 24.9 in is comfortable. Stairs between 30° and 37° feel natural; above 42° they turn into a ladder.
Stringers
The stringer follows the nosings: √(108² + 147²) = 182.4 in. Each notch cuts riser × tread ÷ √(riser² + tread²) into the board — 6 in here — so a 2x12 (11¼ in deep) keeps 5.3 in of throat. Space stringers no more than 16 in apart under 1 in treads: four for a 36 in stair.
Headroom and the floor opening
The IRC asks for 6 ft 8 in of clear height above the nosing line (R311.7.2). With a 10 in floor above, the underside of the floor is at 98 in; the nosing line reaches 98 − 80 = 18 in only 15¾ in from the foot of the stair, so from there on the floor must be open — 10 ft 11¼ in of opening on a 12 ft 3 in run.
Turning stairs
An L-shaped stair gives the stair width of the run to the turn, a U-shaped stair turns back on itself and each flight gets half of the rest. A landing must be at least as deep as the stair is wide and at least 36 in (R311.7.6); winders need 10 in of tread at the walkline and 6 in at the narrow end (R311.7.5.2.1).
Common mistakes
- Measuring the rise to the subfloor. Use finished floors, or the top and bottom steps come out different.
- As many treads as risers. One fewer — otherwise the run is a tread too long.
- Forgetting headroom. The floor opening, not the stair, is usually the limit.
For the lumber, use the Board Foot Calculator; for deck stairs, the Deck Calculator.