How to use the stair calculator
- Measure the total rise: the vertical distance from the finished lower floor to the finished upper floor or deck surface. Measure at the spot where the stairs will land, since floors are rarely level.
- Keep the maximum riser at 7.75 in unless your local code says otherwise. Lower risers make gentler stairs that take more floor space.
- Enter your tread depth. Treads are measured from the front edge of one step to the front edge of the next.
The formula
Risers = total rise ÷ max riser, rounded up. Riser height = total rise ÷ risers. Treads = risers − 1. Total run = treads × tread depth. Stringer length = √(rise² + run²).
Worked example: a 9 ft floor-to-floor rise
108 in ÷ 7.75 = 13.9, so 14 risers of 108 ÷ 14 = 7.71 in. That gives 13 treads; at 10 in each the stairs run 130 in (10 ft 10 in). The stringer is √(108² + 130²) = 169 in, about 14 ft 1 in.
Stairs for common rises
7.75 in maximum riser, 10 in treads.
| Total rise | Risers | Riser height | Total run | Stringer |
|---|---|---|---|---|
| 2 ft 0.0 in | 4 | 6.00 in | 2 ft 6.0 in | 3 ft 2.4 in |
| 3 ft 0.0 in | 5 | 7.20 in | 3 ft 4.0 in | 4 ft 5.8 in |
| 4 ft 0.0 in | 7 | 6.86 in | 5 ft 0.0 in | 6 ft 4.8 in |
| 8 ft 0.0 in | 13 | 7.38 in | 10 ft 0.0 in | 12 ft 9.7 in |
| 9 ft 0.0 in | 14 | 7.71 in | 10 ft 10.0 in | 14 ft 1.0 in |
| 10 ft 0.0 in | 16 | 7.50 in | 12 ft 6.0 in | 16 ft 0.1 in |
Code basics (IRC)
- Maximum riser 7 3/4 in, minimum tread 10 in, and no more than 3/8 in variation between steps (R311.7.5).
- Minimum headroom 6 ft 8 in measured from the tread nosings (R311.7.2).
- Minimum stair width 36 in; handrails 34 to 38 in above the tread nosings (American Wood Council DCA 6).