How do you calculate the number of steps in a staircase?
Divide the total rise (floor-to-floor height) by the maximum riser of 7¾ inches and round UP — that's the minimum number of risers (2018 IRC §R311.7.5.1). Then the actual riser height is the total rise divided by that count. Example: 108 inches ÷ 7¾ = 13.9 → 14 risers, each 108 ÷ 14 ≈ 7.71 inches. Treads equal the risers minus one.
Source: 2018 IRC §R311.7.5.1 · Verify against your AHJ and the adopted code edition.
Start with the total rise — the finished floor-to-finished-floor height the stair has to climb. Then:
- Divide by the maximum riser, 7¾ inches (IRC §R311.7.5.1), and round up. That’s the fewest risers that keep every step at or under the code max.
- Actual riser = total rise ÷ number of risers. Dividing back out gives an even riser height for the whole flight (they must all match within ⅜ inch).
- Treads = risers − 1. The top riser lands on the upper floor, so there’s always one fewer tread than riser.
Example — 9 ft (108 in) floor-to-floor:
- 108 ÷ 7.75 = 13.9 → 14 risers
- 108 ÷ 14 = 7.71 in per riser
- 13 treads
Each tread also needs a minimum 10-inch run (§R311.7.5.2), so 13 treads is about 130 inches of horizontal run — check that it fits before you frame. Codes and amendments vary — verify with your AHJ.
Work it out: the stair calculator does this instantly — enter the total rise and it returns the riser count, exact riser height, run, and pitch.
Sources
- ICC — International Codes (IRC) — 2018 IRC §R311.7.5.1, §R311.7.5.2