How to use the stair calculator
Measure the total rise: the vertical distance from the finished floor at the bottom to the finished floor at the top. Measure with the final flooring in mind. If the bottom will get 3/4 in hardwood later, the rise shrinks by 3/4 in, and your bottom step would come out short.
Enter the riser height you'd like. The calculator rounds the riser count up so no riser is taller than that, then splits the rise evenly. Enter the tread run (the horizontal depth of each step, measured nosing to nosing) and the stair width. The code checks update as you type.
The math
- Risers = total rise ÷ target riser, rounded up
- Riser height = total rise ÷ risers
- Treads = risers − 1 (the top floor acts as the last step)
- Total run = treads × tread run
- Stringer length = risers × √(riser² + tread²)
- Comfort check: 2 × riser + tread should land near 24 to 25 in
Stair layouts by total rise
Target riser 7 1/2 in, tread run 10 1/2 in:
| Total rise | Risers | Riser height | Treads | Total run | Stringer |
|---|---|---|---|---|---|
| 24 in | 4 | 6 in | 3 | 2 ft 7 1/2 in | 4 ft 3/8 in |
| 30 in | 4 | 7 1/2 in | 3 | 2 ft 7 1/2 in | 4 ft 3 5/8 in |
| 36 in | 5 | 7 3/16 in | 4 | 3 ft 6 in | 5 ft 3 11/16 in |
| 48 in | 7 | 6 7/8 in | 6 | 5 ft 3 in | 7 ft 3 13/16 in |
| 96 in | 13 | 7 3/8 in | 12 | 10 ft 6 in | 13 ft 10 7/8 in |
| 102 in | 14 | 7 5/16 in | 13 | 11 ft 4 1/2 in | 14 ft 10 15/16 in |
| 105 in | 14 | 7 1/2 in | 13 | 11 ft 4 1/2 in | 15 ft 5/8 in |
| 108 in | 15 | 7 3/16 in | 14 | 12 ft 3 in | 15 ft 11 in |
| 112 in | 15 | 7 7/16 in | 14 | 12 ft 3 in | 16 ft 1 1/4 in |
| 120 in | 16 | 7 1/2 in | 15 | 13 ft 1 1/2 in | 17 ft 2 7/16 in |
IRC stair limits
| Rule | Limit | Section |
|---|---|---|
| Riser height | 7 3/4 in max | R311.7.5.1 |
| Tread depth | 10 in min | R311.7.5.2 |
| Largest minus smallest riser or tread | 3/8 in max | R311.7.5.1, .2 |
| Nosing (where tread is under 11 in) | 3/4 to 1 1/4 in | R311.7.5.3 |
| Headroom | 6 ft 8 in min | R311.7.2 |
| Stair width | 36 in min | R311.7.1 |
| Handrail height | 34 to 38 in | R311.7.8 |
The layout is a planning aid. Stair dimensions, guards, handrails, headroom and landings are set by your local building code and are checked at inspection. Confirm your plan with your building department before you cut stringers.
Worked example: deck stairs
The deck surface is 31 in above the patio. You'd like risers near 7 1/2 in and 11 in treads (two 5.5 in boards).
- Risers: 31 ÷ 7.5 = 4.13, rounded up = 5.
- Riser height: 31 ÷ 5 = 6.2 in, about 6 3/16 in.
- Treads: 5 − 1 = 4; total run 4 × 11 = 44 in.
- Stringer: 5 × √(6.2² + 11²) = 5 ft 3 1/8 in, so a 8 ft 2 × 12.
- Comfort: 2 × 6.2 + 11 = 23.4 in, inside the comfortable range.
Questions people ask
How many steps do I need for 8 feet?
An 8 ft (96 in) rise is usually 13 risers of 7 3/8 in, which means 12 treads. 14 risers of 6 7/8 in make a gentler stair but need one more tread of floor space. Floor-to-floor height is more than the ceiling height, because it includes the floor framing, so measure the actual rise.
What is the most comfortable riser height?
Between about 7 and 7 1/2 in, paired with a tread of 10 1/2 to 11 in. Use the 2 × riser + tread rule: 24 to 25 in feels natural to most people.
Do treads count the nosing?
The IRC measures tread depth from the front of one nosing to the front of the next, which equals the run of each step. The nosing projection itself (3/4 to 1 1/4 in) is added to the tread board's depth.