Stair Calculator
Enter the floor-to-floor rise and the tread you want. The calculator works out the number of risers, the exact riser height, total run, stringer length and angle, and checks it all against the residential building code.
Your staircase
Code & comfort checks
Checks follow the 2021 International Residential Code (IRC R311.7). Local codes can differ – confirm with your building department.
Stringer layout
| Step | Height above floor | Distance from first riser |
|---|
How to use the Stair Calculator
- Measure the total rise from the finished lower floor to the finished upper floor (or deck surface).
- Enter the maximum riser height and tread run you want, plus the nosing overhang.
- Add width and headroom if you want them checked against code.
- Press Calculate stairs to see the riser count, riser height, total run, stringer length and angle.
- Review the code checks and use the layout table to mark each step on the stringer.
Stair formulas
- Number of risers n = ⌈total rise ÷ maximum riser⌉
- Riser height R = total rise ÷ n
- Treads = n − 1 when the upper floor is the top step (most stairs), otherwise n
- Total run = treads × tread run T
- Stringer length = √(total rise² + total run²)
- Angle = arctan(R ÷ T)
- Comfort: 2R + T ≈ 24–25 in (61–64 cm)
Worked example: total rise 108 in, maximum riser 7.75 in, tread run 10 in. 108 ÷ 7.75 = 13.94, so 14 risers of 7.714 in (about 7 11/16″). 13 treads × 10 in = 130 in of run. Stringer = √(108² + 130²) = √28,564 = 169.0 in, so a 16 ft board. Angle = arctan(7.714 ÷ 10) = 37.6°, and 2R + T = 25.4 in, a little steep, so a 10.5 in tread would be more comfortable.
IRC residential stair requirements
| Item | Requirement | IRC section |
|---|---|---|
| Riser height | 7¾ in (197 mm) maximum | R311.7.5.1 |
| Tread depth | 10 in (254 mm) minimum, nosing to nosing | R311.7.5.2 |
| Riser / tread variation | No more than 3/8 in (9.5 mm) within a flight | R311.7.5 |
| Nosing | ¾ to 1¼ in when treads are less than 11 in | R311.7.5.3 |
| Headroom | 6 ft 8 in (2032 mm) minimum | R311.7.2 |
| Width | 36 in (914 mm) minimum above the handrail | R311.7.1 |
| Rise of one flight | 12 ft 7 in (3835 mm) maximum between landings | R311.7.3 |
| Handrail | Required with 4 or more risers, 34–38 in high | R311.7.8 |
Commercial buildings (IBC) are stricter: 7 in maximum riser and 11 in minimum tread. Always check your local code before building.
Common rise and run combinations
| Total rise | Risers | Riser height | Treads at 10.5 in | Total run |
|---|---|---|---|---|
| 24 in (deck) | 4 | 6.0 in | 3 | 31.5 in |
| 36 in | 5 | 7.2 in | 4 | 42 in |
| 96 in (8 ft) | 13 | 7.38 in | 12 | 126 in |
| 108 in (9 ft) | 15 | 7.2 in | 14 | 147 in |
| 120 in (10 ft) | 16 | 7.5 in | 15 | 157.5 in |
Rows use a 7.5 in maximum riser. Tip: measure total rise at the actual stair location with a level and straightedge, because floors are rarely perfectly flat, and remember to include finished flooring thickness at top and bottom.
Frequently asked questions
How do I calculate the number of stairs I need?
Divide the total rise by the maximum riser height and round up. For a 108 in rise and a 7.5 in maximum riser, 108 ÷ 7.5 = 14.4, so you need 15 risers of 108 ÷ 15 = 7.2 in each. With the upper floor as the top step, you need 14 treads.
What is the maximum riser height allowed by code?
The International Residential Code limits risers to 7¾ in (19.7 cm) and requires treads of at least 10 in (25.4 cm). The largest and smallest riser in a flight may differ by no more than 3/8 in, which is why the calculator makes all risers equal.
How long should a stair stringer be?
The stringer length is the hypotenuse of the total rise and the total run: √(rise² + run²). For a 108 in rise and 147 in run that is about 182.4 in, so you would buy a 16 ft board.
What is the 2R + T rule?
It is a comfort rule: twice the riser height plus the tread run should be 24 to 25 in (61 to 64 cm), matching a normal walking stride. For example 2 × 7.2 + 10.5 = 24.9 in feels comfortable.
How much headroom do stairs need?
The IRC requires at least 6 ft 8 in (80 in, 203 cm) of clear headroom measured vertically from the sloped line connecting the tread nosings.
Is tread run the same as tread depth?
Tread run is the horizontal distance from one riser to the next, which is what the code calls tread depth. The actual tread board is deeper by the nosing overhang, usually ¾ to 1¼ in.