Stair Riser and Tread Calculator
Calculate stair step count, riser height, and tread depth from total rise and run.
Checks against IRC building code limits for safe, comfortable stairs.
Stair layout is one of those calculations where being off by a fraction of an inch matters — uneven risers cause people to trip because they expect every step to feel the same.
The total rise is the vertical distance from the lower floor to the upper floor (or deck surface).
The total run is the horizontal distance the stairs can occupy.
These two numbers determine everything else.
The target riser height for comfortable stairs is 7 to 7.5 inches.
The IRC residential building code sets hard limits:
- Maximum riser height: 7.75 inches
- Minimum riser height: 4 inches
- Minimum tread depth: 10 inches (measured from nosing to nosing)
A useful comfort check: riser + tread should be between 17 and 18 inches.
At 7.5-inch riser + 10-inch tread = 17.5 inches — ideal.
The calculation:
n_steps = round(total_rise ÷ 7.0) riser = total_rise ÷ n_steps tread = total_run ÷ n_steps
The number of treads equals the number of risers.
On an exterior deck stair, the top tread is usually the deck itself, so the stringer typically has one fewer tread than risers.
If the total run is fixed (a tight space), you may need to adjust step count to keep tread depth legal and comfortable.
This calculator shows the result at the calculated step count and one step up and down, so you can choose the best fit.
Stringer length: use the Pythagorean theorem — stringer length = √(total_rise² + total_run²).
Add 6 to 8 inches for the top attachment and cut allowances.