How to use the stair calculator
- Measure the total rise from finished floor to finished floor, including flooring on both levels.
- Enter it. The calculator picks the number of risers that gives a comfortable step, and derives the tread depth from the step formula.
- If your space is limited, enter the available run. The tread depth then comes from the space, and the calculator picks the riser count that fits the step formula best.
- Use − and + to try more or fewer steps. The checks below the result show whether the stair stays comfortable and within code.
The rules behind a comfortable stair
A stair feels natural when rise and tread match the length of an average step. Three rules of thumb describe this, with R as riser height and T as tread depth:
| Rule | Formula | Target |
|---|---|---|
| Step formula | 2R + T | 24–25½ in (59–65 cm), ideal about 24¾ in |
| Safety rule | T + R | about 18 in (46 cm) |
| Comfort rule | T − R | about 4¾ in (12 cm) |
The number of treads is always one less than the number of risers, because the top step is the upper floor itself.
US residential code limits (IRC)
For one- and two-family homes the International Residential Code sets these main limits for stairs:
| Item | Limit |
|---|---|
| Riser height | max. 7¾ in |
| Tread depth | min. 10 in |
| Headroom | min. 6 ft 8 in |
| Width | min. 36 in |
Local codes can differ and change. Treat these values as a planning aid and confirm them with your building department before you build.
Frequently asked questions
How many steps do I need for 9 ft ceilings?
With a 9 ft ceiling and about 12 in of floor structure the total rise is around 108 in. That gives 15 or 16 risers of roughly 7 in each.
Why does the calculator show fewer treads than risers?
You step onto the upper floor with the last riser, so there is one tread fewer than risers.
Can I use metric units?
Yes. Switch between in and cm above the input fields; your values are converted.