Stair rise, run, and riser count
Divide the total finished rise into equal risers, use one fewer tread for a typical floor-to-floor flight, and verify every dimension against the locally adopted code before cutting stringers.
108 in total rise
7.5 in maximum target riser
The stair calculation
Rounding the riser count up lowers the actual riser height. Do not round the actual height to a convenient fraction until the full layout has been checked; small differences accumulate across the flight.
Examples for a 108-inch total rise
| Maximum target riser | Risers | Actual riser | Treads | Run at 10 in |
|---|---|---|---|---|
| 7.75 in | 14 | 7.714 in | 13 | 130 in |
| 7.5 in | 15 | 7.2 in | 14 | 140 in |
| 7.0 in | 16 | 6.75 in | 15 | 150 in |
These are geometry examples, not approval of a particular stair design.
Measure finished surface to finished surface
Total rise is the vertical distance from the finished lower walking surface to the finished upper surface. Flooring, decking, landings, and tread thickness can change that distance. The first and last risers are common trouble spots because material thickness is overlooked.
A typical straight floor-to-floor flight has one fewer tread than risers because the upper floor serves as the final walking surface. Deck stairs, landings, winders, and unusual configurations may require a different layout.
Code and field checks
The 2021 International Residential Code model provisions list a maximum 7¾-inch riser, minimum 10-inch tread depth, and no more than ⅜ inch difference between the largest and smallest riser or tread in a flight. Local jurisdictions may adopt a different edition or amendments, and stair width, headroom, landings, nosings, guards, handrails, lighting, and structural connections have separate requirements.
- Confirm the adopted code. Ask the local building department which rules apply.
- Measure the finished rise. Account for every finish layer.
- Lay out equal risers. Verify the top and bottom conditions before cutting.
- Check available run and headroom. The stair must fit the space as a complete system.
- Have critical work reviewed. Incorrect stairs create serious fall hazards.
Frequently asked questions
Why does the calculator round the number of risers up?
If a division produces a fraction, using fewer risers would make each riser taller than the selected maximum target.
Why are there fewer treads than risers?
In a common floor-to-floor arrangement, the upper floor acts as the final tread surface. Confirm the actual configuration before using that assumption.
Can I use the calculated angle to approve a stair?
No. Angle is a planning aid; code compliance depends on many individual dimensions and conditions.