How to use the stair calculator
- Measure total rise from the top of the finished lower floor to the top of the finished upper floor. If the flooring isn't down yet, add its thickness (or the difference between the two floors) now. A finish-floor mistake is the most common reason the first or last step comes out a different height.
- Enter a tread depth. This is the horizontal run of each step, nosing to nosing, not the width of the tread board. 10" to 11" is typical.
- Read the risers. The calculator divides total rise by the max riser and rounds up, so every riser is equal and under the limit. A flight always has one fewer tread than risers, because the upper floor acts as the last step.
Worked example
A basement stair with 108" total rise and a 10¼" tread:
- 108 ÷ 7.75 = 13.9, so round up to 14 risers.
- 108 ÷ 14 = 7.714", or 7 11/16" per riser.
- 13 treads × 10¼" = 11'-1 1/4" total run.
- Stringer length √(108² + 133.25²) ≈ 14'-3 1/2", so a 16' board gives room for the cuts.
Laying out the stringer
Set stair gauges (or clamps) on a framing square at the riser height on the tongue and tread depth on the blade, then step the pattern down the board. Two adjustments people forget:
- Drop the stringer. Cut the bottom riser short by the thickness of one tread. Once the treads are on, the first step comes out the same height as all the others.
- Top connection. Whether the top tread sits flush with the floor or one step down depends on how the stringer hangs on the header. Sketch it before cutting.
The comfort rule
Two risers plus one tread should land between about 24" and 25". It's an old rule of thumb that matches a normal walking stride. The calculator flags flights that come out steep or shallow, but they can still meet code.
FAQ
How do I calculate how many stairs I need?
Divide the total rise (finished floor to finished floor) by the maximum riser height, 7¾" under the IRC, and round up. That gives the number of risers. The number of treads is one less.
What is the maximum stair riser height?
The 2021 International Residential Code allows a maximum riser of 7¾" and a minimum tread of 10" for houses. Commercial stairs under the IBC are stricter: 7" max riser and 11" min tread. Local codes can amend these.
How long should a stair stringer be?
The stringer is the diagonal of the total rise and total run, √(rise² + run²). Buy stock at least a foot longer than that number to allow for the top and bottom cuts.
Why is my bottom step a different height?
Usually because the stringer was not dropped by the tread thickness, or because finished flooring was not included in the total rise. Measure finished floor to finished floor, and cut the bottom of the stringer down by one tread thickness.
How much can stair risers vary?
Under the IRC, the tallest and shortest riser in a flight can differ by no more than ⅜". The same limit applies to tread depths.