How the stairs calculator works
Risers = total rise ÷ maximum riser height, rounded up
Riser height = total rise ÷ risers
Treads = risers − 1
Total run = treads × tread depth
Stringer length = √(total rise² + total run²)
Worked example
A 108 inch total rise with a 7 3/4 inch maximum needs 14 risers of 7.71 inches. That gives 13 treads; at 10 inches each the total run is 130 inches. The stringer is √(108² + 130²) = 169 inches, or 14.1 feet, so buy 16 foot 2x12 boards.
US residential code limits
| Rule | Limit | IRC section |
|---|---|---|
| Riser height | 7 3/4 in maximum | R311.7.5.1 |
| Tread depth | 10 in minimum | R311.7.5.2 |
| Variation in a flight | 3/8 in maximum between steps | R311.7.5 |
| Nosing | 3/4 to 1 1/4 in when treads are under 11 in | R311.7.5.3 |
Limits summarized from Viewrail's IRC stair code guide. Many cities adopt the IRC with changes, so check with your building department.
Comfortable stairs
A common comfort rule is 2 risers + 1 tread = 24 to 25 inches. A 7.5 inch riser with a 10 inch tread gives 25 inches. The calculator shows this check under the result.
Measuring the total rise
- Measure from the finished lower floor or ground to the finished top surface.
- If the ground slopes, measure where the bottom step will land.
- Include the thickness of any flooring still to be added.
Common questions
How do I calculate the number of stairs?
Divide the total rise in inches by the maximum riser height and round up. For a 108 inch rise with a 7 3/4 inch maximum, 108 ÷ 7.75 = 13.9, so 14 risers of 7.71 inches each.
What is the maximum riser height?
The International Residential Code (IRC R311.7.5.1) allows a maximum of 7 3/4 inches. Your local code may differ, so check before building.
What is the minimum tread depth?
10 inches under IRC R311.7.5.2, measured from the edge of one nosing to the next.
Why is there one less tread than risers?
The top step is the upper floor or deck itself, so it does not need a tread board.
How many stringers do I need?
It depends on the tread material and stair width. A common approach is stringers no more than 16 inches apart, so a 36 inch wide stair needs 4. Check your tread maker's and local code's requirements.