How stairs are calculated
Everything starts with the total rise, the vertical distance from one finished floor to the next. The calculator divides it by your maximum riser height and rounds up to get the number of risers. Then it divides the rise evenly, so every step is exactly the same height.
There's always one fewer tread than risers, because the top floor acts as the last step. Treads times tread depth gives the total run, and the rise and run together give the stringer length.
Example: 9 ft floor to floor
- Total rise: 108 in
- 108 ÷ 7.75 = 13.9, so 14 risers
- Each riser: 108 ÷ 14 = 7.71 in
- 13 treads at 10 in = 130 in of run
- Stringer length: about 14.1 ft
Building code basics
Most U.S. homes follow the International Residential Code. It limits riser height to 7¾ inches and requires treads of at least 10 inches. Within one flight, the tallest riser can't be more than 3/8 inch taller than the shortest, and the same goes for tread depth. Uneven steps are a common cause of trips, so measure carefully.
Local codes can differ, and stairs usually need a permit and inspection. Check with your building department before you cut stringers.
Tips
- Measure total rise from the finished floor surfaces, including flooring that isn't installed yet.
- A comfortable stair often has a riser of about 7 in and a tread of about 11 in.
- Buy stringer lumber longer than the calculated length to allow for the cuts at each end.
Need lumber totals for the job? Use the board foot calculator.
Frequently asked questions
How many steps do I need for a 9 foot ceiling?
With about 9 feet floor to floor including the floor framing, you'd typically need 14 risers at about 7.7 inches each, and 13 treads. Measure your actual total rise to be sure.
What is the maximum stair riser height?
Under the International Residential Code, 7 3/4 inches. Your local code may be stricter.
What is the minimum tread depth for stairs?
The International Residential Code requires at least 10 inches, measured from the edge of one tread to the edge of the next.
How do I calculate stringer length?
Use the Pythagorean theorem: the square root of total rise squared plus total run squared. The calculator does this for you.
Sources
Results are for planning. Stairs must meet your local building code, and many projects need a permit and inspection.