Stair Calculator
Enter the total rise from floor to floor to get the number of steps, riser height, tread depth, total run and a stringer cut list, with a to-scale side view and code checks.
How this stair calculator works
Number of risers: the total rise divided by the maximum riser height (7-3/4 in for homes, 7 in for commercial), rounded up. Comfort mode then picks the count that gives a riser near 7 in. The riser height is the total rise divided by that number, so every riser is equal.
Treads: there is always one less tread than risers, because the top floor is the last tread. Comfort mode sets the tread from the common rule that two risers plus one tread should add up to about 24 to 25 in. It never goes below the code minimum (10 in for homes, 11 in for commercial). If you give a run limit, the tool shortens the tread and, if needed, reduces the number of steps while staying inside the code.
Stringers: total run is the number of treads times the tread depth. The angle is the arctangent of riser over tread. The stringer length from corner to corner is the square root of rise squared plus run squared, and the tool adds about 6 in for the end cuts and rounds up to a stock board. The throat is the wood left under each notch: board depth minus (riser × tread ÷ the length of one step). Cut marks list the height of every tread, how far out it sits, and the distance along the board.
Key stair rules (IRC R311.7, homes)
- Maximum riser 7-3/4 in and minimum tread 10 in.
- Minimum clear width 36 in.
- Minimum headroom 6 ft 8 in, measured straight up from the nose line.
- Handrail 34 to 38 in above the nose line, on at least one side when there are 4 or more risers.
- Nosing of 3/4 to 1-1/4 in when the tread is under 11 in.
- Riser and tread sizes must not vary more than 3/8 in in a flight.
- A landing is needed when the rise is more than about 12 ft 7 in.
For commercial stairs, IBC section 1011 limits risers to 7 in (4 in minimum) and requires treads of at least 11 in. Widths of 44 in are common. These are the values this tool uses in commercial mode. Rules change by edition and city, so verify with your building department or inspector.
Open sides more than 30 in above the ground need a guard, and open risers must stop a 4 in sphere on taller stairs. Ask your inspector about both.
Sources
Estimates only — not engineering, legal or code advice. Check requirements with your local building department and a licensed professional before you build.
Common questions
How many steps do I need for a 36 inch rise?
Divide 36 by the 7-3/4 in maximum riser and round up: 36 ÷ 7.75 = 4.65, so at least 5 risers. This tool uses 5 risers of 7-3/16 in (36 ÷ 5 = 7.2 in) and 4 treads of 10-5/8 in. The total run is 42-3/8 in, or 3 ft 6-3/8 in.
What are the maximum riser height and minimum tread depth?
For homes, the IRC (R311.7) allows a riser up to 7-3/4 in and requires a tread at least 10 in deep and a stair at least 36 in wide. For commercial buildings, the IBC (section 1011) allows a riser of 4 to 7 in and requires a tread at least 11 in deep. Local codes can differ, so check with your building department.
How do I find the riser height?
Divide the total rise by the number of risers. For a 9 ft (108 in) floor-to-floor rise, 108 ÷ 7.75 = 13.9, so at least 14 risers. The comfort setting picks 15 risers of 7-3/16 in, with 14 treads of 10-5/8 in, for a total run of 12 ft 4-3/8 in. Always use the same riser height on every step, within 3/8 in.
What is the 2R + T comfort rule?
Twice the riser plus the tread should be about 24 to 25 in. A 7-3/16 in riser with a 10-5/8 in tread gives 2 × 7.1875 + 10.625 = 25.0 in. A steeper stair has a larger sum than a gentle one, so it feels harder to climb. It is a rule of thumb, not a code requirement.
How long should my stair stringer be?
The straight distance from the bottom corner to the top corner is the square root of rise squared plus run squared. For a 36 in rise and 42.4 in run that is 55.6 in, or 4 ft 7-5/8 in. With the end cuts, an 8 ft 2×12 is enough. A 36 in wide stair uses 3 stringers, spaced no more than about 18 in apart.
Do I need a handrail, and how high should it be?
The IRC requires a handrail on at least one side of a stair with 4 or more risers. It must sit 34 to 38 in above the line along the tread noses. Any open side more than 30 in above grade also needs a guard. Check headroom too: you need 6 ft 8 in straight above the nose line.