Stair calculator
Every set of stairs starts with one number: the total rise from the lower floor to the upper floor. From that the calculator works out how many steps you need, exactly how tall each riser is, how far the stairs will run, how long the stringer boards must be, and whether the result passes residential code.
How the stair calculator works
The calculator divides the total rise by your preferred riser height and rounds to the nearest whole number of risers, then divides again to get the exact riser height, so every step is identical. If that pushes a riser over the 7 3/4 in code maximum, it adds a step. Treads are one fewer than risers when the top riser lands on the upper floor, which is the usual case for deck and basement stairs.
Total run is treads × tread depth. The stringer cut line is the hypotenuse of rise and run, and the board length rounds up to the next 2 ft of lumber with 6 in to spare for the top and bottom cuts. Stringers are spaced no more than 18 in apart for 2 × lumber treads, so a 36 in stair needs three and a 48 in stair four.
Formula
risers = round(total rise ÷ preferred riser)
riser height = total rise ÷ risers
stringer length = √(total rise² + total run²)
Example: a 108 in rise with a 7 1/4 in target gives 15 risers of exactly 7.2 in. With 10.5 in treads and a top landing, 14 treads run 147 in, the stringer measures 182.5 in, and you buy three 16 ft 2 × 12s. The stairs sit at 36.3 degrees and the comfort check comes out at 24.9.
Code limits and layout tips
| Item | IRC residential limit |
|---|---|
| Riser height | 7 3/4 in maximum; vary no more than 3/8 in across the flight |
| Tread depth | 10 in minimum (measured nosing to nosing) |
| Nosing | 3/4 to 1 1/4 in when tread is under 11 in |
| Width | 36 in minimum above the handrail |
| Headroom | 6 ft 8 in minimum |
| Handrail | Required with 4 or more risers; 34 to 38 in above nosings |
- Measure total rise at the actual location of the stairs, and account for finished floor thickness at both ends.
- Drop the first riser cut by the tread thickness so the bottom step ends up the same height as the rest.
- Cut one stringer, test fit it, then use it as the template for the others.
- Deck stairs usually land on a concrete pad: the concrete calculator sizes it.
Building the deck too? Start with the deck calculator.
Common sizes, already worked out
Common questions
How many steps do I need for a 9 foot rise?
108 in divided by a comfortable 7 1/4 in riser is 15 risers of 7.2 in each, with 14 treads if the top riser lands on the floor. At the code-maximum 7 3/4 in you could use 14 risers of 7.71 in.
How long of a board do I need for stair stringers?
Take the square root of total rise squared plus total run squared, add about 6 in for the end cuts, and round up to lumber length. A 9 ft rise with a 12 ft run needs a 16 ft 2 × 12.
How many stringers do I need?
One at each side plus one every 16 in or less in between for 2 × lumber treads: three for a 36 in stair, four for 48 in. Composite treads often require 12 in spacing; check the manufacturer.
What is the most comfortable stair rise and run?
A 7 in riser with an 11 in tread is the classic comfortable stair. The old rule of thumb is that twice the riser plus the tread should land between 24 and 25 in. Steeper combinations are legal up to 7 3/4 by 10 but feel noticeably harder to climb.