About this calculator
Building stairs starts with one measurement — the total rise from the lower floor to the upper one. From there, the number of steps, the exact height of each riser and the length of the stringers all follow. This stair calculator does that math, keeping every riser equal (which codes require) and checking the result against the International Residential Code limits.
It applies to straight interior stairs, basement stairs and deck stairs.
How to use it
- Measure the total rise: the vertical distance between the two finished floor surfaces. For deck stairs, measure from the deck surface to the ground or landing pad.
- Enter the maximum riser height allowed (7¾ in under the IRC; check local code).
- Enter the tread depth you want — at least 10 in, excluding the nosing.
- Read the number of risers and treads, the total run you need to fit, and the stringer length.
The formula
Risers = total rise ÷ maximum riser height, rounded up
Riser height = total rise ÷ risers
Treads = risers − 1
Total run = treads × tread depth
Stringer = √(total rise² + total run²)
There is one fewer tread than risers because the top “step” is the upper floor itself. The stringer figure is the straight diagonal; the board you cut it from needs extra length for the top and bottom cuts.
Worked example
These are the calculator’s starting values and the results they produce:
| Total rise (floor to floor) | 108 in |
|---|---|
| Maximum riser height | 7.75 in |
| Tread depth (run per step) | 10 in |
Tips before you buy
- All risers in a flight must be equal within ⅜ in under the IRC — even small differences trip people.
- Allow for flooring thickness: if the lower floor will get tile and the upper floor carpet, measure between finished heights.
- Stringers for deck stairs are usually cut from 2×12 lumber and spaced no more than 16–18 in apart for wood treads.
- Stairs with four or more risers typically need a graspable handrail.
Frequently asked questions
How many steps do I need for a 9 ft rise?
A 9 ft (108 in) rise divided by a 7¾ in maximum riser needs 14 risers of about 7.71 in each, with 13 treads. At 10 in per tread the stairs run 130 in (10 ft 10 in) horizontally.
What is the maximum stair riser height?
The International Residential Code limits risers to 7¾ in and requires treads of at least 10 in. Commercial buildings (IBC) use 7 in maximum risers and 11 in minimum treads.
How long should a stair stringer be?
The stringer length is the hypotenuse of the total rise and total run. For a 108 in rise and 130 in run, that’s about 169 in (14 ft 1 in), so you would cut it from a 16 ft board.