Stair calculator rise and run
Everything starts from one measurement: the total rise, from the finished lower floor (or the landing pad) to the finished upper floor. Measure it plumb, after flooring is down or with its thickness added.
- Risers = total rise ÷ target riser height, rounded up.
- Exact riser height = total rise ÷ risers. Every riser must be within 3/8 in of the others.
- Treads = risers − 1. The upper floor is the last "step."
- Total run = treads × tread depth — the floor space the stair takes up.
| Total rise | Risers (7-3/4 max) | Riser height | Treads | Run at 10 in | Stringer length |
|---|---|---|---|---|---|
| 24 in (deck) | 4 | 6 in | 3 | 2 ft 6 in | 3 ft 2 in |
| 36 in | 5 | 7.2 in | 4 | 3 ft 4 in | 4 ft 6 in |
| 48 in | 7 | 6.86 in | 6 | 5 ft | 6 ft 5 in |
| 96 in | 13 | 7.38 in | 12 | 10 ft | 12 ft 10 in |
| 108 in | 14 | 7.71 in | 13 | 10 ft 10 in | 14 ft 1 in |
| 120 in | 16 | 7.5 in | 15 | 12 ft 6 in | 16 ft |
Stair stringer calculator
Stringers are the notched boards that carry the treads, usually cut from 2×12s. Their length is the diagonal of the rise and the run. The calculator picks the shortest stock length that leaves at least 6 in for the top and bottom cuts.
How many you need depends on the width and the tread material. For deck stairs, the American Wood Council's DCA 6 guide limits cut stringers to 18 in apart with at least 5 in of solid wood left below the notches — which is why a 36 in stair gets three stringers. Inside a house, follow your plans; a 2×12 with 7-1/2 in risers and 10-1/2 in treads leaves about 5 in of throat.
Lay out the notches with a framing square and stair gauges set to your exact riser and tread, then drop the bottom of the stringer by one tread thickness so the first step isn't taller than the rest once the treads go on.
Is it a comfortable stair?
The code sets limits; comfort is a separate test. A long-used rule says two risers plus one tread should total 24 to 25 in. A 7-1/4 in riser with a 10-1/2 in tread scores 25; a 7-3/4 in riser with a 10 in tread scores 25.5 and feels steep. The calculator shows this score and a short table of the riser counts on either side of yours — adding one riser makes every step lower at the cost of about a tread's depth of extra run.
The code checks the calculator runs, from the 2021 IRC:
| Rule | Limit |
|---|---|
| Riser height | 7-3/4 in maximum |
| Tread depth | 10 in minimum, nosing to nosing |
| Uniformity | Risers and treads within 3/8 in of each other |
| Width | 36 in minimum |
| Headroom | 6 ft 8 in |
| Nosing | 3/4 to 1-1/4 in when treads are under 11 in |
Stairs with four or more risers need a handrail, and an open side more than 30 in above the floor below needs a guard — the baluster spacing calculator handles that. Building concrete steps instead? Use the concrete steps calculator. For a deck, the deck calculator sizes the platform the stairs land on.
