Enter the total rise and either a tread depth or the run you have available, and this stair calculator works out the number of risers, the exact riser height, total run, stringer length, and stair angle. Each layout is checked against the IRC R311.7 limits — 7¾" maximum riser, 10" minimum tread, 36" width, 12' 7" per flight — plus the 2R + T comfort rule, and a scaled side-view diagram shows the rise and run.
Building deck stairs? Pair this with the deck calculator for the framing, or the ramp slope calculator if you need an accessible route instead of steps. The concrete calculator sizes a landing pad at the bottom.
Frequently asked questions
How do I calculate the number of stairs?
Measure the total rise from finished floor to finished floor, divide by the maximum riser height (7¾" under the IRC), and round up. That is your number of risers; the riser height is the total rise divided by that number. There is always one fewer tread than risers, because the upper floor acts as the top step. For a 105" rise: 105 ÷ 7.75 = 13.5, so 14 risers of 7½" and 13 treads. Multiply the tread depth by 13 for the total run. Measure at the stair location itself, since floors are rarely perfectly level.
What are the IRC stair code limits?
For homes, IRC Section R311.7 (2018, 2021, and 2024 editions) sets a maximum riser height of 7¾", a minimum tread depth of 10" measured nosing to nosing, no more than ⅜" difference between the largest and smallest riser or tread in a flight, at least 6' 8" of headroom, a minimum width of 36", and no more than 12' 7" of rise in one flight before a landing. Many states and cities amend these, so always check your local code before building. Commercial stairs follow the stricter IBC.
When does a stair need a nosing?
IRC R311.7.5.3 requires a nosing that projects ¾" to 1¼" past the riser below on stairs with solid risers — unless the tread is at least 11" deep, in which case no nosing is required. Open-riser stairs can omit the nosing, but the opening between treads must not let a 4" sphere through when the stair rises more than 30". The calculator notes which case your tread depth falls in. Consistency matters too: nosings within a flight may not vary by more than ⅜", the same tolerance as risers and treads, so plan the tread material before cutting.
How long should my stringers be?
The stringer line length is the diagonal of the flight: √(total run² + total rise²). A 105" rise with 130" of run gives about 13' 11". Buy stock longer than that, because the top and bottom cuts and the seat at the floor need extra — the calculator suggests the next standard 2×12 length with at least 6" to spare. Remember to trim the bottom of the stringer by the tread thickness so the first step is the same height as the rest. Cut one stringer first, test-fit it, then use it as the template for the rest.
How many stringers does a stair need?
It depends on the tread material. 2× wood treads commonly sit on stringers 16" on center; the American Wood Council's DCA 6 deck guide allows cut stringers up to 18" on center with at least three of them, and many composite treads require 12" spacing. A 36" wide stair gets four stringers at 16" or three at 18". Cut 2×12 stringers in DCA 6 also span no more than about 6 feet horizontally without extra support. Always follow the tread manufacturer's span rating.