How this calculator works
Every riser must be the same height, so the total rise is divided into equal steps that stay under the 7 3/4 in limit of the International Residential Code. Dividing by 7.75 and rounding up gives the number of risers, and the rise divided by that count gives the exact riser height to cut. There is always one fewer tread than riser, because the upper floor or deck surface acts as the last tread.
Formula
n = ceil(total rise ÷ 7.75 in); riser height = total rise ÷ n; total run = (n − 1) × tread; comfort check = 2 × riser + tread, which should land between 24 and 25 in (about 610 to 635 mm).
Worked example
A deck 104.5 in above grade: 104.5 ÷ 7.75 = 13.5, so 14 risers of 7.46 in each. With a 10 in tread the total run is 13 × 10 = 130 in (10.83 ft) and 2R + T is 24.93 in, inside the comfort range. A 96 in interior rise gives 13 risers of 7.38 in and 24.77 in.
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Frequently asked questions
Why one fewer tread than riser?
The walkway at the top is the final tread, so the stair itself needs one fewer board. This assumes the top landing is exactly one riser above the last tread.
Do Canadian stairs use the same limits?
No. The National Building Code of Canada allows risers of 125 to 200 mm and treads of at least 255 mm, and asks for 2R + T between 550 and 700 mm. Check the edition your permit uses; this page uses the US 7 3/4 in limit and flags the result against the 24 to 25 in comfort rule.
What about a landing or a turn?
A mid-run landing replaces a tread and resets the run measurement, so plan the run for each straight flight separately and add the landing depth between them.