How it works
Measure the total rise from the finished lower surface to the finished upper floor. The calculator divides it into the fewest equal risers that stay under your maximum riser height, then works out the total run and the diagonal length of the stringer.
The International Residential Code (IRC) limits riser height to 7 3/4 inches and requires treads at least 10 inches deep, with no more than 3/8 inch variation between steps. A common comfort rule is that two risers plus one tread should total about 24 to 25 inches.
There is always one fewer tread than risers, because the upper floor acts as the top step. Buy stringer stock at least a foot longer than the diagonal to allow for the plumb and level cuts at each end.
Formula
Risers = ⌈total rise ÷ max riser⌉. Riser height = total rise ÷ risers. Total run = (risers − 1) × tread. Stringer = √(rise² + run²).
Questions people ask
What is the maximum stair riser height?
Under the IRC, 7 3/4 inches for residential stairs. Commercial stairs under the IBC are limited to 7 inches.
How many steps for a 9 ft floor-to-floor height?
With 108 in of total rise and a 7 3/4 in maximum, you need 14 risers of about 7.71 in and 13 treads.
What size lumber is used for stair stringers?
Usually 2 × 12 pressure-treated or framing lumber, which leaves at least 3.5 inches of solid wood below the notches. Use three stringers for stairs up to 36 in wide.