Estimates only. Check measurements, product labels and local building codes before you buy or build.
How the Stair Calculator works
Divide the total rise (finished floor to finished floor) by the tallest riser you allow and round up: that is the number of risers. The rise divided by that number is the riser height, so every step comes out equal. There is always one tread fewer than risers, because the upper floor is the last step.
Total run = treads × tread depth. Each step along the stringer is the diagonal of one riser and one tread, √(riser² + tread²); the stringer length is that times the number of treads.
Example: a 108" (9 ft) rise with a 7 3/4" maximum riser gives 108 ÷ 7.75 = 13.94, so 14 risers of 7 3/4" (7.714"). With 10" treads there are 13 treads and a total run of 130" (10' 10"). One step is √(7.714² + 10²) = 12.63", so the stringer is 13 × 12.63 = 164 3/16" (13' 8 3/16") and a 16 ft board leaves room for the cuts.
Check the result against your local code before you cut. The calculator shows the IRC limits it checks (riser ≤ 7 3/4", tread ≥ 10", headroom ≥ 6' 8"); your jurisdiction may differ.
Under the IRC (R311.7.5.1) a residential riser may be at most 7 3/4", and the tallest and shortest riser in a flight may differ by no more than 3/8". Local codes can be stricter, and commercial stairs (IBC) allow 7" at most.
How deep should a stair tread be?
The IRC (R311.7.5.2) asks for at least 10" measured nosing to nosing. A common comfort check is 2 × riser + tread = 24" to 25"; for a 7 1/2" riser that means a tread of 9" to 10", so 10" or a little more feels natural.
Why is there one less tread than risers?
The top riser lands on the upper floor, which acts as the last step. So 14 risers need only 13 treads, and the total run is 13 tread depths, not 14.
How long a board do I need for a stair stringer?
Take the stringer length (number of treads × the diagonal of one step) and add about a foot for the top and bottom cuts, then round up to the next stock length. The calculator does this for you and shows the board to buy.