Stair stringer calculator.
Give it the floor-to-floor rise and it lays out the whole stair: how many steps, the exact riser height, total run, and how long to cut your 2x12 stringers. It checks the numbers against IRC 2021 and flags anything a framer would want to catch before the saw comes out.
Heads up: code wants a minimum 6 ft 8 in of headroom over the whole run. That one you check on the plan, it is not in these numbers.
Side profile of one stringer, drawn to your numbers. Screenshot it and take it to the saw.
See the shorter first riser at the bottom? It is cut down by one tread thickness (1.00 in) on purpose. Once the finished treads land on the stringer, every step comes out equal at 6.81 in. Bottom riser cut to 5.81 in.
The same math you’d do on the tailgate.
Divide the total rise by 7 inches and round to whole steps. Then split the rise back over that many risers to get the exact height. That is why 109 inches comes out to 16 steps at just under 6.85 in each.
Treads are always one less than risers. Multiply treads by your run for total run, then the stringer is the diagonal: the square root of run squared plus rise squared. We add about a foot for the cuts and round up to the next even foot of 2x12.
A 36 in stair carries 3 stringers. Wider than that? We space them 16 inches on center and add one for the far edge, so the treads never sag.
Riser over 7.75 in, tread under 10 in, or the comfort rule (two risers plus a tread) falling outside 23 to 27 in all get flagged. Nothing is blocked, you just see it before it becomes a problem at inspection.
This is the free warm-up. PlumbCut reads your whole plan set and prices the entire frame: walls, headers, sheathing, trusses, and yes, the stairs. The stringer math is the easy part. Let the AI do the takeoff on your next real job and hand you a customer-ready bid.