The math
Stair design is a series of compromises. Total rise is fixed by your deck height. From there, you pick a target riser height, round to the nearest whole number of risers, and recalculate to get equal steps.
num_risers = round(total_rise ÷ target_riser)
actual_riser = total_rise ÷ num_risers
num_treads = num_risers − 1
total_run = num_treads × tread_depth
Number of treads is always one less than risers - the top step lands on the deck itself. The total run is just treads times tread depth.
Frequently asked
Why do my steps need to be exactly equal?
Code requires variation under ⅜″ for safety. People's brains form a pattern after one step - if the second is even slightly different, they trip. This is the leading cause of stair injuries. Build to the calculated actual riser, not the target.
What stringer stock do I need?
For stairs under 8 risers, 2×12 stringers are standard. Over 8 risers, structural engineering gets into play - you may need engineered LVL or doubled-up 2×12s. The stringer length output here tells you minimum stock length.
How do I lay out the stringer?
Use a framing square. Mark the riser height on the tongue and tread depth on the body. Walk it down the stringer, marking each step. Or use stair gauges (small brass clamps) to lock the square at the right positions and step-mark the whole stringer in one pass.