Stair Calculator
Risers, treads, run, and stringer length from the one number that matters — the total rise — with the IRC geometry limits checked on the page.
Geometry and a dimensional check against IRC R311.7 (2021: R311.7.5.1 risers, R311.7.5.2 treads). California and local amendments, headroom, handrails, guards, and landings are separate requirements — the building department has the final word, and stringer lumber sizing is not an output here.
How the math works
Risers = total rise ÷ target riser, rounded UP; the actual riser is the total rise divided by that count, so every step in the flight is identical (the 3/8 in uniformity rule in R311.7 is really a “make them all the same” rule). Treads = risers − 1. Total run = treads × tread depth. Stringer length is the hypotenuse: √(rise² + run²).
The code basis printed here is the 2021 IRC, Section R311.7: maximum riser 7¾ in (R311.7.5.1), minimum tread depth 10 in (R311.7.5.2), greatest-to-smallest variation no more than 3/8 in. California and local amendments control on a real permit.
A worked example
A 96 in floor-to-floor rise at a 7 in target: 14 risers at an actual 6.86 in each, 13 treads at 11 in, a total run of 143 in (11.92 ft), and a stringer of 172.24 in (14.35 ft) at a 33.87° angle. The comfort check (2 × riser + tread) lands at 24.71 in — inside the classic 24–25 in band.
Questions from the field
What is the maximum riser height? 7¾ inches under IRC R311.7.5.1 for residential stairs — and the tallest and shortest risers in a flight may differ by no more than 3/8 inch.
What is the minimum tread depth? 10 inches under IRC R311.7.5.2, measured between the vertical planes of the nosings. Deeper is more comfortable; 11 in is the working default here.
How many steps do I need? Divide the total rise by your target riser and round up — 96 in at 7 in is 14 risers (13 treads). The actual riser is always the total divided by the count, never the target.
Does this size my stringer lumber? No. It gives the stringer LENGTH. What the stringer is cut from is a span and load question for the plans.
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