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Stair Calculator

Code-legal risers and the exact stringer to cut.

Stairs fail inspection on two numbers: riser height (7.75" max under the IRC) and tread depth (10" min). This calculator divides your total rise into equal, code-legal risers, counts treads (one fewer than risers — the upper floor is the last landing), computes total run and stringer length, and checks the 2R+T comfort rule.

Why people use this

Deck builders and basement finishers use this to get equal, code-legal risers on the first layout — the 7.75" IRC limit and the treads-minus-one rule are where stair math goes wrong. The stringer length and 2R+T comfort check come free with the count.

How it works

1. Measure total rise — finished floor to finished floor — in inches.
2. Pick tread depth (10" IRC minimum; 11" is noticeably more comfortable).
3. Read the riser count and exact riser height, tread count, total run, and stringer length.
4. Check the pass/fail line against IRC basics and the 24–26" comfort rule.

Common questions

How many risers for a 105-inch floor-to-floor rise?

⌈105 ÷ 7.75⌉ = 14 risers, which makes each riser exactly 7.5". That yields 13 treads, a 130-inch total run at 10" treads, and a 2R+T of 25" — comfortably in the ideal range.

How many stringers do I need?

Two for stairs under 36" wide cut from 2×12s; add a center stringer at 36" and wider, and composite treads often need stringers at 12" on center regardless of width. Check headroom (6'8" minimum) and handrail rules with your local code before cutting.

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