Stair Calculator
Enter the total floor-to-floor height and this lays out the staircase: the number of steps, the exact riser height, the run it takes up, and the stringer length.
Steps (risers)
14risers
- Riser height
- 7.71 in
- Treads
- 13 treads
- Total run
- 130 in
- Stringer length
- 169 in
Stair geometry estimate only. Riser height, tread depth, headroom, handrails, and landings must meet local building code (IRC: 7-3/4-in max riser, 10-in min tread, uniform within 3/8 in). Verify against your code — this doesn't replace it.
How it’s calculated
Number of risers = total rise ÷ target riser height, rounded to a whole number; the actual riser height is then total rise ÷ that count, so every step is equal. Treads = risers − 1. Total run = treads × tread depth. Stringer length = √(total rise² + total run²) — the diagonal the stringer follows.
Worked example
108-inch total rise, ~7.5-inch target riser, 10-inch tread: 108 ÷ 7.5 = 14.4 → 14 risers at 108 ÷ 14 = 7.71 in each; 13 treads × 10 = 130-inch run; stringer = √(108² + 130²) ≈ 169 in (14 ft 1 in). It rounds to 14, not 13: 13 risers would be 108 ÷ 13 = 8.31 in each — over the 7-3/4-in code maximum — so 14 keeps every step legal at 7.71 in.
FAQs
- Why round the number of risers?
- You can't have a fractional step, so the rise is divided into a whole number of equal risers. The calculator picks the count closest to your target height, then recomputes the exact riser so every step matches — uneven risers are a trip hazard and a code violation.
- What riser and tread are allowed?
- The IRC sets a maximum riser height of 7-3/4 inches and a minimum tread depth of 10 inches, and requires the tallest and shortest riser (and tread) in a flight to be within 3/8 inch of each other (R311.7.5). Your local code governs and can differ — check it before cutting.
- What size board is the stringer?
- Stringers are usually cut from 2×12 lumber, since notching for the treads and risers leaves enough board behind the cuts. The stringer length here is the diagonal; order boards a bit longer to allow for the cuts and attachment.
Sources
- Risers = round(total rise ÷ target riser); riser = total rise ÷ risers; run = (risers − 1) × tread; stringer = √(rise² + run²). Layout geometry — IRC riser/tread limits noted, not designed.