Stair Calculator
From one measurement — the floor-to-floor rise — get the riser count, exact riser height, tread layout, total run, and stringer length, checked against residential code.
floor to floor
IRC code max 7.75"
IRC code min 10"
Risers
14
7.71" each
Treads
13
10.5" each
Total run
11' 4.5"
Stringer length
14' 6.1"
✓ Meets IRC residential code (riser ≤ 7.75", tread ≥ 10"). Comfort check (2 × riser + tread): 25.9" — carpenters target 24–25".
Measure total rise from finished floor to finished floor. Code also limits riser-height variation to 3/8" across the flight, requires 6'8" of headroom, and treats the top floor as the final “riser” — which is why there's one fewer tread than risers. Always confirm with your local building department.
How Stairs Are Laid Out
Divide the total rise by the code-maximum riser (7¾" under the IRC) and round up — that's your riser count, and dividing the rise by it gives each riser's exact height. Every step must match within ⅜", because human feet memorize the first step and trip on any odd one. The classic comfort check is 2 × riser + tread = 24–25": a 7" riser with a 10½" tread walks naturally.
Worked example: 9-foot rise
Total rise 108" ÷ 7.75" max = 13.9 → 14 risers
Riser height = 108 ÷ 14 = 7.71"
Treads = 14 − 1 = 13 × 10.5" = 136.5" run (11' 4.5")
Stringer = √(108² + 136.5²) = 174" ≈ 14' 6"
IRC limits: riser ≤ 7¾", tread ≥ 10", headroom ≥ 6'8",
variation ≤ ⅜" per flightFrequently Asked Questions
How many steps do stairs to a second floor have?
Usually 13–15 risers. The floor-to-floor rise for an 8-foot ceiling is about 105–110 inches once joists and flooring are added, and 108 ÷ 7.75 max riser rounds up to 14. A 9-foot ceiling typically pushes it to 15–16 risers.
What's the most comfortable riser and tread combination?
Around a 7-inch riser with an 11-inch tread — stair builders' favorite — and anything satisfying 2 × riser + tread ≈ 24–25 inches walks naturally. Steeper-and-narrower or shallower-and-deeper both break stride; the formula encodes a century of trial and error about human gait.
Can basement or attic stairs be steeper than code?
Not under the IRC — the 7¾-inch riser and 10-inch tread limits apply to all stairs serving habitable space, basements included. The exceptions are spiral stairs (their own rules) and ship ladders to non-habitable lofts where some jurisdictions allow them. Local amendments vary, so check before framing.
What lumber do I need for stair stringers?
2×12s are the standard — after cutting the notches, code requires at least 3½ inches of solid wood (the 'throat') remaining. A typical 14-riser flight needs 16-foot boards, and three stringers (two outer plus a center) for 36-inch-wide stairs with dimensional-lumber treads.