Stairs are the unforgiving part of framing. A 1/4-inch error compounded across fourteen risers becomes a 3-1/2 inch problem at the top, and code violations on rise or run get flagged on every inspection. This guide walks the math, the IRC limits, and the order of cuts on a stringer.

The four numbers you need first

Before you touch a 2x12, you need: total rise (finished floor to finished floor), unit riser height, unit tread depth (the run), and the diagonal stringer length. Total rise is the only one you measure; the other three come from it.

  • Pick a riser count. Divide total rise by 7 to estimate. A 9-ft (108") rise wants ~14 risers (108 ÷ 14 = 7.71" — under the 7-3/4" code max).
  • Set the run. 10" is the IRC minimum. 10-1/2" or 11" steps feel better and give more room for nosed treads.
  • Solve the diagonal. Total run = (risers − 1) × unit run. Stringer length = √(rise² + run²).

The stair stringer calculator runs all four steps and flags rise/run combos that fail IRC.

IRC R311.7 — what passes inspection

  • Maximum riser height: 7-3/4"
  • Minimum tread depth: 10" (nose to nose)
  • Riser variation in a flight: max 3/8" between smallest and largest
  • Tread variation: same 3/8" rule
  • Headroom: 6 ft 8 in minimum from nose diagonal
  • Nosing projection: 3/4" minimum, 1-1/4" maximum

The "rule of comfort" — 2R + T between 24 and 25 inches — is not code, but it predicts which stairs feel right underfoot. A 7" riser with an 11" tread hits 25". A 7-3/4" riser with a 10" tread is legal but feels steep at 25.5".

Laying out the stringer

Mark the rise and run on the long edge of a 2x12 with a framing square. Most framers attach stair gauges (small brass clamps) at the rise and run dimensions on the square, then walk the square down the board, marking each step. Walk it the same number of times as treads, because the bottom of the stringer gets a rise-thickness cut at the shoe and the top gets a run-thickness cut at the upper framing.

Subtract the tread thickness (typically 1") from the bottom riser cut so all your finished risers land on the same height. This is the "drop" — the most-missed cut on a DIY stringer.

Stringer stock and throat

Always cut from a 2x12. After cutting the rise and run notches, the remaining "throat" of wood must be at least 3-1/2" wide (some inspectors require 5"). A 2x10 leaves barely 2-3/4" of throat on a typical stair — fragile, and will fail many inspections. The calculator suggests a stock length one foot longer than the diagonal to account for seat cuts and to leave clean ends.

Use 3 stringers under standard 36" stairs (left, middle, right). Add one stringer per 16" of additional width for stair widths over 36".

Why you don't eyeball the math

The classic homeowner mistake: 13 risers across a 96" total rise gives 7.38" per rise, fine. But if total rise is actually 99" (an extra ¾" of subfloor at the top), 13 risers becomes 7.62" — still legal, but it changes every mark on the stringer. Always re-measure total rise after subfloor and finish floor are dry-fit, before cutting.

Common layout errors

Forgetting the drop. If you cut all 14 rises at 7.71" and forget to subtract the 1" tread thickness from the bottom, your first step ends up 1" taller than the others — instant code fail.

Wrong stringer count. Two stringers under a 36" residential stair will let the treads sag and crack. Three minimum, more for wider stairs.

Skipping the headroom check. Cathedral stairs into basements often violate the 6'8" headroom rule because the basement ceiling beam is too low. Catch this before you frame the rough opening.

Quick FAQ

What is the maximum stair rise per code? 7-3/4" under IRC R311.7.5 in most jurisdictions. Some local codes allow 8" in non-residential or remodel work — check locally.

Can I use 2x10 stringers? Sometimes, but most inspectors prefer 2x12 because the throat thickness is stronger and meets minimum requirements with margin.

How wide should residential stairs be? 36" minimum clear width above the handrail, per IRC R311.7.1. Many builders frame at 38–40" rough opening to leave room for finish work.

Estimate only. Stair stringers are a structural element. The numbers in the stair stringer calculator are for planning; before cutting, verify rise, run, and headroom with your local building inspector or licensed contractor. ProjectCalc is not responsible for code violations or installation defects.