Stair Calculator

Rise, run, and stringers from one measurement — checked against the IRC 2021 limits.

Your stairs

Finished deck surface straight down to the landing.
Two 5/4×6 deck boards give an 11 in tread. Code minimum is 10 in.
2-by lumber treads span further — 3 stringers at 18 in spacing is common practice.

Materials list

Generated by AllUnits.io — allunits.io/calculators/stair-stringer

How this calculator figures it

Riser count is total rise divided by the IRC 2021 maximum riser of 7¾ inches, rounded up, which yields equal risers within the code's 3/8-inch uniformity limit (R311.7.5.1). Treads use your run — at least the 10-inch code minimum (R311.7.5.2) — and stringer length is the hypotenuse of total rise and total run. Local amendments can be stricter; the permit office has the final word.

How it works

risers = ceil(rise ÷ 7.75)  ·  riser height = rise ÷ risers  ·  treads = risers − 1
stringer = √(rise² + total run²) + ~12" end-cut margin, bought as the next 8/10/12/14/16 ft 2×12

Worked example — a 36-inch rise with a 10.5-inch run: 36 ÷ 7.75 rounds up to 5 risers of 7.20 inches each; 4 treads make a 42-inch total run; the stringer hypotenuse is √(36² + 42²) ≈ 55.3 inches, cut from an 8 ft 2×12. A 36-inch-wide stair with 5/4 treads takes 4 stringers (16 in on center max), and each step takes two 5/4×6 boards at the stair width.

Stair FAQs

What is the maximum riser height?

7¾ inches — IRC 2021 R311.7.5.1. Risers in a flight must not vary more than 3/8 inch.

What is the minimum tread depth?

10 inches with nosing — IRC 2021 R311.7.5.2.

How many stringers for 36-inch stairs?

Four for 5/4 or composite treads — 16 inches on center max; three at 18-inch spacing is common with 2-by lumber treads.

Do deck stairs need a permit?

Usually, as part of the deck permit — check locally.

Embed this calculator

Put this calculator on your own site — free. Paste this where HTML is accepted.

Free to embed on any site. Keep the "Calculator by AllUnits" link with the embed.

Related calculators & sources

Deck the stairs land on: Deck Board Calculator. Tally the 2×12s with the Board Foot Calculator, and set the posts with the Fence Post Concrete Calculator. Code limits are cited at IRC 2021 R311.7; convert measurements with the length and angle converters.

Estimates for planning and purchasing. Not engineering or code-compliance advice — confirm structural work with your local building department.