stair-calculator.com does one thing: it turns two measurements (total rise and run) into a complete, code-checked stair specification you can take to the lumber yard.

How the math works

The calculator divides your total rise by a target riser height of 7.5 inches, rounds to a whole number of risers, then recomputes the exact riser so every step is identical. Stringer length is the Pythagorean hypotenuse of rise and run. Every configuration is checked against four rules: the IRC maximum riser (7.75″), minimum tread (10″), the 2R+T comfort band (24-26″), and the 30-40° angle range. The advanced mode adds fixed-riser or fixed-step-count constraints, tread thickness, headroom checks, and flush-mount geometry.

What it assumes

• Straight flights only. No winders, spirals, or L-shaped landings (treat each flight separately).
• US IRC R311.7 residential limits. Commercial stairs (IBC) and many local amendments are stricter.
• Measurements are finished-floor to finished-floor. Flooring thickness changes your first and last riser. Measure after finishes, or account for them.

Data basis & code version

The code checks reference the International Residential Code (IRC), section R311.7. The core residential limits used here — 7¾″ (196 mm) maximum riser, 10″ (254 mm) minimum tread, 36″ (914 mm) minimum width, 6′8″ (2032 mm) minimum headroom, and the ⅜″ (9.5 mm) uniformity tolerance — are unchanged across the 2015, 2018, and 2021 editions of the IRC. The 2R + T comfort band (24–26″) follows Blondel's classical stride formula. The section number behind each check is listed in the code guide.

What it is not

It is not a substitute for your local building department or a structural engineer. Codes vary by jurisdiction and change over time. Use the output for planning and material estimates, then verify against the code that actually applies to your project before cutting.

How I keep this accurate

Every formula and code figure on this site is checked against the published IRC text and standard carpentry references before it ships, and each worked example is recomputed to confirm it matches the calculator's own output. Guides carry a Published and Updated date so you can see when each was last reviewed; this page and the code reference were last reviewed in July 2026. If you spot an error in a formula or a code reference, tell me — corrections ship fast and the change date is noted on the affected page.

Who runs this

stair-calculator.com is an independent, free tool built and maintained by Ben, who has an academic background in civil and architectural engineering and got tired of re-deriving stair math on scrap plywood. It has no paywall, no sign-up, and no affiliation with any code body, lumber supplier, or manufacturer. It is a planning aid, not an authority — the IRC and your local building department are.

Sources & references

• International Residential Code (IRC), Section R311.7 — Stairways (2021 edition; core limits consistent with the 2015 and 2018 editions).
• François Blondel, Cours d'architecture (1675–1683) — origin of the 2R + T comfort rule.