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Stairs Calc

Blondel formula (2R + T)

The comfort rule that twice the rise plus the run should equal 24–25 in.

The Blondel formula is the classic stair-comfort rule: twice the riser height plus the run should fall in a comfortable band — about 24–25 in (610–635 mm), reflecting the natural length of a human stride on an incline. Example: a 7 in riser and an 11 in run give 2 × 7 + 11 = 25 in, right in the sweet spot. Many builders also check the rise-plus-run rule (≈ 17–18 in) and the rise-times-run rule (≈ 71–74 in²) alongside it. None of these is a hard code requirement, but a layout that satisfies all three feels natural to climb. Stairs Calc scores every result against Blondel automatically so you can tune the riser and run for comfort.

Try the calculator Stair Rise and Run Calculator

Related terms

Riser height The vertical height of a single step; every riser must be equal within 3⁄8 in. Run (going) The horizontal depth of one step, measured nosing-to-nosing, not counting the overhang. Stair angle (pitch) The slope of the flight in degrees; comfortable stairs sit around 30–37°.

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Stair Rise and Run Calculator Turn one total‑rise measurement into equal risers and runs that meet code — with the comfort rules (2R+T and Rise+Run) checked for you.
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Written by the Stairs Calc editorial team. Methodology and code references: see our methodology.

Built and maintained by builders, drafters and engineers who plan stairs for a living — every code limit is transcribed from the published standard and cited to its exact section.

Last reviewed 2026-06-20 against IRC 2021/2024

Stairs Calc gives accurate geometry and checks it against published building-code limits, but results are estimates for planning. Codes are adopted and amended locally and change over time. Always confirm dimensions against your local adopted code and a licensed professional before you build.