🪜 Stair Calculator (Blondel Rule)

Floor-to-floor height and available space: get the number of risers, their height, the going, the pitch and the stairwell opening of a straight stair, with a dimensioned drawing that updates live.

📐 Dimensions

Floor-to-floor height

Total run

Stair width

Upper floor thickness

Target headroom

Nosing overhang

Floor-to-floor height: from the finished lower floor to the finished upper floor. Run: floor length taken by the stair. Headroom: clear height above the nosings (1.90 m minimum, 2 m advised).

📊 Your stair

Line of nosings
Headroom
Typical step

Possible solutions

Risers	h	g	2h + g	Pitch	Run	Verdict	Choose

⚠️ Indicative calculation for a straight stair. It does not replace a professional’s drawings: check dimensions on site, the structure (stringers, opening, trimmers) and the rules that apply where you live (building code, accessibility, fire safety).

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About the Stair Calculator

A comfortable stair is not only about riser height: the height and depth of the steps must match the length of a natural stride. That is the Blondel rule, used by joiners and architects since the 17th century. This calculator finds every straight-stair solution for your floor-to-floor height, ranks them by comfort, works out the stairwell opening needed so nobody hits their head, and draws the stair to scale.

How to Use the Calculator

Measure the floor-to-floor height between the two finished floors (coverings included).
Leave the run free to get the most comfortable stair, or fix the floor length available.
Enter the upper floor thickness and the target headroom to get the length of the stairwell opening.
Compare the solutions in the table and click “Show” to draw another option.

Rules Used

Blondel rule: 2 × h + g between 60 and 64 cm (ideal around 63 cm), h being the riser height and g the going.
Riser height: ideally 16 to 18 cm; the French timber stair standard (NF DTU 36.3) caps it at 21 cm.
Going: 25 to 32 cm for a comfortable stair; below 24 cm the stair becomes steep.
Calculation: number of risers n, h = height ÷ n, n − 1 treads, run = (n − 1) × g, pitch = arctan(h ÷ g).
Opening: length = g × (headroom + floor thickness) ÷ h, measured from the top nosing, headroom being taken vertically above the nosings.
Public buildings: in France, public buildings (ERP) require h ≤ 16 cm and g ≥ 28 cm (decree of 20 April 2017, given for information).

Tips

Aim for 17 cm risers and a 28 to 29 cm going for a main house stair.
Keep 2 m of headroom if you can: 1.90 m is a minimum.
Allow 80 cm of width or more for a main stair, more to carry furniture.
Limited space: a quarter-turn or winder stair saves run, but balancing the winders is beyond this tool.

Common Mistakes and Limits

Measuring between bare slabs: a screed or a floor covering changes the first or last riser.
Confusing going and tread depth: the going is measured nosing to nosing; the nosing comes on top.
Forgetting the floor thickness when working out the opening.
Straight stairs only: turning stairs need a winder layout.

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