Stair Blondel Formula Calculator

Stair geometry by the Blondel rule

Back in 1675 François Blondel wrote down the rule that designers still lean on: 2·riser + tread = 63-64 cm, roughly how far an adult steps when walking comfortably. The Brazilian standard NBR 9050 leans toward a riser near 17 cm and a tread near 28 cm, which lands inside Blondel (2·17 + 28 = 62) and works out to a slope of about 31°. You can go steeper, say a 19 cm riser with a 25 cm tread, and service stairs often do, but it wears people out sooner.

Take a typical floor with a total rise of 2.80 m: the number of risers comes out to 2.80/0.17 ≈ 16-17, and the run projects 16 × 0.28 = 4.48 m on the horizontal. NBR 9077 caps a single flight at 16 steps before a landing has to break it up, and stair width is sized by the same egress-capacity rules used for corridors.

Applications

NBR 9077 fire egress, NBR 9050 accessibility, residential and commercial design, and spiral stairs (which NBR 9077 admits with restrictions).

FAQ

Why 63-64 cm? That is about how long an adult's natural stride is, and the figure goes back to 17th-century French stair design.

Spiral stair as main stair? No. NBR 9077 lets you use a spiral stair only as a secondary one, and in small areas, never as the building's main egress.

Open risers? For accessibility, NBR 9050 asks for at least a 7.5 cm tread overhang and steers away from open risers.