What is Blondel's formula
Created by French architect François Blondel in the 17th century, this formula relates the riser height (the vertical part of a step) to the tread depth (the horizontal part you step on), aiming for a comfortable and safe walking rhythm: 2 × riser + tread = 62 to 64 cm.
How the calculator works
From the total height to climb, the calculator estimates a number of steps with a riser close to 17 cm (considered comfortable), computes the exact riser height, and, via Blondel's formula, the ideal tread depth.
Comfortable reference ranges
| Measurement | Recommended range |
|---|---|
| Riser (step height) | 16 to 18.5 cm |
| Tread (step depth) | 25 to 32 cm |
Example
Height to climb: 280 cm. The calculator suggests 16 or 17 steps with a riser of roughly 16.5-17.5 cm each.
Frequently asked questions
Why does Blondel's formula use 62 to 64 cm?
Because it corresponds, on average, to the length of a comfortable human stride when climbing stairs.
What happens if the riser is too tall?
The stairs become tiring and more dangerous to climb — it's best to keep the riser between 16 and 18.5 cm.
Does this calculator replace an engineering design?
No — it's an initial sizing reference; real staircases should follow a proper design and local building codes.
Can I use a different number of steps than suggested?
Yes, but check that the resulting riser and tread still fall within the comfortable ranges recommended.