The formula
2 × rise + tread = between 60 and 66 cm (Blondel's rule)
Where it comes from
The rise is the height of each step and the tread its depth. François Blondel observed in the seventeenth century that a human stride climbing stairs follows a constant relationship, and from that comes the formula still in use: twice the rise plus the tread should fall between 60 and 66 centimetres.
How to work it out by hand
- Divide the total height by the maximum rise and round up
- That is the number of steps
- Divide the total height between them: every rise must be identical
- Check Blondel's rule against the tread you have chosen
What is worth knowing
Every step has to measure exactly the same. A centimetre of difference in a single one is enough to cause trips, because climbing, the foot repeats the movement from memory. Domestic building rules commonly cap the rise around 18.5 cm and require a tread of at least 28 cm, though the exact limits vary by country — check your local building regulations before building.