Blondel formula
The main rule of stair ergonomics: 2 step heights + depth = stride length (60–64 cm).
01 /What the Blondel formula is
The Blondel formula (also known as the "step formula") is the main mathematical rule of stair ergonomics, derived by the French architect François Blondel in 1675. It relates the height and depth of steps to the length of a person's average stride.
The formula itself: 2H + D = 60–64 cm, where H is the riser height and D is the tread depth.
02 /The logic behind the formula
When a person climbs, they spend roughly twice as much effort as when moving horizontally. That is why Blondel doubled the vertical component (2H).
If the staircase parameters fall within this range, a person does not lose their natural walking rhythm while climbing — neither taking too-frequent steps nor stepping unnaturally wide.