Discover François Blondel’s classic 1675 formula (2R + T = 63 cm / 25 in) and why it remains the gold standard for comfortable residential staircases.
M
Mark Henderson, Master Carpenter
Carpentry & Construction Author
In 1675, French architect François Blondel discovered the mathematical relationship between the energy required to lift one’s foot vertically versus walking horizontally.
The Comfort Equation
2 × Riser Height + Tread Depth = 24 to 25 inches (610 to 635 mm)
When a stair layout satisfies this formula, walking up and down feels natural, effortless, and minimizes trip hazards for people of all ages.
Ready to Calculate Your Stairs?
Use our free online stringer calculator to get exact rise, run, throat depth, and board sizing.
