How to size a staircase (Blondel formula)
Updated on June 30, 2026 · by Rafael Rossi
A good staircase follows a proportion between each step's height (riser) and depth (tread).
Step 1: number of steps
Divide the total height (floor to floor) by an ideal riser of ~17.5 cm and round. E.g., 280 cm ÷ 17.5 = 16 steps.
Step 2: real riser
Height ÷ number of steps. Aim for 16 to 18 cm for comfort.
Step 3: tread (Blondel formula)
2 × riser + tread = 63 to 65 cm. With a 17.5 cm riser, the ideal tread is ~29 cm. This rule, by 17th-century architect François Blondel, matches the length of a human stride going up.
Step-by-step example (280 cm floor-to-floor)
- Steps: 280 ÷ 17.5 = 16 steps
- Real riser: 280 ÷ 16 = 17.5 cm ✔
- Tread (Blondel): 64 − 2 × 17.5 = 29 cm
FAQ
What's a comfortable slope? Between 30° and 37°. Too steep is tiring and unsafe; too shallow wastes space.
Must all steps be equal? Yes — a safety rule. Uneven riser heights are the leading cause of trips on stairs.