Stair calculator (Blondel rule)
Enter the height to climb (floor to floor) and the floor length available. The number of steps aims at a 17.5 cm rise, or you can set it.
Result
16Number of risers
- Rise per step (R)
- 17.5 cm
- Going, tread depth (G)
- 26.67 cm
- Number of treads
- 15
- Blondel 2R + G
- 61.67 cm
- Comfortable: 2R + G is between 60 and 65 cm.
- Slope
- 33.3°
- Stringer length (approx.)
- 488.3 cm
Inputs
- Total rise (floor to floor)
- 280
- Total run (floor length)
- 400
- Number of risers (optional)
- 0
How it is calculated
R = height ÷ number of risers. By default, risers = height ÷ 17.5, rounded.
Result
Number of risers
- Rise per step (R)
- 17.5 cm
- Going, tread depth (G)
- 26.67 cm
- Number of treads
- 15
How it is calculated
- R = height ÷ number of risers. By default, risers = height ÷ 17.5, rounded.
- There is one tread fewer than risers: the top floor is the last step. G = run ÷ (risers − 1).
- Blondel rule: 2R + G between 60 and 65 cm gives a comfortable stride.
- Slope = arctan(R ÷ G); stringer ≈ √(height² + run²).
Example
280 cm high over 400 cm: 16 risers of 17.5 cm, 15 treads of 26.67 cm, 2R + G = 61.67 cm (comfortable), slope 33.3°, stringer about 488.3 cm.
Good to know
- Straight stairs only (no winders or mid-landing).
- It does not check local building codes (headroom, handrails, accessibility): have a professional confirm.
- Tread and stringer thickness are not included.
Frequently asked questions
What is this tool used for?
Enter the height to climb (floor to floor) and the floor length available. The number of steps aims at a 17.5 cm rise, or you can set it.
How is the result calculated?
R = height ÷ number of risers. By default, risers = height ÷ 17.5, rounded.
What should I check before using the result?
Straight stairs only (no winders or mid-landing).