Stair calculator (steps and Blondel formula)
Calculate the number of steps, riser height and tread depth from total height, using the Blondel rule.
The Blondel rule (2 × riser + tread = 63–65 cm) ensures comfortable steps. Ideal riser 16–18 cm; tread at least ~25 cm for the foot.
How the calculation works
Divide the total height by the desired riser and round to get the number of steps. The real riser is height ÷ steps. The tread comes from the Blondel rule: 2 × riser + tread = 64 cm, which matches a comfortable human stride.
Examples
- Floor-to-floor 280 cm: 280 ÷ 17.5 = 16 steps; riser 17.5 cm, tread ~29 cm.
- Blondel: tread = 64 − 2 × riser. With riser 18 → tread 28 cm.
Frequently asked questions
What is the ideal step height?
A riser of 16–18 cm with a tread of 25–30 cm. Outside that, stairs become tiring or unsafe.
Must all steps be equal?
Yes — a safety rule. Uneven risers are the leading cause of trips on stairs.
What is the Blondel rule?
2 × riser + tread = 63 to 65 cm. It matches the human stride going up.
What is the ideal step height?
Between 16 and 18 cm is the most comfortable range for homes, accepting up to 19 cm when space is tight.
How much floor space will the stairs take?
Multiply the tread depth by the number of steps minus one. With 16 steps at 29 cm, that is 15 × 29 = 4.35 m of floor length.
Why must all steps be identical?
Because your body memorises the rhythm after the first steps and climbs without looking. A few millimetres of difference in one step is the leading cause of trips.
Does this replace a professional design?
No. It solves the geometry of the flight. Structure and code compliance require a qualified engineer or architect.
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Updated on June 30, 2026 · by Rafael Rossi · Methodology & sources