Comfort
Using the Blondel formula for comfortable stairs
What the Blondel formula (2h + g) measures, why the target sits near 600 to 640 mm, and how to use it to balance riser height and tread depth.
What the Blondel formula measures
The Blondel formula adds two riser heights to one going. In metric design the target usually sits around 600 to 640 mm, depending on local practice and how the stair will be used.
A result in that range keeps the walking rhythm close to a natural stride.
Why it helps
A stair can fit on the plan and still feel awkward to climb. The Blondel value flags many of those combinations before they turn into expensive mistakes on site.
When the result is too low, the stair tends to feel flat and long. When it is too high, the stair can feel steep and tiring.
Use it alongside real constraints
Check the Blondel value together with local code, headroom, landing dimensions and the opening you actually have.
On a renovation, compare several layouts before you commit to cutting the stairwell or setting out the stringers.
Put this into practice
Open the editor and test these numbers on your own staircase, with live drawings and a comfort score. No account needed.
Related guides
Comfort
Stair comfort grade explained
How a stair comfort grade combines riser height, going, pitch and the Blondel value, what the score tells you, and where your own judgment still matters.
Stair calculations
How to calculate stair rise and run
Work out stair riser height, going, pitch and total run from your floor-to-floor height, with the typical ranges to aim for before you build.
Planning
Stair headroom: what to check before building
How stair headroom depends on the floor opening, slab thickness and finishes, and the layout mistakes that cause clearance problems near the top of a flight.