Blondel's rule
This relationship, first formalised by 17th-century French architect François Blondel, approximates a comfortable human stride: taller risers naturally pair with shorter treads and vice versa. The calculator rounds your floor-to-floor height into a whole number of equal risers close to your target, then derives the matching tread depth from the 630mm midpoint of the rule.
Reading the result
Most building codes also set independent minimums — commonly a riser between 150-200mm and a tread of at least 250mm. The calculator flags whether your resulting dimensions fall in the generally comfortable/code-compliant range, but always check your governing local code, since exact limits vary by jurisdiction and occupancy type.
Frequently asked questions
What does '2R + T ≈ 630mm' actually mean?
It's a rule of thumb balancing riser height (R) and tread depth (T): as one goes up, the other should come down to keep the stride length comfortable. 630mm is the commonly used midpoint of the accepted 600-660mm range.
Why doesn't my floor height divide evenly into my target riser?
The calculator rounds to the nearest whole number of risers, then recalculates the exact riser height needed to hit your floor-to-floor height precisely — risers must be equal, so the target is a starting point, not the final figure.
Does this check headroom clearance?
No — this tool only checks riser/tread proportions, not headroom, which depends on the stair's slope and any structure above it. Check headroom separately against your code's minimum (commonly around 2m/6'8").