How riser and tread are calculated
Blondel's Rule (1675) sets a comfortable relationship between how high you step and how far you reach — twice the riser height plus the tread depth should equal about 63 cm, matching a natural walking stride.
- R — riser height (cm) — height of one step
- T — tread depth (cm) — depth of one step
- 63cm — Blondel's constant, an average walking stride
More detail
Why 63 cm?
French architect François Blondel proposed this comfort formula in 1675: a taller riser means a shorter reach is needed, so the tread can be shallower, and vice versa. The 63 cm target keeps the up-and-forward motion of climbing close to a natural walking stride, which is why it still underlies stair design guidance today.
When the numbers don't work
If the total height is small relative to the step count, the riser gets very low and the formula pushes the tread depth past what's practical. In the opposite case — too few steps, so the riser exceeds 31.5 cm — the tread depth comes out zero or negative and no result is shown. Adjust the step count until the riser lands roughly in the 15–20 cm range (tread 23–33 cm).
Practical tip. Most comfortable staircases land around a 17–18 cm riser, which pairs with a 27–29 cm tread under 2R + T = 63. If your result is far outside that, try a different step count before finalizing a design.
Frequently asked questions
What is Blondel's Rule?
A stair-comfort formula published by French architect François Blondel in 1675: 2 × riser height + tread depth ≈ 63 cm. It balances the vertical and horizontal effort of each step to match a natural stride, and is still referenced in architectural design guidance.
What's a comfortable riser height for stairs?
Most comfortable stairs use a riser between about 15 and 20 cm. For a total height of 210 cm split into 14 steps, this calculator gives a 15 cm riser and a 33 cm tread — enter your own height and step count above to check yours.
Why does my tread depth come out negative or zero?
That means the step count is too low for the given height — the riser is too tall for Blondel's formula to yield a positive tread. Try increasing the number of steps to bring the riser down.
What is 'total run' in a staircase?
It's the total horizontal length the staircase occupies: tread depth × (number of steps − 1), since the top step meets the upper floor level and needs no additional tread. This is the number to check against available floor space.
Is Blondel's Rule a legal building code requirement?
No — it's a comfort guideline used in architectural design, not a substitute for your local building code, which sets binding minimum and maximum riser/tread dimensions. Always verify against your local code before construction.