Stair calculator
A badly proportioned stair is obvious from the first step: you either trip or climb it like a ladder. The two numbers that matter are riser and tread, and the relation between them is three hundred years old.
The 2h + g rule
Blondel's rule says twice the riser plus the tread should land between 61 and 66 centimetres. Below that the stair is tiring because the step is cramped; above it, the climb feels forced.
A comfortable riser is 16-19 cm and a tread 25-32 cm. Every step in a flight must be identical: one centimetre of difference on the last step is the most common cause of tripping.
What else to check
- Headroom: at least 200 cm measured vertically from the step nosing.
- Stair width: 90 cm indoors; below 80 cm you cannot carry furniture up.
- If the run is too short, split the stair into two flights with a landing rather than raising the risers.
- A handrail is required above three steps, at 90-100 cm height.
Why do I get a fractional step count?
The step count is always whole: the calculator rounds it and recomputes the riser so every step is equal.
Can I make the stair steeper to save space?
Above 42 degrees a stair becomes uncomfortable and dangerous on the way down. A flight with a landing is the better answer.
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