The 63 cm rule: why proportion decides, not any single measurement

What the rule actually says

2 × riser height + tread depth ≈ 63 cm

The idea is simple: climbing shortens your stride, and the higher you lift your foot the shorter it gets. The rule turns that trade-off into one number — every centimetre of height “costs” roughly two centimetres of stride, so twice the height plus the depth stays around a constant.

Our tool measures how far you are from 63 cm and shows that distance as a signed number: −0.06 cm means slightly tighter than 63, +1.94 cm slightly roomier. The words next to it describe only the distance, never a verdict: nowhere does it say the stair is good, bad, correct or acceptable, because none of that follows from the sum.

The rule is attributed to the 17th-century French architect Nicolas-François Blondel, who derived it from the length of a human stride. Different sources today quote 62, 63, 64 or 65 cm — and that disagreement is the clearest sign of what it is: a convention carried through practice, not a figure prescribed anywhere.

The same sum, completely different stairs

The most useful thing to learn about this rule is where it stops speaking. Take five flights on the same 280 cm storey. The one with a 20.00 cm riser and a 22 cm tread sums to 62.00 cm; the one with a 14.00 cm riser and a 35 cm tread sums to exactly 63.00 cm. By the rule both are near the top, and their pitches are 42.3° and 21.8° — almost twenty degrees apart.

The difference shows up in space too: the first flight takes 2.86 m of floor, the second 6.65 m. Those are two entirely different decisions about a house, and the sum does not distinguish them.

So read the sum as a proportion check, not as a grade. A sum far from 63 almost certainly means something is out of balance — a tall step with a shallow going, or the reverse. A sum close to it means only that the two measurements agree with each other, not that the staircase is comfortable, roomy, or right for your house.

The riser is twice the lever the tread is

Because the height enters the formula multiplied by two, one centimetre of height moves the sum as much as two centimetres of depth. That is the most practical takeaway here: if the sum is not where you want it, the height limit shifts it faster than the tread does.

In the table below the tread is fixed at 30 cm and only the height limit moves. The sum runs from 58.00 cm at a 14.00 cm riser to 70.00 cm at a 20.00 cm riser — a 12 cm spread, while the pitch moves by less than nine degrees.

The second table is the reverse: the riser stays at 16.47 cm and the tread moves from 24 to 36 cm. The sum runs from 56.94 to 68.94 cm — the same 12 cm spread, for twice as much change in the measurement.

When the sum means nothing at all

If you do not enter a tread depth, the tool derives it from the rule itself: a 16.47 cm riser suggests 30.06 cm. The sum then reads exactly 63.00 cm and the deviation 0.00 cm — but that is not a property of the staircase, it is a property of the arithmetic. The number is what it is because it was built that way.

The suggestion is a useful starting point, but you only get real information once you enter a depth you can actually build — the one that fits your space, or the one the joiner quoted. Only then does the sum say something about your stairs rather than about the formula.

The same holds in reverse: if the space dictates a 25 cm tread, the sum will be 57.94 cm and there is nothing the sum can fix. The answer is more room or a different layout, not a different number in the formula.

How to work out the height and depth of a single step →

What the rule does not say

It says nothing about safety. The sum does not know whether there is a handrail, how much headroom there is, whether the walking surface is slippery, or who will be using the stairs.

It says nothing about which dimensions are permitted where. Heights, depths, widths and balustrades vary by country, by building type and by the use of the space, and we have researched them for no market and do not check them in this tool.

And it says nothing about construction: the nosing overhang, the thickness of the tread, notching the stringer and fixing it are not part of this calculation. For all of that, ask a designer or your local building authority — the geometry is only the first step.

Check your own numbers

Open the calculator, enter your own storey height and limit, then try two or three depths. Each one immediately shows the sum, the distance from 63 cm, the pitch and the floor space the flight takes — and those four together are what you decide on, not one number.

The values in this article are examples, not recommendations: both the height limit and the depth stay editable, because the real measurement is on your storey, not in the formula.

Open the calculator with this article's example →