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Stair Calculator

Calculators · Added

Every riser in a flight has to be the same height, and the total rise is fixed by the building — so the riser height is not something you choose, it is the rise divided by a whole number. This finds the number that lands closest to your target, shows the alternatives either side, and checks each against the code limits.

Units

Finished floor to finished floor, not floor to joist

The solver picks the nearest whole number of risers

Horizontal depth of one step, excluding the nosing

Adds to the board width, not to the run

Leave blank if the stair is not constrained horizontally

How to use the stair calculator

  1. 1Measure the total rise from finished floor to finished floor, not floor to joist.
  2. 2Enter your target riser height — around 175 mm or 7 inches suits most flights.
  3. 3Enter the tread going, which is the horizontal depth of a step without the nosing.
  4. 4Read the chosen flight, then compare it with the other riser counts in the table.

Examples

A standard domestic floor height

Input
2700 mm rise, 175 mm target riser, 260 mm going
Result
15 risers at 180 mm · 14 treads · 3640 mm run · 36.6° pitch

16 risers would give a shallower 168.8 mm step, but 180 mm is nearer the target.

Where the space runs out

Input
The same flight with only 3500 mm of floor available
Result
Drops to 14 risers at 192.9 mm and a 3380 mm run — the only count that fits

It fits, but 2R + G is now 646 mm, outside Blondel's range, and the page says so.

About the stair calculator

The arithmetic of an equal-riser flight

Start with the total rise and divide by the riser height you would like. The answer is almost never a whole number, so you round it — and the two candidates either side give two different flights. Rounding up gives more risers, each one shorter, and a longer flight along the floor. Rounding down gives fewer, taller risers and a steeper stair in less space.

That is the whole trade-off, and it is why this shows both rather than picking silently. Where floor space is the constraint, the taller-riser option may be the only one that fits; where comfort matters more, the shallower one usually walks better.

What the stringer length does not include

The figure here is the hypotenuse of the rise and run — the length of the sloping line the treads sit on. A real stringer needs more material than that: the notches remove depth, the top and bottom connections need their own allowance, and the board has to be wide enough that what remains after cutting is still structural.

Use the calculated length to work out what stock to buy and add a generous margin. Cutting a stringer short is unrecoverable; cutting it long costs one trim.

Frequently asked questions

Why is there one fewer tread than riser?
Because the upper floor is the last tread. A flight climbing to a landing has a riser between every pair of surfaces, and the top surface is the landing itself, so the tread count is one short of the riser count. It is the off-by-one that ruins a set of cut stringers, and it is why the tread figure here is always derived rather than entered.
What is the 2R + G figure for?
It is Blondel's rule, published in 1675 and still the most useful single check on whether a flight will be comfortable. Two riser heights plus one going should come to roughly the length of a walking pace — 600 to 640 mm. A stair can satisfy every code limit and still fall outside that range, and it will feel wrong to climb.
Are the limits shown the ones that apply to my stair?
Only if your local authority has adopted them. The figures quoted are from the International Residential Code, which is the basis for most residential work in the United States and a reasonable reference elsewhere. Other codes differ — the UK's Approved Document K permits a steeper private stair, for one — so check what is in force where you are building. Nothing here replaces the code or the inspector.
How much can the risers vary in practice?
Codes generally allow about 9.5 mm, or three eighths of an inch, between the tallest and shortest riser in a flight. The reason is not tidiness: people stop looking at the steps after the second one, and a riser out of step with its neighbours is where a fall happens. It is the most enforced rule in stair construction.

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