Stair Calculator
Riser height, tread depth, stringer length and the code checks, from total rise.
Geometry and the common limits. Your local code governs, and requirements for handrails, guards, landings, and winder geometry are not covered here.
How it works
A flight of stairs has one more riser than it has treads, because the top riser lands on the upper floor and needs no tread of its own. Getting that off by one is the classic stair mistake and it puts every riser in the flight fractionally wrong.
The method is fixed: divide the total rise by your preferred riser height, round to a whole number, then divide the total rise by that number to get the real riser height. Every riser in a flight must be the same within a few millimetres — people climb stairs by proprioception, not by looking, and a single odd riser is what trips them.
The comfort rule is 2R + G, twice the riser plus the going, and it should land between about 600 and 640 mm — 24 to 25 inches. It encodes the geometry of a human stride: a taller riser needs a shallower tread to keep the step natural. A flight that satisfies both limits individually but fails 2R + G still feels wrong to walk on, and everyone notices even if they cannot say why.
Codes then bracket it from both ends. UK domestic: maximum 220 mm rise, minimum 220 mm going, maximum 42° pitch. US IRC: maximum 7¾ in rise, minimum 10 in tread, and a rule that the largest and smallest riser in a flight may not differ by more than 3/8 in.
Headroom is where stairs get expensive. Two metres measured vertically from the nosing line means the floor opening above has to be considerably longer than the run of the flight, and discovering that after the joists are cut is a bad day.
Common questions
Why one more riser than treads?
Because the top riser brings you up to the finished upper floor, which serves as its own tread. A flight of 15 risers has 14 treads. Counting them as equal is the single most common stair error.
Do all the risers really have to match?
Yes, and codes enforce it — the IRC allows 3/8 in of variation across a whole flight. People climb stairs without looking. An odd riser at the top or bottom, which is where the fudge usually ends up, is exactly where falls happen.
My stair is too steep. What do I change?
Only the total run, in practice. The rise is fixed by the building. A shallower pitch needs more risers or a deeper going, both of which eat floor length — which is why steep stairs exist in old houses and why a winder or a landing is often the real answer.
What is the stringer length for?
It is the diagonal of the flight, so it tells you the minimum board length to buy. The minimum board width matters too: cutting the triangles out of a stringer removes a lot of timber, and what is left under the notches has to still be a beam.