How it works
The fewest risers that meet the rules are chosen first: the floor-to-floor height divided by the most a rise can be, rounded up, then more if the pitch or twice the rise plus the going is out. The height is shared equally between them, and there's one tread fewer than risers, as the top riser steps onto the floor above. The table lets you try a riser more or less.
For headroom, the line through the nosings (the pitch line) is followed up the flight. The floor above has to stop where the space between that line and its underside drops below 2.0 m (80 inches in America); from there to the top riser is the hole it needs. Its thickness is the joists, the ceiling below and the floorboards together.
These are the figures for a straight flight in a house. Winders, landings, handrails and balustrades have their own rules, and building control has the last word.
Questions
What are the Part K rules for a house's stairs?
For a private stair in England: each rise 150 to 220 mm, each going 220 to 300 mm, twice the rise plus the going between 550 and 700 mm, and a pitch no steeper than 42°. Every step in a flight the same. Wales has its own Approved Document K, and Scotland its own rules, so check where you're building.
How much headroom does a staircase need?
2.0 m, measured straight up from the pitch line (the line through the nosings) to anything above. In a loft conversion where that can't be had, 1.9 m at the middle of the stair is accepted, coming down to 1.8 m at its side.
How do I work out the number of steps?
Divide the floor-to-floor height by the most a rise can be (220 mm), and round up: that's the fewest risers. Share the height equally between them. There's one tread fewer than risers, because the top riser steps onto the floor above. Then check the pitch and twice the rise plus the going with the going you want.
Can the stairs have open risers?
Yes, in a house, as long as each tread overlaps the one below by at least 16 mm and a 100 mm ball can't pass through the gaps.
Is this enough to build from?
It's for planning the flight and the hole in the floor above. Building control has the last word, and the strings, handrail, balustrade and landings all have rules of their own.