How the risers are worked out
Divide the total rise by the riser you would like, then round to the nearest whole number. That is how many risers the stair will have.
Dividing the total rise by that whole number gives the actual riser, which every step must match. A 2,600 mm rise with 190 mm preferred risers gives 13.68, so 14 risers of 185.7 mm each.
You cannot have a different riser at the top or the bottom. Building control treats a varying riser as a trip hazard, and it is the most common reason a stair fails inspection.
Worked example
A 2,600 mm rise with 190 mm preferred risers gives 14 risers at 185.7 mm. With a 220 mm going the pitch is 40.2 degrees, and twice the rise plus the going is 591 mm.
That satisfies Part K on all three counts: the riser is under 220 mm, the going is at least 220 mm, and the pitch is under 42 degrees.
The pitch and the comfort rule
Pitch is the angle of the line joining the nosings: the arctangent of the riser divided by the going. Steeper stairs are harder to descend, which is why the 42 degree limit exists for a private stair.
The comfort rule, twice the rise plus the going, comes from the way people walk up stairs. Between 550 and 700 mm suits most adults; below that feels cramped and above it feels like climbing.
A stair that satisfies the maxima can still feel wrong. A 220 mm riser with a 220 mm going passes every limit and sits at 42 degrees, which is steep for daily use.
Limits of this calculator
It calculates the geometry of one straight flight. It does not check headroom, which needs 2,000 mm measured vertically from the pitch line, or the width, which is 900 mm for a dwelling.
It does not design the structure, the stringers, the balustrade or the fixings, and it does not check the landing at the top or bottom.
It uses the limits for a private stair in a dwelling. A common stair in a block of flats has different limits, and so does a utility stair.
For US riser and tread limits in inches, use the US stair calculator.
Inputs for the stair calculator
Total rise
Floor to floor height in millimetres, which is what the stair has to climb.
Preferred riser height
The riser you would like. The stair is built from a whole number of them.
Going
Depth of each tread from nosing to nosing. Part K asks for at least 220 mm.
CALCULATOR QUESTIONS
UK stair calculator questions
How many stairs will I need?
Divide the rise by about 190 mm and round to the nearest whole number, then divide again to get the exact riser.
Can the top step be a different height?
No. Every riser must be equal, or the stair becomes a trip hazard and fails inspection.
What is the maximum pitch?
42 degrees for a private stair in a dwelling. That is roughly a 220 mm riser with a 220 mm going.
Does this check headroom?
No. Headroom needs 2,000 mm measured vertically from the pitch line, and it is a separate check.