Stair rise and run explained (and how to get them right)
6 min read · Updated 2026-10-12 · BuildCalc
Stairs look complicated and are actually governed by one rule: every step must be identical. That requirement shapes the whole calculation, because it means you start with the total rise and divide it into a whole number of equal risers rather than choosing a riser height and seeing what happens. Get that right and the rest follows.
Divide the total rise into equal risers
Measure the total rise from finished floor to finished floor, then divide by your preferred riser height to get a number of steps, and round it to a whole number. Divide the rise by that number and you have the exact riser height, which will rarely be a round figure — and it does not need to be.
Around seven inches is comfortable for most people, and building codes typically allow between roughly four and eight inches for a private stair. Slightly shallower is easier on the knees; steeper saves floor space at the cost of comfort, and a stair that is too steep is genuinely dangerous to descend.
Treads, run and the comfort rule
The number of treads is one fewer than the number of risers, because the top step is the floor above. Multiply the tread depth by the tread count to get the total run — the horizontal distance the stair occupies. Ten-inch treads are typical, with the nosing overhanging slightly at the front of each step.
A widely used comfort rule says twice the riser plus one tread should come to about twenty-five inches. A seven-inch riser with an eleven-inch tread fits the rule nicely. Deviating far from it produces a stair that feels either abrupt or endlessly shallow, and both are tiring in daily use.
Stringers and the practical checks
The stringer is the sloped beam that carries the treads, and its length is the hypotenuse of the rise and the run. Add a little for the notches and for the ends that sit on the floor and the landing. Two stringers suit a stair up to about three feet wide; wider stairs want a third in the middle.
Before cutting anything, check headroom — you need roughly six feet eight inches of clearance above the nosing line — and check the local code, because permitted riser and tread dimensions, handrail heights and landing requirements are all regulated. Stairs are the most injury-prone part of a house, and the rules exist for good reasons.
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Common questions
Why must all risers be the same height?
An uneven step, especially a taller final one, is a trip hazard. Codes require risers to match closely, so you choose the number of steps to make them come out even.
How do I find the stringer length?
It is the diagonal of the total rise and total run: the square root of rise squared plus run squared, plus a little for notches and end cuts.
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