Stair calculator
Measure floor to floor — finished surface to finished surface — and this divides it into risers that are all exactly equal, which is the part that matters.
Equal risers are a safety requirement, not a preference
People climb stairs by feel, not by looking. A single riser that differs from the others by even a quarter of an inch is a genuine trip hazard, and building codes almost everywhere limit the variation between the tallest and shortest riser in a flight. That is why the number here comes out to three decimal places: you divide the total rise by a whole number of risers and build to that figure, rather than rounding to something convenient and absorbing the error at the top or bottom step.
The rise-plus-run check is the old carpenter's rule. Riser height plus tread depth landing between 17 and 18 inches produces stairs that feel natural to walk; drift outside it and they feel either cramped or stretched, even if every individual dimension passes code.
Two things this deliberately leaves to you: your local code's exact maximum riser and minimum tread, which vary by jurisdiction and by whether the stair is interior, exterior, or serving a deck; and the thickness of the tread material at the top and bottom, which changes total rise if the finished floor is not already in place when you measure.
Questions
Why is there one fewer tread than riser?
The top riser lands on the finished upper floor, which serves as the last tread. A flight with 14 risers has 13 treads.
What is the maximum riser height?
Commonly around 7¾ inches for interior stairs, with minimum tread depth around 10, but this varies by jurisdiction and stair type. Check your local code before cutting stringers.
What if my total rise gives an awkward riser height?
Try one more or one fewer riser. Adding a riser lowers each one and lengthens the total run; removing one does the opposite. The run figure above updates so you can check it still fits the space.