Stair arithmetic starts with one measurement and one constraint. The total rise, floor to floor, is divided into a whole number of equal risers, and the riser height and going are governed by regulation, so the number of steps is chosen to bring both inside the limits rather than to suit the space. Everything else, the total run, the stringer length and the headroom, follows from that division, which is why a stairs calculator is really a divider with rules attached.
Divide the total rise into equal risers
Measure floor to finished floor, divide by an approximate target riser height, round to a whole number of risers and divide back to get the exact riser. Every riser must be equal: an uneven one is a trip hazard and a regulatory failure, and it is the most common fault in site-built stairs.
Run and stringer follow from the riser: the stair stringer calculator
The number of goings is one less than the risers, and the total run is that multiplied by the going. The stringer length is the diagonal of the total rise and run, which is the number a stair stringer calculator gives you and the reason a long flight needs a longer board than people expect.
Finishing the same space: the tiling calculator and plank flooring layout calculator
A tiling calculator and a plank flooring layout calculator answer the neighbouring question on the landing and the hall: area divided by unit, plus the waste the layout produces and the cuts at the perimeter. All three calculations share the lesson that the waste depends on the pattern rather than on the area.
Questions people ask about stairs calculator
What riser height and going are allowed?
They are set by the building regulations that apply where you are, and they differ between domestic and other buildings. Take the limits from the applicable code rather than from a rule of thumb, because this is one of the details an inspector checks.
Why must all risers be equal?
Because people climb stairs without looking, and a single different riser is where they fall. It is both a safety matter and one of the easiest things for an inspector to measure.
What about headroom?
It is governed too, and it is decided by the flight's position rather than by the stair arithmetic. Check it before cutting anything, because a flight that works on paper and fails on headroom has to move.