Stair run calculator

Total run is the horizontal footprint of the stair, measured from the first riser face to the last, and it's what shows up on a floor plan as the length of the stairwell. It's independent of stair width, which is a separate, perpendicular measurement.

Total run = tread count × tread depth, and tread count is always riser count minus one. A stair with more, shorter risers takes less height but a similar or larger run, depending on the tread depth you choose to pair with it.

Total run for stair math purposes is measured nose to nose, so nosing overhang doesn't add to it, it just adds to how far the physical tread board extends past the riser below. Two stairs with the same 10.5 in tread depth need the same total run whether the nosing is ¾ in or 1¼ in.

Where nosing does matter for space planning is at the very top and bottom of the flight, where the first or last nosing can overhang slightly past the calculated run line and needs clearance from a wall or door swing.

When your stairwell opening is fixed, work backwards: divide available run by the tread count you expect (based on riser count minus one) to see what tread depth that gives you, and check it against commonly cited minimums before committing.

130 in of available run with 14 risers gives 13 treads: 130 ÷ 13 = 10 in, right at the commonly cited IRC minimum for residential treads. If your opening were 120 in instead, you'd land at 120 ÷ 13 = 9.2 in, below that minimum, and you'd need a different approach.

1. Measure the available run in the floor plan or existing opening.
2. Pick a riser count from your total rise and target riser height.
3. Subtract one from risers to get tread count.
4. Divide available run by tread count to check the resulting tread depth against your target and against commonly cited minimums.

If available run is too short for a comfortable, code-minimum tread depth, you have a handful of real options: steepen the stair with a taller riser and fewer treads, add a landing partway up and split the flight into two shorter runs, switch to winders or an L-shaped layout, or, on the outdoor side, use an alternating tread stair for very tight spaces.

None of these fixes is free. A steeper stair is less comfortable and may exceed commonly cited riser limits. A landing takes floor space in a different direction. Winders concentrate tread depth unevenly. Pick based on what your floor plan can actually absorb.

Using riser counts from 12 to 16 (11 to 15 treads):

If height, not floor space, is your starting point, use the stair height calculator. For the diagonal stringer length rather than the flat floor run, use the stair length calculator, and for layouts that split run across a landing, see the stair calculator with landing.