Use this calculator to determine the required stairwell opening size in metric units. Input the stair rise, run, nosing projection, floor thickness, and required headroom. The calculator provides the total stairwell opening length in metres and the number of stair treads at the point where the required headroom is reached.

Calculator

1. enter the riser height (unit rise) in millimetres
2. run depth (unit run) in in millimetres
3. thickness of floor structure (floor finish material plus joist plus plywood plus ceiling finish) in in millimetres
4. required headroom metres
   Note: Keep in mind that the required headroom must allow for finished surfaces.
5. nosing, the amount the stair tread projects past the vertical riser in millimetres

Results

1. Opening Size - the length of the rectangular hole to cut in the floor, shown in metres
2. Tread # at head-room - The first tread whose nosing sits directly under the required head-room line

"Tread # at head-room" equals the number of steps down from the landing (the floor itself is not counted) to the first tread whose nosing falls wholly below the required head-room clearance line.