Abstract: A slide rule comprises, to calculate the parameters of a stairway to be installed between lower and upper floor surfaces, (a) a first scale on a stationary body denoting the number of risers, (b) a second scale on a first slide denoting the elevation of the upper floor surface above the lower one and associated with a first arrow on the body, (c) a third scale on the body denoting tread depths and associated with a mark on the first slide, (d) a fourth scale on a second slide denoting the horizontal spread of the stairway and associated with a second arrow on the body, (e) a fifth scale on the body denoting the number of treads, and (f) a sixth scale on the second slide denoting the length of the stairwell and associated with a third arrow on the first slide. The first arrow indicates on the second scale the height of the risers when the number of risers on the first scale is aligned with the elevation of the upper floor surface on the second scale.