Assuming a regular stair case:

Measure the distance from the front of the bottom stair to the wall.

If you can, measure the height of stairs up the wall.

The slope is the Height/Base.

If you cannot measure the height then measure from the floor in front of the bottom stair to the top stair and calculate the height.

Height^2 + Base^2 = Stair measurement^2 (^2 = squared)

if base = 30 and stair measurement = 50 then

Height^2 = 50^2 - 30^2

Height^2 = 2500 - 900

Height^2 = 1600

Height = 40

Slope = 40/30 = 1.33333

Measure the distance from the front of the bottom stair to the wall.

If you can, measure the height of stairs up the wall.

The slope is the Height/Base.

If you cannot measure the height then measure from the floor in front of the bottom stair to the top stair and calculate the height.

Height^2 + Base^2 = Stair measurement^2 (^2 = squared)

if base = 30 and stair measurement = 50 then

Height^2 = 50^2 - 30^2

Height^2 = 2500 - 900

Height^2 = 1600

Height = 40

Slope = 40/30 = 1.33333