If you have a chair rail angling up your staircase, you may need to calculate its angle for a variety of reasons, such as cutting wallpaper at an angle to fit above or below the rail. Because the stairs below the rail disrupt the horizontal line you want to use to measure your angle, it may seem difficult to compute. You only need a small portion of the horizontal line to calculate your angle, however, because the angle of the chair rail is consistent for its entire length.
Place a bubble level flat against the wall below the chair rail so the bottom corner of the chair rail meets the top edge of the bubble level.
Tilt the bubble level slowly up or down, keeping it flat against the wall, until the bubble on the level is exactly centred between the level's guidelines. Hold the level in place firmly with one hand.
Place a protractor against the level so the middle mark on the protractor's flat side and the "0" mark at the curved side's centre both line up with the top edge of the bubble level. The middle mark on the flat side should also line up with where the chair rail meets the level.
Read the angle of your chair rail by looking through the translucent curve to see where the bottom edge passes through the angle markings. The protractor will most likely list two angle possibilities for that marking. The angle that is less than 90 degrees is your chair rail's angle above the horizontal plane (the floor). The angle larger than 90 degrees is the amount your chair rail angles off the vertical plane (the edge of the wall).
Opaque protractors can also be used, but the translucent or transparent types, often found in the drafting sections of office supply or craft stores, are easier to read in this instance.