Measure the shelf's depth, which is the distance it extends from the wall. For example, suppose that a shelf extends 12 inches from the wall.
Hold a carpenter's level below the shelf perpendicular to the wall. The level's bubble will float to the center of its tube when it is horizontal.
Depending on the shelf's angle, the level may make contact with the shelf's back or front.
Measure the horizontal distance along the level between the wall and the front of the shelf. For example, suppose that this distance measures 115 inches.
Divide the second measurement by the first. In the example, 115 divided by 12 is 0958.
Find the inverse cosine of this ratio on a scientific calculator. The inverse cosine of 0958 is 1666.
Identify whether the level touched the front or back of the shelf in step 2. If the level touched the front of the shelf, the brace must be less than 90 degrees.
Subtract 1666 from 90, giving 7333 degrees. If the level touched the back of the shelf, the brace must be more than 90 degrees.
Add 1666 to 90, giving 10666 degrees.
Things You Will Need
- Measuring tape
- Carpenter's level
- Scientific calculator
- Shelves most often meet the wall at right angles, which measure 90 degrees. In this case, you can use a 90-degree brace without calculating the shelf's angle.