How to Measure Corner Angles for a Miter Saw

Hold a framing square against the corner to be measured. If the sides of the square are flush against the wall, you can be sure that the two walls that form the corner meet at a 90-degree angle. Set your miter saw to 45 degrees to make the cut.

Open an angle finder and press its two legs against the corner. The hinge of the angle finder should be inside the angle at the corner. In the case of an outside corner, the hinge should be placed against the corner.

Lock the finder in place. The locking mechanism of different finders varies. Remove the finder from the wall, being careful not to bump the finder or change the position of its arms.

Place the angle finder on a piece of scrap lumber, hanging the lip of the angle finder over the factory-cut edge of the scrap. Trace along the inside of the finder angle with a pencil.

Insert your pencil into the hole on one of the finder arms. Drag the pencil across the board, pulling the metal arm away from the finder. This will create an arc on the scrap wood. Repeat with the second arm, creating a center point in the two arcs you have drawn.

Close the finder and align it with the center point made by the two arcs and with the center point of the angle. Draw a straight line connecting the two using a pencil.

Position the scrap board on the miter saw and align the saw blade with the traced straight line. Cut along this line to test the set of the saw blade. This is the exact angle you need for the miter cut for the corner.

Flip over one of the pieces of the cut scrap board. Place both of them against the corner to check their fit.