Measuring most cabinets is a simple process of measuring the length, width and height of each unit. Corner cabinets, however, require a few extra measurements because they aren't rectangular. Precisely measuring a corner cabinet is crucial when you are installing a set of cabinets. The corner cabinet is the first to be installed, and an improperly sized corner cabinet can cause the entire run of cabinets to be off. Measuring a corner cabinet is easiest if the unit is not installed, but as long as you can find the seams where the cabinet's edges are, you can measure an installed corner cabinet.
Place the end of the measuring tape at the back of one side of the cabinet, right next to the corner. Pull out the tape in a straight line until you reach the side edge of the cabinet.
Repeat the process, but start the measurement from the other side of the corner. These first two measurements often are the same, but they don't have to be.
Place the end of the tape measure against the back edge of the cabinet, along one of the ends. Measure from the back of the cabinet to the front edge. This will give you the width of the cabinet, and it will be the same for both sides.
Measure from the floor to the top of the cabinet, if the cabinet hasn't been installed. For a hanging cabinet, measure from the top of the cabinet to the bottom. This will give you the cabinet's height.