Sketch out the shape of the corner cabinet on a piece of paper so you can properly record the measurements. Look at the bottom edges of the cabinet to determine its exact shape.
Measure the depth of the cabinet by measuring the side edges of the cabinet from where it's attached to the side walls to the front of the cabinet, as well as a measurement from the corner of the wall straight across the cabinet to the front edge.
Measure the front edge of the cabinet. Hold the tape measure against the bottom edge of the cabinet and not across the door.
Measure the height of the cabinet from the top edge to the bottom edge. Measure down one of the rails on the edge of the cabinet and not over the door.
Measure the sides of the cabinet that are attached to the wall by using the distance from the corner to the edges of the cabinet where it meets the adjoining cabinets.
Calculate the surface area of the cabinet by multiplying the length and height of each side, then adding the surface area of each side together to determine the total surface area.