How to Calculate Corrugated Outside Dimensions

Find the thickness of the corrugated cardboard. The manufacturer should provide you with the thickness, or you can use calipers to measure it yourself.

Add the measurement of two thicknesses of the cardboard to the length and width dimensions for the sides of the box. For example, if the thickness is 1/8 inch and your box is 12 inches long and 14 inches wide, add 2/8, which simplifies to 1/4 inch, to the length and width to arrive at 12 1/4 inches long and 14 1/4 inches wide.

Add four thicknesses to the height of the box to accommodate two sets of flaps on the top and two on the bottom. If the inside height of the box is 16 inches, add 4/8 inch, which reduces to 1/2 inch, to arrive at a height of 16 1/2 inches.