Measure the width of your box with a tape measure.
Measure the height of your box.
Add the width of your box to its height, and multiply the result by two; this will give you the girth of the box. Expressed as a formula, this calculation is (width + height) x 2. If, for example, your box is 12 inches wide and 10 inches high, add these figures together, and double the result. In this scenario, your box's girth would be 44 inches.