Corrugated boxes have three dimensions: length, width and depth.The length and the width are the opening dimensions and the length is equal to or greater than the width. The depth is the height of the box and is perpendicular to the box opening. The length, width and depth relate to the inside dimensions of the box.
Milk Cartons with 2 layers
In order to determine the inside dimensions you will need to measure the outside dimension of your product and how many you want to fit into the box.
If you are going to have additional packing material in the box, such as foam peanuts or bubble wrap, you will need to allow for extra space.