Corrugated boxes have three dimensions: length, width and depth.
Measured along the opening of the box, the length is always the longer dimension while the width is always the shortest dimension. The depth is the distance between the opening and opposite panel. The length, width and depth relate to the inside dimensions of an assembled box. Inside dimensions are used for measuring as the corrugated board thickness may vary. However, when measuring the inside of an existing box, be sure to measure from the center of the score (the crushed fold line).
Pads, liners, partition pieces, die-cut pads, scored blanks and sheets of corrugated or solid fibre board, with only two dimensions, have their size specified with the first dimension parallel to the corrugations, or perpendicular to the grain of solid fibre board.
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.