Use internal dimensions when checking whether a product will fit. External dimensions are useful for shipping cartons and shelf space, but they do not tell you the usable space inside the box.
Start with the product arrangement
Measure the product in its intended orientation, including any protective wrapping. For a gift set, arrange every item as it should appear when opened, then measure the overall footprint and height.
Allow space for the insert
A fitted insert occupies part of the interior. Its material thickness, walls and cavities can change the required box size. Ask for the insert and box to be checked together rather than sizing them independently.
State the units and order of dimensions
Send length × width × height, explicitly marked as internal dimensions, in centimetres, millimetres or inches. Include a sketch or photograph to remove ambiguity about orientation.
Confirm with a sample
A physical fit check helps reveal tight clearances, lid interference and difficult product removal. Agree dimensional tolerances and the necessary clearance with your supplier before approving production.