Measure the product first
Measure the product in the orientation it will occupy inside the box. Include caps, protrusions and any protective wrapping. For multiple items, arrange them together before taking measurements.
Internal versus external dimensions
Internal dimensions describe usable space; external dimensions include the structure and board thickness. Suppliers may use different dimension conventions, so confirm that your brief and dieline use the same definition.
Length, width and depth
Write every measurement with a unit and label it. A common convention is length × width × depth, but do not rely on the order alone. Provide a drawing or product photo when orientation is ambiguous.
Allow space for protection
Inserts, dividers and cushioning need room. Too much empty space can let products move, while too little can make assembly difficult or put stress on the product. Confirm tolerances and fit through a physical sample.
Check the shipping package
A retail carton and its shipping box are different design decisions. Ask for the external packed size and weight when estimating freight. Confirm packing arrangements with the supplier before production.