Use internal dimensions to check the contents fit. Use the packed external dimensions and total weight to check the postal service you plan to buy.
Postal is a use, not a guaranteed rate
“Postal box”, “mailing box” and “letterbox box” can describe several shapes and styles. The name alone does not establish that a packed item qualifies for a particular carrier’s service or price band.
Check the closed external size
Allow for the thickness of the board, the assembled shape and anything that makes the package bulge. Measure after packing and closing it, and include the box and protection when you weigh it.
Check the current size, weight and content rules directly with your chosen carrier. We have not assigned any proposed Box Supply Co. products to postal price categories.
Choose a usable internal fit
Measure the contents with the protection you intend to use. Keep enough usable space for that complete arrangement and check how the box closes. Test the finished pack with your actual product before making it a repeat packing method.
Know what is in the pack
Check whether the listing includes only flat boxes or also inserts, tape or protective material. Compare packs using their actual quantities and total delivered costs.
Inside versus outside: the box size guide
Where do I check Royal Mail parcel sizes?
Use Royal Mail’s weighing and measuring guide, which links to its current UK and international size guides. Recheck the rules for your specific service before buying postage.
A supplier’s internal box size cannot establish your postage format. Weigh the complete package and measure it after closing, including any bulge.
Can a box be too tight even if the measurements match?
Yes. A folded flap, insert or closure may use some space, and manufacturing tolerances can affect the fit. Ask which measurements describe the usable space and test a sample with the real contents.