Postage is priced by size before it is priced by weight, which means the box you pick can quietly cost you more than the thing inside it. A product that would have slipped through a letterbox in a slim mailer goes out as a parcel instead, because the box had 20mm more depth than it needed. One order, that is pennies. A few hundred orders a month, and it is a line on your P&L.
Getting box size right is not complicated, but it does need doing properly once, with a tape measure rather than a guess. Here is how we would go about it.
Size costs you three times, not once
An oversized box is not just a postage problem. It costs you in three separate places.
First, postage. UK carriers band parcels by dimensions, and the jump between one band and the next is where the money goes. Second, void fill. Every extra centimetre of empty space has to be filled with something, and shredded paper, bubble wrap or air packaging all cost money and take time to pack. Third, damage. A product rattling around inside a box that is too big for it moves in transit, and movement is what breaks things. Replacing a broken order costs far more than the box did.
Undersizing has its own problems. Forcing a product into a box that does not comfortably take it stresses the corners, splits the flaps and makes the parcel look like a rushed job.
Measure the product, not the box you already use
The most common mistake we see is a business measuring the box they currently use and asking for the same again. That only makes sense if the current box was right in the first place, and often it was chosen in a hurry when the product was different.
Start with the product itself. Measure length, width and height at its widest points, with any retail packaging, tissue or card insert already on it. Then add a protection allowance on each dimension for whatever void fill you intend to use. For a robust item that might be 10mm; for something fragile it will be considerably more. That total is your internal box size, and internal is the number that matters. Box dimensions in our range are internal, so you are comparing like with like.
Depth is the measurement that decides your postage
Length and width tend to get all the attention, but depth is usually what pushes a parcel from one postage band into the next. UK postal pricing separates flat items from parcels largely on thickness, so a few millimetres in the wrong direction changes the price of every single order you send.
This is worth understanding properly before you commit to a size, and we have written it up in full in our guide to Royal Mail PiP size categories. If your product is genuinely flat, a shallow box or a wrap-style mailer will almost always beat a conventional box on postage. Our book and magazine wrap mailers exist for exactly this reason: they fold down to the depth of the contents rather than a fixed height, so books, prints and flat garments do not get charged as parcels.
For everything with genuine bulk, work through the postal cardboard box range and find the smallest size that comfortably takes your product plus its protection. Smallest that works, not smallest that fits.
Allow for manufacturing tolerance
Here is something most suppliers will not tell you until it is a problem. Cardboard boxes are manufactured to a tolerance of roughly 10 to 20mm either way. That is normal across the industry, not a defect, and it is a consequence of how corrugated board is cut and folded.
For most products it makes no practical difference. But if your product is 148mm across and the box is 150mm internal, that tolerance can turn a fine fit into a fight. Where the margin is that tight, build in more room or check the actual box before you order in volume.
Order a blank sample before you commit
The cheapest way to settle a sizing question is to put the real product in the real box. A blank sample costs a couple of pence, and holding one tells you more in thirty seconds than any spec sheet will. You can see whether the flaps close flat, whether the product sits still, and how much void fill you actually need rather than how much you assumed.
Seeing how your artwork prints is a separate sample, and we cover both in our guide to ordering a packaging sample. We do not print a full order until you have approved a printed sample.
Sizing affects your artwork too
Worth flagging while you are still choosing: box size changes what your branding will look like. We print onto pre-manufactured stock packaging in pre-defined print areas, so the printable area scales with the box. A design that reads well on a large parcel box can look cramped on a small letterbox-friendly one, and fine detail that survives on a big panel closes up when it is scaled down.
Check the print areas for the size you have settled on before you finalise artwork. Which areas of a box you can actually print on covers what is available and why, and it is a quicker read than redrawing a logo twice.
Do not standardise on one box too early
There is a real temptation to buy one box size for everything, and for a very small range that can be the right call. But the moment you have products of genuinely different shapes, one box means most of your orders travel in something too big, and you pay the oversize penalty on all of them. Two or three sizes covering the bulk of your orders is usually the sweet spot.
A sensible order of operations
- Measure the product itself, at its widest points, with any retail packaging already on it.
- Add a protection allowance for the void fill you plan to use.
- Check whether depth keeps you in a cheaper postal band, and consider a wrap mailer if the item is flat.
- Order a blank sample and test it with the real product.
- Confirm the print area on that size before you finalise your artwork.
Spend an hour on this once and it pays for itself on every order afterwards. If you are stuck between two sizes, ask us, and there is more detail on sizes, materials and print in our frequently asked questions.
