Most people pick a shipping box by eye, find one the product fits into, and fill the gap with paper. That works, and it quietly costs money on every single parcel. Box size is a pricing decision before it is a packing decision.
Why an oversized box costs more than it looks
Carriers do not bill on weight alone. They bill on the greater of actual weight and dimensional weight, because a van fills up on volume long before it hits its weight limit. Dimensional weight turns the size of your box into a billable number.
The formula is the outer dimensions multiplied together, divided by a figure the carrier sets:
Dimensional weight = (length x width x height) / DIM divisor. UPS and FedEx commonly use 139 for domestic parcels in inches, but divisors differ by carrier, service and negotiated contract. Check your own rate card rather than assuming.
The consequence is that air is billed. A box two inches larger in every direction than it needs to be can move a parcel into a higher band without a gram of extra product in it. Over a few thousand shipments that is a real line on the accounts, and it is invisible unless you go looking.
How much space to actually leave
Clearance depends on what is inside and what is protecting it, not on a universal number.
| Contents | Clearance per side | Why |
|---|---|---|
| Rigid, robust item | 5–10mm | Enough to get it in and out without forcing |
| Fragile item | 25–50mm | Room for genuine cushioning on every face |
| Soft goods, apparel | Minimal | The product absorbs shock itself |
| Multiple loose items | Varies | Space for dividers, not just fill |
Note that these are interior figures. Board has thickness, so the exterior is larger than the interior by roughly the board thickness on each face, and dimensional weight is calculated on the exterior. On double wall that difference stops being trivial.
Stock sizes against custom
A stock box is cheaper per unit and available immediately. Its weakness is that it is sized for the nearest common dimension rather than for your product, so you pay for the gap in two places: the void fill that goes into it, and the dimensional weight of the air.
- Stock wins on low volumes, irregular or changing products, and anything urgent.
- Custom wins when one product ships repeatedly at a predictable size.
- Custom removes most void fill, which is a per-parcel material and labour cost.
- Custom lets the box be a brand surface rather than a plain brown carton.
The break-even is lower than most people expect, because the saving is per parcel and compounds. If you ship the same item week after week, work out the void fill and dimensional weight you are paying on a stock size before assuming custom is the expensive option.
Getting the measurement right
All of this depends on measuring properly, and the convention trips people up more often than the arithmetic does. Boxes are quoted length x width x depth, interior unless stated. We cover the full convention, and the interior-versus-exterior trap, in a separate guide.
If you would rather not work it out, send the product dimensions and roughly how many you ship. We size the box around the product, return a free structural dieline inside 24 hours, and you keep it whether or not you order.