Shipping box sizes and postage bands: how to avoid paying for air
Why a few millimeters can change your postage, and how dimensional weight works.
Carriers charge by weight and by size. A box that is a few millimeters too thick can jump to the next price band, and a box that is too large can be charged on dimensional weight instead of the real weight. Measure the packed parcel and check the limits before you order.
UK: Royal Mail size bands
Royal Mail has several size bands. Large Letter limits are 353 x 250 x 25 mm and 750 g. Small Parcel limits are 450 x 350 x 160 mm and 2 kg. Anything bigger goes into a Medium Parcel band. Prices and limits change, so check the current figures with Royal Mail.
US: weight and dimensional weight
US carriers charge by actual weight or dimensional weight, whichever is greater. Dimensional weight is the volume of the box divided by a divisor. UPS and FedEx commonly use 139 for cubic inches. USPS applies dimensional weight to larger parcels. Always check the carrier's current rules.
Example
A 12 x 10 x 6 in box has a volume of 720 cubic inches. Divided by 139, that is about 5.2 lb of dimensional weight. If the parcel weighs 2 lb, you pay as if it weighed about 6 lb. A tighter box would cut the bill.
What to measure
- Outside length, width and height of the packed parcel.
- Actual weight including packing material.
- Whether the lid bulges when the box is full.
Making a box that fits a band
If you want to stay under 25 mm to qualify as a Large Letter, remember that E-flute adds board thickness and the lid adds a bit more. Work backwards from the outside limit, then check the inside size with your product. The postage checker on the Mailer category page does the arithmetic.
Frequently asked questions
Do you list carrier rates?
No. Rates change often and depend on your account. We show size and weight so you can check them with your carrier.