What is "dim weight"?
Most parcels are billed by their actual weight, but large ones can be billed by their dimensional weight if it's higher. Each carrier has a formula. The units are inches.
USPS: LxWxH/166 FedEx Ground: LxWxH/194 FedEx 2 day, Express, Overnight: LxWxH/225 UPS: LxWxH/166 DHL: LxWxH/166 (only for >1lb packages)
Dimensional restrictions
Some methods prohibit parcels over a certain size, or apply surcharges. Information about DHL's dimensional restrictions can be found here: DHL SmartMail Parcel
Example
I have a package with 2lbs actual weight, in a 16 x 8 x 10" box.
Each carrier would rate the package as though it were 6–8lbs:
USPS: 7.7lbs
FedEx: 6.6lbs
FedEx Express: 5.7lbs
UPS: 7.7lbs
DHL Express: 7.7lbs
USPS: 7.7lbs
FedEx: 6.6lbs
FedEx Express: 5.7lbs
UPS: 7.7lbs
DHL Express: 7.7lbs