Dimensional weight reflects package density, which is the amount of the space a package occupies in relation to its actual weight. Dimensional weight can apply to all UPS domestic package services.

To determine the dimesional weight packages are measured in inches. Each dimension is measured at the longest point, with each measurement rounded to the nearest whole number. Multiply the package length by the width by the height. The result is the cubic size in inches. 

For Domestic Shipments divide the cubic size in inches by 166 to determine dimensional weight in pounds. Increase any fraction to the next whole pound.

For International Shipments divide the cubic size in inches by 139 to determine the dimensional weight.

That's how you get the dimenisional weight

Packages are also weighed. The package's actual weight is compared to its dimensional weight. The greater of the two is the billable weight and is used to calculate the rate.