Carriers use pounds per cubic foot to calculate the dimensional weight of all shipments. If the dimensional weight is higher than the scale weight the dimensional weight is always used.
Each carrier uses a different rate of pounds per foot and some carriers used different weights for different services.
The easiest way to determine your dimensional weight is to first find the dimensional divisor. This is done by taking (1728/pounds per foot).
Ground = 1728/12.4 = 139.4
Select = 1728/15 = 115.2
Next you will take (Length x Width x Height) / dimensional divisor.
20 x 20 x 20 (Ground)
20 x 20 x 20 = 8,000
8,000 / 139.4 = 57.4 lbs.
Your item might only have a scale weight of 5 lbs but when it is quoted, we use 57.4 lbs.