I use USPS to ship packages and wanted to clarify a couple of points:
Dimensional Weight only applies to delivery to zones 5-8.
It applies to parcels that exceed 1 cubic foot (1,728 cubic inches).
Dimensions are rounded to the nearest inch, not up.
This information comes from the regulations on the USPS website listed below:
Dimensional Weighting
Parcels addressed for delivery to zones 5-8 that exceed 1 cubic foot (1,728 cubic inches) are charged based on the actual weight or the dimensional weight (as calculated in 123.1.4), whichever is greater.
123
1.4 Dimensional Weight Rate for Low-Density Parcels to Zones 5-8
Postage for parcels addressed for delivery to Zones 5-8 and exceeding 1 cubic foot (1,728 cubic inches) is based on the actual weight or the dimensional weight (as calculated in 1.4.1 or 1.4.2), whichever is greater.
1.4.1 Determining Dimensional Weight for Rectangular Parcels
Follow these steps to determine the dimensional weight for a rectangular parcel:
a. Measure the length, width, and height in inches. Round off (see 604.7.0) each measurement to the nearest whole inch.
b. Multiply the length by the width by the height.
c. If the result exceeds 1,728 cubic inches, divide the result by 194 and round up (see 604.7.0) to the next whole number to determine the dimensional weight in pounds.
1.4.2 Determining Dimensional Weight for Nonrectangular Parcels
Follow these steps to determine the dimensional weight for a nonrectangular parcel:
a. Measure the length, width, and height in inches at their extreme dimensions. Round off (see 604.7.0) each measurement to the nearest whole inch.
b. Multiply the length by the width by the height.
c. Multiply the result by an adjustment factor of 0.785.
d. If the final result exceeds 1,728 cubic inches, divide the result by 194 and round up (see 604.7.0) to the next whole number to determine the dimensional weight in pounds.
e. If the dimensional weight exceeds 70 pounds, the parcel pays the 70-pound rate.
604
7.1.3 Rounding Numerical Values
For these standards:
a. Round off requires increasing by 1 the last digit to be kept if the digit to its right, which is not to be kept, is 5 or greater. If that digit is 4 or less, the last digit kept is unchanged (e.g., 3.376 rounded off to two decimal places is 3.38, 3.374 is 3.37).
b. Round up requires increasing by 1 the last digit to be kept if there are any digits to its right, regardless of significance (e.g., rounding up either 3.3701 or 3.379 to two decimal places yields 3.38).