Billable Weight

How carriers work it out

Two numbers go in. Actual weight is what the packed box weighs on a scale. Dimensional weight is a weight the carrier assigns for the space the box takes up: length × width × height in inches, divided by a number called the divisor or DIM factor. The carrier compares the two and rates the package on whichever is higher. A dense box bills on its scale weight; a big, light one bills on its size.

Both numbers are rounded up before they're compared. UPS rounds each measurement up to the next whole inch (1.01 to 1.99 inches becomes 2) and any fraction of a pound up to the next whole pound. FedEx and USPS also round each side up to the next inch and the DIM result up to the next pound. One exception on the scale side: at retail, USPS Ground Advantage prices parcels under a pound in ounce steps (4, 8, 12 and 15.999 oz), so pound rounding there starts above 1 lb.

The divisors carriers use

A smaller divisor gives a higher DIM weight, so the divisor decides how hard a carrier charges for empty space.

• UPS: 139 for Daily Rates and 166 for Retail Rates, per UPS.
• FedEx: 139 for U.S. shipments, in its U.S. packaging guidelines.
• USPS: 139 for Priority Mail and Ground Advantage, applied only when the rounded volume is over 1,728 cubic inches (one cubic foot). USPS moved from 166 to 139 on July 12, 2026, per the Domestic Mail Manual. Domestic Flat Rate boxes and envelopes are the exception: they're priced at one flat rate up to 70 lb, whatever they weigh.

One box, three answers

Take a 12 × 12 × 12-inch box that weighs 2 lb packed, with the box and filler included. Its volume is 1,728 cubic inches.

• FedEx and UPS Daily Rates: 1,728 ÷ 139 = 12.43, rounded up to 13 lb. Billable weight is 13 lb.
• UPS Retail Rates: 1,728 ÷ 166 = 10.41, rounded up to 11 lb. Billable weight is 11 lb.
• USPS: at standard weight-based rates, the box is exactly 1,728 cubic inches, not over it, so DIM weight doesn't apply. Billable weight is 2 lb.

Now make one side 12.25 inches. Every carrier rounds it up to 13, so the box is 13 × 12 × 12 = 1,872 cubic inches. That's over the USPS line, and 1,872 ÷ 139 = 13.47, so the billable weight becomes 14 lb at USPS, FedEx and UPS Daily Rates alike. A quarter inch took the USPS number from 2 lb to 14 lb.

To run your own box through all three carriers, use our dimensional weight calculator. It shows the DIM weight and billable weight for FedEx, UPS and USPS side by side.

Why the bill can come in higher than the scale

If a label is bought on scale weight but the box is big enough for DIM weight to apply, the label was priced on the wrong number. That gap can be billed after the package ships. USPS, for example, runs Automated Package Verification on Click-N-Ship and PC Postage labels and adjusts postage when the weight or dimensions on the label don't match the package that was shipped.

The fix is to measure the way the carrier does. Measure the outside of the packed box at its widest points, including any bulge, round each side up, and work out the DIM weight before you buy the label. If the DIM weight is well above the scale weight, a smaller box is the direct fix. For small, heavy USPS parcels, cubic pricing is another option worth checking.