Dimensional weight calculator
Billable weight for a parcel: the greater of what it weighs and what its size says it weighs. With the break-even point, so you can see exactly when a box starts costing you for air.
Inputs
Result
What the rounding costs
The same box, measured exactly and rounded up. Since August 2025 both FedEx and UPS round every dimension up to the next whole inch before dividing.
| Measured | Dimensions | Cubic in | ÷ divisor | Billable lb |
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Why carriers charge for size
A trailer fills up long before it reaches its weight limit. A box of pillows weighs nothing and takes the space of a box of books, so carriers price the space: every parcel is charged on whichever is greater, its actual weight or a weight worked out from its volume.
The divisor is the whole argument
A divisor of 139 cubic inches per pound means the carrier expects about 139 cubic inches for every pound — roughly 12.4 lb per cubic foot. Anything lighter than that pays for the gap. FedEx uses 139 across domestic and international services; UPS uses 139 on daily rates and 166 at retail counters. The metric equivalents are 5,000 and 6,000 cm³ per kilo, and 6,000 is the IATA standard for air freight.
A larger divisor is better for you: at 166 the same box bills less than at 139. Divisors are negotiable on volume, and moving from 139 to 166 is often worth more than a headline discount on rates.
Rounding up twice
Since August 2025 both FedEx and UPS round each dimension up to the next whole inch before multiplying, then round the resulting weight up to the next whole pound. An 11.1 by 8.5 by 6.2 inch box is calculated as 12 by 9 by 7. That is 585 cubic inches measured and 756 as billed, a 29% increase from rounding alone.
Which is a straightforward argument for boxes that are a hair under a whole inch rather than a hair over.
The break-even weight
Every box has a weight above which it is charged on the scale and below which it is charged on its size. That figure is simply the cubic size divided by the divisor, and it is shown in the results. If your product weighs less than its box's break-even, you are paying to ship air, and a smaller box is worth more than a lighter one.
Questions
What is dimensional weight?
A weight worked out from a parcel's size rather than what it weighs. Multiply length by width by height in inches and divide by the carrier's divisor, usually 139. You are billed on whichever is greater, that figure or the actual weight.
What is the DIM divisor for FedEx and UPS?
FedEx uses 139 cubic inches per pound for domestic and international services. UPS uses 139 on daily rates and 166 at retail. In metric those are 5,000 and 6,000 cm³ per kilogram. Large shippers can negotiate a higher divisor, which lowers the billable weight.
How do I calculate billable weight?
Work out the dimensional weight, compare it with the actual weight, and take the greater. A 12 inch cube is 1,728 cubic inches, which at 139 is 12.43 rounding up to 13 lb. If the box weighs 5 lb you are billed for 13.
Do carriers round the dimensions up?
Yes. Since August 2025 both FedEx and UPS round every dimension up to the next whole inch before calculating, and then round the resulting weight up to the next whole pound. A box measuring 11.1 inches is treated as 12.
How do I avoid dimensional weight charges?
Make the box smaller, not the contents lighter. Cutting a box from 12 inches to 10 on every side removes 42% of its cubic size. Right-sized cartons, less void fill and avoiding dimensions just over a whole inch all help, since each inch is rounded up.
What is the break-even weight for a box?
Its cubic size divided by the divisor. A 12 inch cube at divisor 139 breaks even at 12.43 lb: heavier than that and you pay on the scale, lighter and you pay for the space.