How to measure a parcel and work out its dimensional weight
Carriers price a parcel on the greater of its actual weight and its dimensional (DIM) weight, a number derived from the box size. A light but bulky box can be billed as if it were heavier. This guide shows how to measure, do the arithmetic by hand and check the result. You do not need any software.
What you need
- A tape measure (inches, if you are quoting US carriers) and a scale that shows ounces or tenths of a pound.
- The box as it will ship: packed, closed and taped. Padding and bulging sides count.
- The carrier's divisor. It differs by carrier and sometimes by service or contract. See the table below and the linked rate guides.
Step by step
- Weigh the packed box. This is the actual weight.
- Measure length, width and height at the longest point of each side. By convention length is the longest side. If a side bulges, measure the bulge.
- Round each measurement up to the next whole inch (USPS and UPS say round up; FedEx's pages differ, see the note below). 12.2 becomes 13.
- Multiply length × width × height to get cubic inches.
- Divide by the carrier's divisor and round up to the next whole pound. That is the DIM weight.
- Compare: the billable weight is the larger of actual weight and DIM weight.
Worked example
A packed box measures 12.2 × 9.5 × 6.1 in and weighs 3 lb 4 oz.
| Step | Result |
|---|---|
| Round up each side | 13 × 10 × 7 in |
| Volume | 13 × 10 × 7 = 910 cubic inches |
| Divide by 139 | 910 ÷ 139 = 6.55, rounded up to 7 lb |
| Actual weight | 3.25 lb |
| Billable weight (UPS Daily Rates style rules) | 7 lb, the DIM weight is higher |
For the same box, USPS Ground Advantage uses DIM weight only when the volume exceeds 1,728 cubic inches (1 cubic foot). At 910 cubic inches it would not apply, and the price would follow the actual weight. That is why the same box can bill differently on different carriers.
Divisors by carrier, as published when we read them
| Carrier | Divisor | Notes |
|---|---|---|
| USPS Ground Advantage (commercial) | 139 | Applies only above 1,728 cu in; each side rounded up; DIM weight capped at the 70 lb price; non-rectangular parcels multiply by 0.785 first. DMM 283 marks these as effective July 12, 2026. |
| UPS Daily Rates | 139 | Each side rounded up (minimum 1 in); DIM weight rounded up to the next whole pound. |
| UPS Retail Rates | 166 domestic, 139 export | Same rounding as Daily Rates. |
| FedEx | 139 | FedEx's explainer page says to round to the nearest whole inch. Its Service Guide, which we read as the contract document, says fractions are rounded up. Check your own agreement. |
Divisors, rounding and thresholds change, and your own contract can differ from the published rates. Treat any number you get from this arithmetic as an estimate until you compare it with the carrier's own quote or invoice. This guide is not legal, tax or customs advice.
Check your work
Weigh and measure a few real boxes, calculate by hand, then compare with the weight on the carrier's label receipt or invoice. If they differ, the usual causes are measuring before the box was filled, bulging sides, or a different divisor on your account.
If you would rather not do the arithmetic for every box, Dimloom runs these steps in your browser and shows the divisor and rounding it used, so you can compare it with the sources below. It works on the numbers you type and you enter your own rates.
Sources
- USPS, "USPS Ground Advantage": https://www.usps.com/ship/ground-advantage.htm (read 2026-10-27)
- USPS, Domestic Mail Manual 283, "USPS Ground Advantage — Commercial Prices and Eligibility", section 1.4: https://pe.usps.com/cpim/ftp/manuals/dmm300/283.pdf (read 2026-10-27)
- UPS, "Shipping dimensions and weight": https://www.ups.com/us/en/support/shipping-support/shipping-dimensions-weight (read 2026-10-27 as a search-index excerpt; ups.com blocks automated fetching)
- FedEx, "What is dimensional weight?": https://www.fedex.com/en-us/shipping/packaging/what-is-dimensional-weight.html (read 2026-10-27)
Related tool: Dimloom. Estimates and guidance only; check the source documents before relying on a figure.