Install on ShopifyCarriers sell space on a truck as well as weight. When a box is large for what it weighs, they charge a dimensional weight (DIM weight) worked out from its size. You pay for whichever is greater, the scale weight or the dimensional weight. That's the billable weight.
How USPS works it out
- It applies to Priority Mail and Ground Advantage packages larger than 1 cubic foot (1,728 cubic inches).
- Round each side up to the next whole inch. A side of 12.2 inches counts as 13.
- Multiply length × width × height and divide by 139. Round up to the next whole pound.
- If that's more than the actual weight, you pay the dimensional weight price.
Older guides use a divisor of 166 for USPS; USPS's current rules use 139 for rectangular boxes. Nonrectangular parcels, like tubes, use an adjusted formula. And if dimensions are missing or wrong when DIM weight applies, USPS charges a dimension-noncompliance fee.
How UPS works it out
- UPS says dimensional weight may apply to all its domestic and international package services, not only big boxes.
- Round each measurement up to the next whole inch, and multiply length × width × height.
- Divide by 139 for Daily Rates or 166 for Retail Rates. Round any fraction up to the next whole pound.
- UPS bills the greater of the actual and dimensional weight.
A worked example
A product that weighs 4 lb, packed two ways:
| 16 × 12 × 10 in box | 14 × 12 × 8 in box | |
|---|---|---|
| Cubic inches | 1,920 | 1,344 |
| USPS billable weight | 14 lb (1,920 ÷ 139 = 13.8, rounded up) | 4 lb (1 cubic foot or less, so no DIM) |
| UPS billable weight (÷ 139) | 14 lb | 10 lb (1,344 ÷ 139 = 9.7, rounded up) |
Same product, same weight. The bigger box is billed as 14 lb at both carriers. The smaller one drops USPS back to the real 4 lb and cuts the UPS billable weight to 10 lb. Whether that makes one carrier cheaper for this order depends on the rates to that address, so compare.
How to pack to avoid it
- Use the smallest box that protects the item. Leave room for padding, not empty space.
- Keep a few box sizes on hand, and match each product to one. A box that's one size too big is paid for on every order.
- Use mailers for soft goods. Clothing and other items that can't break often fit a poly mailer or padded envelope instead of a box.
- Stay at or under 1 cubic foot for USPS when you can. A 12 × 12 × 12 inch box is exactly 1,728 cubic inches.
- Watch length too. USPS adds nonstandard fees for packages longer than 22 inches, longer than 30 inches, or bigger than 2 cubic feet.
- Measure the packed box, not the product, and round up the way the carrier does. If the carrier measures something different, you get an adjustment.
- Consider Flat Rate for small, heavy items. USPS Flat Rate boxes are one price for any weight up to 70 lb, so the box size is already priced in.
Box profiles in Muvo
In Muvo you save a box and weight for each product once. For every order, Muvo builds the package from those box profiles, so each label is bought with real dimensions, then compares USPS and UPS rates and picks the cheapest qualifying one.
Want to see where the money goes today? Muvo's Shipping X-ray reads your store's last 60 days of orders and shows what shipping really cost.
Muvo has no monthly fee on the Free plan. See pricing for the label fee and what Pro adds, or create a free account and connect your store.
Sources: USPS, USPS Ground Advantage and Priority Mail; USPS Domestic Mail Manual 283 and 223; UPS, Package dimensions, size limits and weight guide. Carrier rules change, so check the carrier's site before you rely on a detail.
