Why box size changes the price
Carriers charge for the space a package takes, not only its weight. They compare the actual weight with a dimensional weight worked out from the box size and bill whichever is greater. That is why a big box of something light can cost more than a small heavy one.
How to measure
- Measure the length (the longest side), width and height in inches, at the widest points, including any bulge.
- Round each measurement up to the next whole inch. A side of 11.2 inches counts as 12.
- Weigh the packed box and round up to the next pound. USPS prices packages under 1 lb by the ounce.
The formula for each carrier
Multiply length × width × height, then divide by the carrier divisor:
| Carrier | Dimensional weight | When it applies |
|---|---|---|
| UPS | L × W × H ÷ 139 | Every package |
| FedEx | L × W × H ÷ 139 | Every package |
| USPS (Priority Mail, Ground Advantage) | L × W × H ÷ 166 | Boxes larger than 1 cubic foot (1,728 cubic inches) |
Round the result up to the next pound. These are the carriers’ standard published divisors and they can change. The rates you see in VinShips come from the carriers, so they already use the current rules.
Example
A 14 × 12 × 10 inch box that weighs 5 lb:
- UPS and FedEx: 14 × 12 × 10 = 1,680 ÷ 139 = 12.1, rounded up to 13 lb. You pay for 13 lb, not 5 lb.
- USPS: 1,680 cubic inches is under 1 cubic foot, so the actual 5 lb applies.
The same box can be much cheaper with one carrier than another, which is why VinShips shows all three side by side.
How to pay less
- Use the smallest box that fits the item with enough padding.
- Enter the exact box size when you create the shipment, so the rate matches what the carrier measures.
- Compare carriers for large, light packages. The divisor rules above can change which one is cheapest.
If the carrier measures a different size or weight than you entered, it can bill an adjustment after delivery. See Carrier adjustments.