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Quick answer: carriers bill the higher of a parcel’s actual weight and its dimensional (DIM) weight: length × width × height in inches, divided by a divisor. On 12 July 2026 USPS cut its divisor from 166 to 139 and started rounding every fractional inch up, matching UPS and FedEx. The same box can now cost about 19% more in billable weight.
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Sources: Ship.com, Optioryx, Rollo DIM calculator.
The formula
DIM weight (lb) = L × W × H (inches) ÷ divisor, rounded up to the next pound. You pay for whichever is higher: DIM weight or scale weight.
Divisors in 2026
| Carrier | Divisor | When it applies |
|---|---|---|
| USPS | 139 (since 12 July 2026) | Only parcels over 1 cubic foot (1,728 cubic inches) |
| UPS daily rates | 139 | All sizes |
| UPS retail (UPS Store) | 166 | All sizes |
| FedEx | 139 | All sizes |
Worked example
A 14 x 12 x 10 in box holding a 6 lb product: 14 × 12 × 10 = 1,680 cubic inches.
- UPS / FedEx: 1,680 ÷ 139 = 12.1, so you are billed for 13 lb, not 6.
- USPS: the box is under 1 cubic foot, so USPS bills actual weight: 6 lb.
Go one inch bigger on each side (15 x 13 x 11 = 2,145 cubic inches) and USPS DIM applies too: 2,145 ÷ 139 = 15.4, billed as 16 lb.
How to cut DIM charges
- Right-size the box. Keep a few box sizes instead of one large size for everything.
- Use poly or bubble mailers for soft and non-fragile items.
- Stay under 1 cubic foot for USPS where you can, so you pay actual weight.
- Measure and weigh every parcel, rounding each side up to the next inch, so carrier adjustments do not surprise you.
- Rate-shop. With all three carriers on 139, the cheapest option depends more on weight and zone.