How-to · USPS prices effective July 12, 2026 · Updated October 2, 2026
How does USPS dimensional weight work?
Direct answer
Dimensional weight applies only when the box is over 1 cubic foot (1,728 in³). Then dimensional weight = L × W × H ÷ 166, rounded up. You pay the higher of that and the actual weight. A 12×10×8 box (960 in³) does not trigger it. An 18×14×12 box is billed as 19 lb even if the item weighs 4 lb.
Boxes to zone 4
| Outside size | Scale | DIM applies | Rated weight | Ground Advantage |
|---|---|---|---|---|
| 12 × 10 × 8 | 5 lb | No | 5 lb | $15.80 |
| 16 × 12 × 10 | 4 lb | Yes, 12 lb | 12 lb | $21.30 |
| 18 × 14 × 12 | 4 lb | Yes, 19 lb | 19 lb | $26.40 |
| 20 × 14 × 10 | 6 lb | Yes, 17 lb | 17 lb | $25.40 |
Post Office postage, zone 4, effective July 12, 2026. These four boxes are under 22 inches on the longest side and under 2 cubic feet, so the price is the rated-weight postage only. 18×14×12 Ground Advantage is $26.40; Priority Mail is $30.30. A side over 22 inches, or volume over 2 cubic feet, adds a separate nonstandard fee.
What does not use this rule
- Boxes at or under 1,728 cubic inches, including a typical 12×10×8.
- Priority Mail Flat Rate, if the item fits in the box or envelope. The price does not change with zone or dimensional weight.
- Media Mail, which is priced on actual weight for qualifying books and media.
Limits
- Measure the outside, including bulges. A soft mailer that is under 1 cubic foot stays on actual weight.
- UPS and FedEx use a divisor of 139 and apply dimensional weight at smaller sizes. Do not reuse this 166 rule for those carriers.
- The price on the label is the one USPS charges. These figures are Notice 123 estimates.
Use the calculator
Flat Rate prices: how much are USPS Flat Rate boxes? The full postage steps: how to calculate USPS postage. Enter a box in the USPS shipping calculator.