Key takeaways
- 1USPS uses dimensional weight only for parcels larger than 1 cubic foot (1,728 cubic inches); smaller parcels pay by actual weight.
- 2Since July 12, 2026 the USPS DIM divisor is 139, and each side is rounded up to the next whole inch before multiplying.
- 3One inch can change a lot: a 5 lb item in a 12 × 12 × 12 box is billed at 5 lb, but in a 13 × 12 × 12 box at 14 lb.
- 4Cubic pricing charges by size, not weight: Priority Mail Cubic up to 0.5 cubic foot and Ground Advantage Cubic up to 1 cubic foot, both up to 20 lb.
- 5Long and large parcels pay extra: $4.50 over 22 inches, and $21 on Ground Advantage parcels over 2 cubic feet.
The short answer
USPS bills Priority Mail Express, Priority Mail, Ground Advantage and Parcel Select by actual weight unless the parcel is larger than 1 cubic foot. Above 1 cubic foot it works out a dimensional weight, compares it with the actual weight and charges for the greater one. That one rule is why USPS is often the cheapest carrier for small, light boxes: UPS and FedEx can apply dimensional weight at any size.
The rules below are taken from the USPS Domestic Mail Manual and Notice 123 price list, checked November 10, 2026. Postage prices change in January, so check the current price list for amounts. For the general method that applies to every carrier, see our guide to dimensional weight.
How USPS calculates dimensional weight
Measure the packed parcel, round each side up to the next whole inch, and multiply length by width by height. If the result is 1,728 cubic inches or less, stop: you pay by actual weight. If it is more, divide by 139 and round up to the next whole pound. That is the dimensional weight. Your billable weight is the greater of that and the actual weight.
USPS changed its DIM divisor to 139 on July 12, 2026, and at the same time began rounding each measurement up rather than to the nearest inch. Both changes raise the billable weight of bulky parcels. For parcels that are not rectangular, such as tubes, USPS multiplies the cubic size by 0.785 before dividing. If the dimensional weight is over 70 lb, you pay the 70 lb price.
If L × W × H ≤ 1,728 in³: billable weight = actual weight If L × W × H > 1,728 in³: billable weight = greater of actual weight and (L × W × H ÷ 139, rounded up)
Free toolDimensional weight calculatorThe 1 cubic foot cliff, with examples
Because the rule switches on at 1,728 cubic inches, a box just over the line can cost far more than one just under it. The table shows the billable weight USPS charges for common carton sizes.
- The 12-inch cube is billed at 5 lb. Add one inch to one side and it jumps to 14 lb. If an item fits a box under 1,728 cubic inches, use it.
- At 2026 commercial prices, the 12-inch cube with 5 lb inside costs $14.53 by Ground Advantage to Zone 5; UPS and FedEx bill the same box at 13 lb.
| Box (inches) | Cubic inches | Actual weight | Over 1 cubic foot? | USPS billable weight |
|---|---|---|---|---|
| 10 × 8 × 6 | 480 | 3 lb | No | 3 lb |
| 14 × 10 × 6 | 840 | 2 lb | No | 2 lb |
| 12 × 12 × 12 | 1,728 | 5 lb | No (exactly 1) | 5 lb |
| 13 × 12 × 12 | 1,872 | 5 lb | Yes | 14 lb |
| 16 × 12 × 10 | 1,920 | 4 lb | Yes | 14 lb |
| 18 × 14 × 10 | 2,520 | 6 lb | Yes | 19 lb |
Cubic pricing: paying by size instead of weight
Cubic pricing is a commercial option for small, heavy parcels, available when you buy postage through software or an online service that offers it. Instead of weight, the price depends on the parcel’s size tier and the zone. To find the tier, round each side down to the nearest quarter inch, multiply, and divide by 1,728.
Example: a box measuring 11.8 × 8.9 × 5.7 inches rounds down to 11.75 × 8.75 × 5.5, which is 565.5 cubic inches or 0.33 cubic foot. It falls in the 0.4 tier, the next tier up, whatever it weighs up to 20 lb.
- Cubic pays off when the item is dense: books, tools, supplements, parts. For light items, weight-based Ground Advantage is usually cheaper.
| Priority Mail Cubic | Ground Advantage Cubic | |
|---|---|---|
| Largest size | 0.5 cubic foot | 1 cubic foot |
| Tiers | 5 (0.1 to 0.5) | 10 (0.1 to 1.0) |
| Maximum weight | 20 lb | 20 lb |
| Longest side | 22 inches | 22 inches |
| Soft packs and padded envelopes | Priced on length + width, up to 36 inches | Priced on length + width, up to 36 inches (5 tiers) |
Fees for long and large parcels
On top of the postage, USPS adds nonstandard fees for parcels that are long or very large. These fees are the same for retail and commercial prices and do not apply to Flat Rate boxes. USPS can also charge a Dimension Noncompliance Fee when the size on the label is missing or wrong.
- Maximum weight is 70 lb. Maximum size is 108 inches in length plus girth, or 130 inches for Ground Advantage bought at retail.
- Ground Advantage parcels between 108 and 130 inches in length plus girth pay an oversized price regardless of weight: $112.94 to $295.93 commercial in 2026.
| Condition | Priority Mail Express | Priority Mail | Ground Advantage |
|---|---|---|---|
| Longest side over 22 in, up to 30 in | $4.50 | $4.50 | $4.50 |
| Longest side over 30 in | $21.00 | $21.00 | $10.00 |
| Over 2 cubic feet (3,456 in³) | $35.00 | $35.00 | $21.00 |
How to pay less with USPS
Most savings come from choosing the box with the USPS rules in mind.
- Keep parcels at or under 1,728 cubic inches where you can, so they are billed by actual weight.
- Keep the longest side at 22 inches or less to avoid the $4.50 fee.
- Use Cubic pricing for small, dense items, and Flat Rate boxes for heavy items going far.
- Measure the packed box and round each side up before you buy the label, so the label matches what USPS measures.
- Ship soft goods in a padded envelope or soft pack instead of a box.
