What dimensional weight is
A parcel takes up space in a truck or a plane whether it is heavy or not. So the big carriers do not only weigh a parcel. They also work out a weight from its size, called dimensional weight or DIM weight, and charge whichever of the two is greater. The FedEx page on the subject puts it in one line: you are charged on the dimensional weight or the actual weight, whichever is greater. The USPS Domestic Mail Manual uses the same rule: actual weight or dimensional weight, whichever is greater.
The greater of the two is the billable weight. For a heavy, compact parcel it is the scale weight. For a light parcel in a large box it is the dimensional weight. That is how a pillow or a lampshade can end up billed at a higher weight than a small, heavy box of books.
The formula
All three carriers covered here use the same three steps. Only the divisor and the rounding differ.
The length is the longest side. UPS describes the order: measure the longest side first, turn the box 90 degrees for the width, turn it again for the height. UPS also says to start each measurement at the longest point, so a bulging side counts at its widest. The multiplication is the same as for any box; the volume calculator does it if you want a check.
The divisor is a number of cubic inches per pound. A divisor of 139 means the carrier treats every 139 cubic inches of box as at least one pound.
The published divisors and rounding rules
These are the figures on each carrier's own pages as of October 2026. Rates and rules change, and UPS notes that its divisor varies by rate type, so treat the table as a starting point and check the divisor for the rates you actually pay.
| Carrier, service | Divisor | Size threshold | Measurements | Result |
|---|---|---|---|---|
| USPS Priority Mail and USPS Ground Advantage, retail and commercial (Domestic Mail Manual, revised July 12, 2026) | 139 | Only parcels over 1,728 cu in (1 cu ft) | Each side rounded up to a whole inch | Rounded up to the next pound, capped at 70 lb |
| UPS, Daily Rates (help page, viewed October 2026) | 139 | None stated on the page | Rounded up, 1.01 to 1.99 becomes 2 | Any fraction to the next whole pound |
| UPS, Retail Rates (same page) | 166 | None stated on the page | Rounded up | Any fraction to the next whole pound |
| FedEx, U.S. shipments (dimensional weight page, viewed October 2026) | 139 | None stated on the page | Each side rounded to the nearest whole inch | Not stated on that page |
Two details in the table are easy to miss. First, the USPS threshold: a parcel of 1,728 cubic inches or less is priced on its actual weight alone, however light it is. Second, the rounding: USPS and UPS round every side up, so a side of 12.1 inches counts as 13. The FedEx page rounds to the nearest inch, so 12.4 counts as 12.
The Domestic Mail Manual defines the words precisely. To round up means any digit after the last one kept pushes it up by one, so 12.01 becomes 13. To round off means 5 or more goes up and 4 or less stays, the ordinary school rule.
One parcel, step by step
Say you measure a parcel at 17.25 by 13.5 by 11.1 inches and it weighs 5.4 lb packed. Under the USPS rules:
- Round each side up: 17.25 becomes 18, 13.5 becomes 14, 11.1 becomes 12.
- Multiply: 18 × 14 × 12 = 3,024 cubic inches. That is over 1,728, so dimensional weight applies.
- Divide by 139: 3,024 ÷ 139 = 21.76, rounded up to 22 lb.
- Round the actual weight up too, as the retail price rules do with any fraction of a pound: 5.4 lb is charged as 6 lb.
- Compare: 22 lb against 6 lb. The parcel is billed at 22 lb.
Notice what the rounding did. The exact volume of the parcel is 2,585 cubic inches. Rounding each side up added 439 cubic inches before any division happened, about 17 percent more.
Worked examples: which weight is billed
Six common box sizes, each with a packed weight, run through the USPS rule. The last two columns show the same box at the two UPS divisors, where no size threshold is stated, so even the small boxes get a dimensional weight.
| Box (in) | Actual | Cubic inches | USPS DIM | USPS billed | DIM at 139 | DIM at 166 |
|---|---|---|---|---|---|---|
| 10 × 8 × 6 | 2 lb | 480 | none, ≤ 1,728 | 2 lb | 4 lb | 3 lb |
| 12 × 12 × 12 | 4 lb | 1,728 | none, ≤ 1,728 | 4 lb | 13 lb | 11 lb |
| 18 × 14 × 12 | 6 lb | 3,024 | 22 lb | 22 lb (DIM) | 22 lb | 19 lb |
| 20 × 16 × 12 | 30 lb | 3,840 | 28 lb | 30 lb | 28 lb | 24 lb |
| 24 × 18 × 18 | 12 lb | 7,776 | 56 lb | 56 lb (DIM) | 56 lb | 47 lb |
| 30 × 24 × 20 | 15 lb | 14,400 | 70 lb (cap) | 70 lb (DIM) | 104 lb | 87 lb |
What the rows show:
- The 12 inch cube is exactly 1,728 cubic inches. USPS applies dimensional weight only to parcels that exceed that, so it is billed at its 4 lb. At a divisor of 139 with no threshold the same box comes to 13 lb.
- The medium box holds 6 lb but takes 3,024 cubic inches, so it is billed at 22 lb under the USPS rule. That is 3.7 times the scale weight.
- The heavy box is dense enough that the scale wins: 30 lb actual against 28 lb dimensional. Dimensional weight does not matter for it at all.
- The bulky box works out to 104 lb of dimensional weight. The Domestic Mail Manual caps that at 70 lb, so it pays the 70-pound price.
When dimensional weight kicks in
There is a quick way to tell, without measuring every box. Divide the cubic inches in a cubic foot by the divisor:
- At 139: 1,728 ÷ 139 = 12.4 lb per cubic foot.
- At 166: 1,728 ÷ 166 = 10.4 lb per cubic foot.
A packed parcel that weighs less than about 12.4 lb for every cubic foot of box is billed on its size at a divisor of 139 (at USPS, only once the box is over 1 cubic foot). Heavier than that, the scale decides. The rounding of sides and pounds moves the exact line a little for each box, so use it as a rule of thumb and the formula for the real number.
Turned around, the formula tells you the lightest weight at which a given box is billed on the scale. For the 18 × 14 × 12 inch box under the USPS rule, anything up to 22 lb is billed as 22 lb, because the dimensional weight is 22 lb either way. Adding a second item that fits in the same box adds nothing to the billed weight until the total passes that line.
What a smaller box changes
Because the billed weight of a light parcel comes from its volume, the box is the one number you control. Here is the same 6 lb item in five boxes, from loose to snug, under the USPS rule and at the two UPS divisors.
| Box (in) | Cubic inches | USPS billed | DIM at 139 | DIM at 166 |
|---|---|---|---|---|
| 20 × 16 × 14 | 4,480 | 33 lb (DIM) | 33 lb | 27 lb |
| 18 × 14 × 12 | 3,024 | 22 lb (DIM) | 22 lb | 19 lb |
| 17 × 13 × 11 | 2,431 | 18 lb (DIM) | 18 lb | 15 lb |
| 16 × 12 × 10 | 1,920 | 14 lb (DIM) | 14 lb | 12 lb |
| 14 × 12 × 10 | 1,680 | 6 lb (actual) | 13 lb | 11 lb |
Going from the 20 × 16 × 14 box to the 18 × 14 × 12 box cuts the billed weight from 33 lb to 22 lb. Taking just one more inch off every side, to 17 × 13 × 11, removes 593 cubic inches and brings it to 18 lb. One inch on three sides is a lot of volume, because the three savings multiply.
The last row is the interesting one. At 14 × 12 × 10 inches the box is 1,680 cubic inches, under the 1 cubic foot line, so USPS bills only the 6 lb on the scale. At a divisor of 139 without a threshold the same box still comes to 13 lb. So the size line in the USPS rules can matter as much as the divisor.
What follows from the math, and nothing more: for a light item, the smallest box that the item and its protection fit in gives the lowest billed weight, and every inch you take off any side counts. How much cushioning an item needs is a separate question that this guide does not cover.
Rounding can push a box over the line
The size threshold is checked after rounding. A parcel measured at 12.1 × 11.6 × 11.8 inches has an exact volume of 1,656 cubic inches, under 1,728. Rounded up it becomes 13 × 12 × 12 = 1,872 cubic inches, which is over the line, so it gets a dimensional weight of 14 lb against 3 lb on the scale. A box that measures just under a whole inch on each side is billed as if it were a full inch longer.
Rounding up versus to the nearest inch
The carriers' rounding rules differ, so the same parcel can have two cubic sizes. At 12.4 × 10.4 × 8.4 inches, rounding every side up gives 13 × 11 × 9 = 1,287 cubic inches. Rounding to the nearest inch, as the FedEx page describes, gives 12 × 10 × 8 = 960. That is 34 percent more volume from the rounding rule alone, before the divisor.
Tubes and other odd shapes
The Domestic Mail Manual has a separate rule for parcels that are not boxes. Measure length, width and height at the extreme points, multiply them as if the parcel were in a box, then multiply by an adjustment factor of 0.785. The retail sections and the USPS Ground Advantage commercial section then divide by 139. The Priority Mail commercial section, as published in October 2026, divides odd shapes by 166 instead and rounds those measurements off rather than up.
A shipping tube 36 inches long and 8 inches across has a box size of 36 × 8 × 8 = 2,304 cubic inches. Times 0.785 that is 1,809, which is over 1,728, so divided by 139 and rounded up it gives 14 lb. Treated as a plain box it would be 17 lb. UPS says to measure an irregular package as if it were in a rectangular box, from its extreme points; its page gives no adjustment factor.
Centimeters and kilograms
U.S. rates are worked in inches and pounds. If your tape measure or scale is metric, convert each side before you round, not the finished result. One inch is exactly 2.54 centimeters and one pound is exactly 0.45359237 kilograms. The length converter and the weight converter do the arithmetic.
Take a box of 40 × 30 × 25 cm weighing 3.2 kg:
- Sides in inches: 15.75, 11.81 and 9.84. Rounded up: 16, 12 and 10.
- Cubic size: 1,920 cubic inches. Dimensional weight at 139: 13.81, rounded up to 14 lb.
- Actual weight: 3.2 kg is 7.05 lb, charged as 8 lb.
- Billed: 14 lb, which is 6.35 kg.
Outside the U.S. you will also see a divisor in metric units. The FedEx help page for shipments from China, for example, divides cubic centimeters by 5,000 to get kilograms. For this box that is 30,000 ÷ 5,000 = 6.00 kg, or 13.23 lb. The two divisors are close but not the same: 139 cubic inches per pound equals 139 × 16.387064 ÷ 0.45359237 = 5,022 cubic centimeters per kilogram. The rest of the gap comes from rounding each side up to a whole inch. If you want the box volume in liters or cubic feet for comparison, the volume converter handles it.
A quick checklist before you buy boxes
- Measure the packed parcel, not the item, at its widest points.
- Round each side the way your carrier says: up for USPS and UPS, to the nearest inch on the FedEx page.
- Multiply, then check the USPS line of 1,728 cubic inches if you ship with USPS.
- Divide by the divisor that applies to your rate: 139 or 166 on the pages quoted here, or the one in your contract.
- Round the result and the scale weight up to whole pounds, as USPS and UPS do, and take the greater.
- Try the next smaller box size and run it again. The table above shows how fast the billed weight falls.
This guide explains how the billed weight is worked out. It does not compare prices, and the cheapest option for a given parcel depends on the rate tables, the zone and any contract you have.
Frequently asked questions
What is the difference between actual weight and dimensional weight?
Actual weight is what the packed parcel weighs on a scale. Dimensional weight is a weight worked out from the size of the box: length times width times height, divided by the carrier's divisor. The carrier charges the greater of the two, which is called the billable weight.
Does a small box pay dimensional weight at USPS?
Not under the Domestic Mail Manual rules for Priority Mail and USPS Ground Advantage. Dimensional weight applies only to parcels that exceed 1 cubic foot, which is 1,728 cubic inches, after each side has been rounded up to a whole inch. A 10 by 8 by 6 inch box is 480 cubic inches, so it is priced by its actual weight.
Why does UPS show two divisors?
The UPS help page gives 139 for Daily Rates and 166 for Retail Rates. A larger divisor gives a lower dimensional weight for the same box. The 18 by 14 by 12 inch box in this guide comes to 22 lb at 139 and 19 lb at 166.
Do I round the box measurements up or to the nearest inch?
It depends on the carrier. The USPS Domestic Mail Manual and the UPS measuring page say to round each measurement up to the next whole inch. The FedEx U.S. dimensional weight page says to round each measurement to the nearest whole inch. The rounding happens before you multiply.
Can I measure in centimeters?
For U.S. rates the formula works in inches and pounds, so convert first. One inch is exactly 2.54 centimeters. Divide each side by 2.54, round up to a whole inch, then multiply. Converting the finished volume or the finished weight skips the rounding step and can give a different result.
What happens if the dimensional weight is over 70 pounds at USPS?
The Domestic Mail Manual says the customer pays the 70-pound price. A 30 by 24 by 20 inch box works out to 104 lb of dimensional weight but is charged at 70 lb.
How are tubes and other odd shapes measured?
USPS measures them at their extreme points, multiplies the three sides, then multiplies by an adjustment factor of 0.785 before dividing. UPS says to measure an irregular package as if it were in a rectangular box, from its extreme points.
Sources
- USPS, Domestic Mail Manual 123: Priority Mail retail prices, dimensional weight in 1.3 (divisor 139, rules revised July 12, 2026) and per-pound rounding in 1.1
- USPS, Domestic Mail Manual 133: USPS Ground Advantage retail, dimensional weight in 1.4 (divisor 139, revised July 12, 2026)
- USPS, Domestic Mail Manual 223: Priority Mail commercial, dimensional weight in 1.4 (139 rectangular, 166 nonrectangular)
- USPS, Domestic Mail Manual 283: USPS Ground Advantage commercial, dimensional weight in 1.4 (divisor 139, revised July 12, 2026)
- USPS, Domestic Mail Manual 604.7.1.3: what "round up" and "round off" mean
- UPS, Package dimensions, size limits and weight guide: rounding up, divisor 139 for Daily Rates and 166 for Retail Rates (viewed October 2026)
- FedEx, What is dimensional weight? Nearest whole inch, divide by 139 for U.S. shipments (viewed October 2026)
- FedEx China, How do I calculate dimensional weight of a package? Centimeters divided by 5,000 for kilograms (viewed October 2026)
- NIST Handbook 44 (2026), Appendix C: 1 inch = 2.54 cm and 1 pound = 0.45359237 kg exactly
General information based on the sources above, not tax, legal or financial advice.