What Dimensional Weight Is and Why It Matters
Dimensional weight (often called "dim weight") is a pricing method shipping carriers use when a package takes up a lot of space but doesn't weigh much. Instead of charging based only on actual weight, carriers charge based on whichever is larger: the package's real weight or its calculated dimensional weight. If your box is large and light, you'll pay as if it were heavier.
This matters because carriers want to charge fairly for the space your package occupies on their trucks and planes. A box of pillows might weigh 5 pounds but take up as much room as a box of books that weighs 30 pounds. Without dimensional weight pricing, the carrier loses money on the pillow shipment.
Most major carriers — UPS, FedEx, USPS (for certain services), and DHL — use dimensional weight calculations. The formula is the same across all of them, though the divisor (called the "dim factor") varies slightly by carrier and service type.
Key Takeaways
- Dimensional weight is calculated by multiplying length × width × height in inches, then dividing by the carrier's dim factor (usually 139 to 166).
- You pay based on whichever is higher: the package's actual weight or its calculated dimensional weight.
- Different carriers and service levels use different dim factors, so check your carrier's specific rate card before shipping.
- Measuring to the nearest whole inch and using the package's outer dimensions (including any padding) gives you the most accurate calculation.
- Knowing your dim weight before you ship lets you decide whether to use a smaller box, a different carrier, or a different service level.
The Basic Dimensional Weight Formula
The calculation is straightforward: Length × Width × Height ÷ Dim Factor = Dimensional Weight.
Measure the longest side, the widest side, and the tallest side of your package in inches. Round each measurement up to the nearest whole inch — a box that measures 11.2 inches counts as 12 inches. Multiply those three numbers together. Then divide by your carrier's dim factor.
For example: a box measuring 16 inches long, 12 inches wide, and 8 inches tall, divided by UPS's standard dim factor of 166, gives you (16 × 12 × 8) ÷ 166 = 1,536 ÷ 166 = 9.25 pounds of dimensional weight. If the box actually weighs 5 pounds, UPS charges you for 9.25 pounds.
Dim Factors by Carrier and Service
The dim factor is the number you divide by, and it varies. A lower dim factor means packages are charged as heavier, so the same box costs more to ship. Here's what the major carriers typically use:
| Carrier | Service | Dim Factor |
|---|---|---|
| UPS | Ground, 3-Day Select | 166 |
| UPS | Next Day Air, 2nd Day Air | 139 |
| FedEx | Ground | 166 |
| FedEx | Express (overnight, 2-day) | 139 |
| USPS | Priority Mail Express | 166 |
| DHL | Most services | 166 |
These numbers change periodically and may differ for international shipments or special services. Always check your carrier's current rate card before calculating, especially if you ship regularly. The difference between a dim factor of 139 and 166 can add $5 to $15 to a single shipment.
How to Measure Your Package Correctly
Measurement errors are the most common reason dim weight calculations are wrong. Measure the outside of the box, not the inside. Include any padding, bubble wrap, or protective material that's part of the package as it will be shipped. If you're shipping something in a padded mailer, measure the mailer itself.
Use a rigid measuring tape or ruler, not a soft tape measure that can sag. Measure each dimension at its widest or longest point. For an irregularly shaped box, measure the longest length, the widest width, and the tallest height — don't try to average or estimate.
Round up to the nearest whole inch. A box that's 11.1 inches becomes 12 inches. A box that's exactly 11 inches stays 11 inches. This rounding works in the carrier's favor, so they won't round down for you.
When Dimensional Weight Kicks In
Dimensional weight only matters if it's higher than actual weight. A 20-pound box measuring 10 × 10 × 10 inches has a dim weight of (10 × 10 × 10) ÷ 166 = 6 pounds. Since 20 pounds is higher, you pay for 20 pounds. The dim weight calculation doesn't explore.
Dim weight becomes the charge when you're shipping something light and bulky: pillows, blankets, foam, packing peanuts, clothing, or anything with a lot of air inside. A 2-pound box of winter coats measuring 20 × 16 × 12 inches has a dim weight of (20 × 16 × 12) ÷ 166 = 23 pounds. You'd pay for 23 pounds even though the coats weigh 2 pounds.
Check both numbers before you ship. If dim weight is significantly higher than actual weight, you have options: use a smaller box, compress the contents, choose a different carrier with a higher dim factor, or select ground shipping instead of express (ground usually has a higher dim factor).
Practical Steps to Calculate Before Shipping
Start by weighing your package on a scale that reads in pounds. Use a bathroom scale, a kitchen scale, or a postal scale — any scale that gives you the actual weight. Write that number down.
Next, measure length, width, and height in inches using a ruler or measuring tape. Round each measurement up to the nearest whole inch. Multiply the three numbers together. Write down that result.
Divide the result by your carrier's dim factor. If you don't know the exact dim factor, use 166 as a conservative estimate for ground shipping and 139 for express. This gives you the dimensional weight.
Compare the actual weight to the dimensional weight. Whichever is higher is what you'll be charged for. Use that number to get a shipping quote from your carrier before you pay.
Frequently Asked Questions
Do I measure the box or the contents?
Measure the box as it will be shipped, including all padding and protective material inside. If you're using a padded mailer or envelope, measure that. The carrier charges based on the space the package occupies, not what's inside it.
What if my package is an odd shape?
Measure the longest length, widest width, and tallest height, even if those measurements are on different sides of the package. A triangular or cylindrical package still gets measured as if it fits in a rectangular box.
Does USPS use dimensional weight for all services?
USPS uses dimensional weight for Priority Mail Express and some international services, but not for Priority Mail or First Class. Check USPS's current rate card for your specific service. Ground carriers (UPS, FedEx, DHL) use it for almost all services.
Can I reduce my shipping cost by using a smaller box?
Yes, if dimensional weight is the problem. A smaller box with the same contents will have a lower dim weight. However, you need to may support the contents still fit safely. Compressing items too tightly can damage them or make the box heavier overall.
What's the difference between dim factor 139 and 166?
A lower dim factor (139) charges you more for the same box size. Express services use 139 because planes have limited space. Ground services use 166 because trucks hold more volume. The same 10 × 10 × 10 box is 6 pounds at 166 but 7.2 pounds at 139.