How to Calculate Dimensional Weight in lbs: E-Commerce Shipping

Dimensional weight (DIM weight, or volumetric weight) is the weight a carrier assigns a package based on how much space it takes up. Multiply the box's length, width and height in inches, divide by the carrier's DIM divisor, and you get a weight in pounds. The carrier then bills whichever is higher: that number or what the box weighs on a scale. The higher one is your billable weight.

How to calculate dimensional weight in pounds

1. Measure the outside of the box in inches: length, width and height.
2. Round each measurement up to the next whole inch. An 11.1-inch side counts as 12.
3. Multiply the three numbers to get cubic inches.
4. Divide by your carrier's divisor and round the result up to the next whole pound.
5. Round the actual weight up to the next pound too, then compare. The higher number is what you pay for. One exception: USPS Ground Advantage prices packages under a pound in ounce steps, not whole pounds.

The rounding in step 2 used to be to the nearest inch. FedEx now tells shippers to round every dimension up to the next whole inch, USPS does the same in its dimensional weight rules, and UPS switched to rounding up in August 2025. A fraction of an inch on each side can add a pound or more to a mid-size box.

DIM divisors by carrier

The divisor is the number that turns cubic inches into pounds. A lower divisor produces a higher DIM weight.

• UPS: 139 for Daily Rates and 166 for Retail Rates, per UPS's package dimensions and weight guide. Which rates you get depends on your UPS account and how you ship.
• FedEx: 139, per FedEx's dimensional weight guide.
• USPS: 139, but only when the package is larger than 1 cubic foot (1,728 cubic inches) after rounding each side up, per the Domestic Mail Manual. At or under 1,728 cubic inches there's no DIM weight, and USPS charges its regular price for the package's weight. USPS Ground Advantage has a few exceptions to that: packages under a pound are priced in 4, 8, 12 and 15.999-ounce steps, commercial shippers can use cubic pricing, which prices small boxes by size tier instead of weight, and packages over 108 inches in length plus girth pay an oversized price. The divisor was 166 until July 12, 2026. Flat Rate boxes and envelopes are priced by the box, so DIM weight doesn't apply to them.

These are the published defaults for standard weight-based parcel pricing. A negotiated carrier contract can set a different divisor, so check yours if you have one.

For a box that isn't rectangular (a tube or an irregular shape), USPS multiplies the cubic inches by 0.785 first. DIM pricing applies only if that adjusted volume is over 1,728 cubic inches, and then the adjusted volume is divided by 139. If the result comes out above 70 pounds, USPS charges the 70-pound price.

Worked example: one box, three carriers

Take a box that measures 11.5 x 7.8 x 5.9 inches and weighs 4 pounds. Rounded up, that's 12 x 8 x 6 inches, or 576 cubic inches.

• UPS daily rates or FedEx: 576 / 139 = 4.14, which rounds up to 5 pounds. You're billed for 5 pounds, not 4.
• UPS retail: 576 / 166 = 3.47, which rounds up to 4 pounds. That ties the actual weight, so you're billed for 4.
• USPS: 576 cubic inches is under the 1,728 threshold, so there's no DIM weight. At the regular weight-based price, you're billed for 4 pounds.

Now a bigger, light box: 16 x 12 x 12 inches (2,304 cubic inches) weighing 6 pounds.

• UPS daily rates or FedEx: 2,304 / 139 = 16.58, so 17 pounds.
• UPS retail: 2,304 / 166 = 13.88, so 14 pounds.
• USPS: the box is over 1,728 cubic inches, so the 139 divisor applies: 17 pounds.

The small box costs you a pound at most. The big one gets billed at more than twice its weight by every carrier. That's why DIM weight hits bulky, light products like pillows, apparel and anything packed with a lot of void fill hardest.

To run your own box sizes, our free DIM weight calculator applies these divisors and rounding rules for UPS, FedEx and USPS and shows the billable weight for each.

International shipments use metric

Many international services work in centimeters and kilograms: multiply the dimensions in centimeters and divide by 5,000 to get kilograms, which is the factor on FedEx's dimensional weight guide. A 30 x 20 x 15 cm box is 9,000 cubic centimeters, or 1.8 kg at 5,000 before the carrier rounds it up.

How to lower your DIM weight

DIM weight is one of the few shipping costs you control directly, and it mostly comes down to the box.

• Right-size the box. Every inch you take off one side cuts the cubic volume, and a smaller box often drops a full billable pound or more. Keep a few box sizes on hand that fit your best sellers closely.
• Use a poly mailer when the product allows it. Soft goods like apparel ship in a mailer that hugs the product, so there's little empty space to pay for.
• Watch the USPS 1-cubic-foot line. A box at or under 1,728 cubic inches has no USPS DIM weight. A box just over it gets DIM pricing.
• Combine items when you can. One right-sized box for a two-item order usually bills less than two separate boxes.
• Ask about your divisor. Higher-volume shippers can sometimes negotiate a higher divisor into their carrier contract, which lowers DIM weight on every package.
• Make box choice a packing rule. If whoever packs orders grabs the nearest box, small items end up in big ones. A simple chart of which box each product goes in prevents most of that.

For a plain definition of the term, see our dimensional weight glossary entry.