How to calculate dimensional weight
If you've ever paid to ship a light package and been billed for something much heavier, dimensional weight is the reason. Carriers charge for the space a package takes up, not only what it weighs, and the calculation takes about thirty seconds once you know the steps.
What dimensional weight is
A truck or plane runs out of space long before it runs out of weight capacity when it's full of light, bulky parcels. Dimensional weight (DIM weight) turns a package's size into a notional weight so carriers can charge fairly for that space. You're billed on whichever is higher, the actual weight or the dimensional weight. That figure is the billable weight.
The formula
In the US, measure in inches and the result is in pounds. The divisor depends on the carrier and service. UPS and FedEx commonly use 139 for daily and account rates, and USPS uses 166 for packages over one cubic foot. With metric measurements, divide cubic centimeters by 5,000 (or 6,000 for some air services) to get kilograms.
Step by step
- Measure the outside of the packed box. Include any bulge from overfilling. Carriers measure what they receive, not the box's catalog size.
- Round each side. Carriers generally round each measurement to a whole inch. Rounding up is the safe way to estimate.
- Multiply the three sides. That gives the cubic size in cubic inches.
- Divide by the divisor. Then round the result up to the next whole pound.
- Compare with actual weight. The higher of the two is what you pay for.
Worked examples
A light but bulky package
A pillow ships in an 18 × 14 × 10 inch box and weighs 3 lb. The cubic size is 2,520 cubic inches. Divided by 139, that's 18.1, which rounds up to 19 lb. You'll pay for 19 lb, more than six times the actual weight.
A dense package
A set of tools ships in a 12 × 8 × 6 inch box and weighs 15 lb. The cubic size is 576 cubic inches, and 576 ÷ 139 = 4.1, so 5 lb. The actual weight is higher, so you pay for 15 lb.
The same box at USPS
USPS generally applies dimensional pricing only above one cubic foot (1,728 cubic inches). The 576 cubic inch tool box is under that, so it's charged on actual weight. The pillow box, at 2,520 cubic inches, is over it: 2,520 ÷ 166 = 15.2, so 16 lb.
How to lower your dimensional weight
Because size drives the charge, the box is the lever. A few practical moves:
- Use the smallest box that protects the product. Work out the size from the product plus the cushioning it needs, rather than defaulting to a box you have on hand.
- Watch the rounding. A box measuring 12¼ inches is billed as 13. Trimming a quarter inch can drop a full inch on one side.
- Ship soft goods in poly mailers. Clothing and textiles rarely need a box.
- Consider custom sizes for your best sellers. If you ship the same product hundreds of times a month, a box made to fit can pay for itself quickly.
Common mistakes
Using inside dimensions instead of outside dimensions is the most common error; board thickness makes the outside larger, and rounding magnifies it. Another is forgetting that rates and divisors differ between services and accounts. Check your own carrier agreement, because negotiated contracts sometimes use different divisors.