Measuring a box for shipping takes two minutes when you know the conventions, and getting it wrong is how sellers end up with adjusted shipping bills weeks later. Here is the exact method every major carrier expects.
The three measurements
- Length: the longest side of the box
- Width: the second-longest side
- Height (depth): the shortest side
Measure the outside of the sealed, packed box, not the product inside and not the flat unassembled box. Use a tape measure, read to the nearest fraction, and note that USPS now rounds every fractional inch up to the next whole inch (a 12.2 inch side counts as 13).
What is girth, and when does it matter?
Girth is the distance around the thickest part of the package: girth = 2 x width + 2 x height. USPS uses "length plus girth" with a 108-inch limit for most services (130 inches for oversize Ground Advantage). If length + girth exceeds the limit, the package cannot ship by that service at all, so check this before promising a delivery method.
Weigh the packed box
Weigh the box after it is fully packed and sealed, including all padding and the label. A kitchen scale works up to its limit; beyond that use a postal or luggage scale. Carriers compare actual weight against dimensional weight and bill the greater, so both numbers matter.
Now calculate what you will pay
Measurements in hand? Enter them in our free dimensional weight calculator to see whether your package bills by size or by weight, then use our box size calculator to check whether a smaller box would drop you into a cheaper bracket. Two minutes of measuring plus one minute of math beats a surprise adjustment every time.