What exactly is the girth measure, and what is it used for?
In general terms, the girth (sometimes also called belt size) is a unit of measurement which, in addition to the maximum package dimensions and weight, is used by delivery services to determine the final shipping class of a package and thus also the postage.
The girth measurement is used by most providers, including DHL, DPD, GLS and UPS.
Why is the girth measurement so important?
The girth measurement limits the maximum size of a package beyond the maximum dimensions specified by the shipping provider—it therefore represents an additional restriction on the maximum package size.
An example (explanation follows below):
If the maximum dimensions for the sides are specified as 120 x 60 x 60 cm (length x width x height), this does not mean that all sides can reach these maximum dimensions at the same time: the girth of this box would be 360 cm, but usually only 300 cm is allowed!
It follows that at least one of the dimensions must be significantly smaller than the maximum allowed size in order to remain below 300 cm. And, yes, this is a problem - not only for retailers, but also for private customers. This is because if the permitted girth measurement is exceeded, the package automatically ends up in the next higher shipping and postage class. At least! In the worst case, it may even be classified as bulky goods, and then things become really expensive!
Conclusion:
The specified maximum dimensions of a box, even in conjunction with the weight, are not sufficient to determine the correct postage: without the girth measurement, you are very likely to be wrong - especially if you want to send a package that is already close to the maximum permitted dimensions.
How is the girth measure calculated?
The girth is calculated from the circumference and length of the package; it has nothing to do with the length of a physical strap or belt that could be wrapped completely around the box.
The length is the longest side of the box; the circumference is calculated as twice the width plus twice the height. This results in the following formula:
Girth = length + 2x width + 2x height
Admittedly, this is not particularly intuitive, but it is the industry standard that is used uniformly by all parcel services for this purpose.
Fortunately, the formula is not really complicated: the length, height, and width of the package should already be known when purchasing the box; and even if not, they can be easily and quickly measured yourself.
When shopping, you just have to be careful that boxes usually specify the internal dimensions—i.e., the space available for the goods to be packed - but the external dimensions, which are important for shipping, are usually listed in the product description.
If in doubt, there's only one thing to do: grab a ruler and calculator and work it out yourself!
