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Shipping a light, bulky thing can cost far more than its weight suggests, because couriers bill by size as well as scale weight. They charge for whichever is greater, the real weight or the dimensional weight worked out from the box size. This gives you both and tells you which one governs, so you can price shipping right and pack to keep it down.
Why a light box can ship heavy
A delivery truck fills up on space before it fills up on weight, so couriers protect themselves by charging for the room a parcel takes, not just its mass. They do that with dimensional weight: length times width times height, divided by a set number called the divisor, which turns a volume into an equivalent weight. If that comes out heavier than the scale weight, they bill the dimensional figure. A pillow in a big box can bill like a brick.
The divisor is the whole game
The divisor sets how harshly size is charged, and it depends on the units and the service. For inches and pounds it is commonly 139 for ground service, with 166 used for some others; for centimeters and kilograms it is 5000, sometimes 6000. A smaller divisor means size costs more. Carriers change these numbers and they differ by country and service level, so use your carrier's current divisor, and the calculator will apply it to your box.
Pack tight to pay less
Because the billed weight is the greater of real and dimensional, the way to cut shipping on a light item is to shrink the box. Drop to the next size down, trim excess void fill, and avoid shipping air. On items where dimensional weight governs, a smaller box lowers the billed weight directly and the postage with it. Weigh and measure your real packed box, not a guess, because a half inch here and there crosses price breaks.
Build it into your price
If you sell and ship, the billed weight, not the scale weight, is what you or your customer pays, so quote shipping from it. Work out the billed weight for your usual box and build it into the price or the shipping charge, and you will not get caught short when a bulky order costs more to send than it weighed. Knowing which factor governs also tells you where to put effort: smaller boxes if dimensional rules, lighter materials if real weight does.
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