Your parcel
Carriers bill the larger of two weights
A courier van fills up by space long before it reaches its weight limit, so light, bulky parcels pay for the room they occupy: the carrier compares actual weight with volumetric weight and bills whichever is larger. That is how a box of foam can cost more to ship than a dumbbell.
Length × width × height, divided by a constant
Volumetric weight in kg is the three sides in centimetres multiplied together, divided by a divisor the carrier sets — 5000 is the common figure for international express, and 6000 appears at some services. A 40 × 30 × 20 cm box is 24,000 cm³: at 5000 that is 4.8 kg, so a 2 kg parcel inside it is billed as 4.8 kg.
Shrinking the bill
What you pay for is the air in the box. Dropping one side of that box from 20 to 10 cm halves the volumetric weight. Many carriers also cap the sum of the three sides separately, so a long thin box can be refused even when its volume is modest — the calculator shows that sum too.
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Frequently asked questions
Q. What is the 139 divisor US carriers quote?
The same rule in imperial units: cubic inches divided by 139 gives pounds. Converted, it matches a metric divisor of roughly 5000 — the two figures describe the same pricing, not a different system.
Q. Why does this system exist at all?
A plane or truck runs out of space before it runs out of weight capacity. Without volumetric weight, light bulky freight would ride almost free while taking the room of cargo that pays — the divisor is how the cost of space gets shared out.
Q. Will the number here match my invoice?
Nearly: most carriers round up — to the next half or full kilogram — before applying the tariff, so the billed figure can sit slightly above this one. Divisors and size limits also differ from carrier to carrier.