Dimensional weight vs. actual weight

Shipping isn't billed purely by how much a package weighs. Carriers charge based on billable weight — whichever is higher between the package's actual scale weight and its dimensional (volumetric) weight, a weight calculated from how much space it takes up.

Why volume matters as much as mass

A truck or cargo plane runs out of physical space before it runs out of weight capacity, on most routes. A large, light box takes up room that a denser, heavier package could otherwise occupy — so carriers price for the space a shipment actually consumes, not only its mass.

The formula

Dimensional weight is generally calculated as:

(length × width × height) ÷ divisor

Length, width, and height are the package's outer dimensions (commonly in centimeters), and the divisor is a carrier- and service-specific constant — a smaller divisor produces a higher dimensional weight, and therefore a higher potential cost, for the same box size. This is why the same package can price differently across carriers even before accounting for their base rates.

Which weight actually gets billed

Whichever number is larger — actual or dimensional — is the billable weight the rate gets applied to. A dense item (electronics, books, metal parts) is usually billed on its actual weight, since its dimensional weight comes out lower. A bulky, light item (a lampshade, an inflatable, packed foam) is usually billed on dimensional weight instead.

What this means when comparing quotes

Two packages with the same actual weight can have very different shipping costs if their dimensions differ — and two carriers can rank differently for the same package depending on which one's divisor is more forgiving of its shape. Reducing packaging size, when the product allows it, is one of the few changes a seller can make that lowers cost across every carrier at once.

Enter your package's real weight and dimensions in the shipping cost comparator to see its billable weight and an estimated cost across carriers.