01The rule and the arithmetic
Every parcel has two weights: the one on the scale, and the one its size implies. Carriers compute dimensional (volumetric) weight from the parcel's dimensions and bill whichever is greater. The formula is universal; only the divisor varies by carrier and service:
Dimensional weight = (Length x Width x Height) / divisor — with dimensions in centimetres and the divisor typically 5000 for express and air services, and 6000 in some road and economy services, giving kilograms. In inches and pounds the equivalents are divisors of 139 and 166. A 60 x 40 x 40 cm parcel at divisor 5000 has a dimensional weight of 19.2 kg; if it actually weighs 7 kg, it bills at 19.2.
Two consequences follow. First, packaging air is billed air: the divisor prices every empty centimetre inside the box. Second, the measurement that matters is the carrier's, taken on their dimensioner — unless you have your own record to bill and dispute by, which is the commercial reason dimensioners exist.
02Why carriers price this way
Vehicles cube out before they weigh out. A truck or aircraft filled with pillows reaches its volume limit at a fraction of its weight limit, and the carrier's costs — fuel, crew, the vehicle itself — were spent moving that volume. Dimensional pricing aligns the bill with the resource actually consumed, and it has spread from air express into road networks and LTL as carriers installed dimensioners across their hubs. The direction of travel is one-way: more freight, measured more precisely, billed on the greater weight.
For shippers the practical stance is not to resent the rule but to engineer around it: measure everything yourself, pack to the product, and choose boxes from your real size data. The step-by-step calculation guide covers the working method, and the calculator runs any parcel through both standard divisors.
03Controlling dim weight cost
Three levers, in order of return:
- Measure. You cannot manage what the carrier measures for you. A dimensioner at packing or dispatch gives you the true billable weight before purchase, defends you in disputes, and produces the dataset the other two levers run on.
- Right-size packaging. Box ranges built from measured parcel data — not habit — remove the billed air. Even one carton size change across a high-volume SKU outweighs most rate negotiations.
- Buy services on billable weight. With real dimensional data at order time, service selection (express vs economy, carrier A vs B) can be made on the weight that will actually bill, including each carrier's divisor.