Dimensional weight,
worked out in the open.

Dimensional weight is a billing weight worked out from how much space a shipment occupies rather than how heavy it is. Carriers charge on it because a trailer or an aircraft runs out of room long before it runs out of payload. A pallet of insulation and a pallet of steel plate take the same floor position; only one of them pays for it by weight.

The calculation is three steps and there is nothing hidden in it.

First, measure the packaged shipment and round each dimension to the nearest whole unit. Second, multiply the three dimensions together to get the cubic size. Third, divide that figure by the divisor the carrier publishes. The result is the dimensional weight, in pounds if you worked in inches, in kilograms if you worked in centimetres.

Then compare it against the actual weight. The larger of the two is the chargeable weight, sometimes called billable weight, and that is the number the rate is applied to. You are never charged on the sum of the two and never on the smaller.

A worked example. A skid measuring 48 by 40 by 36 inches has a cubic size of 69,120 cubic inches, which is 40 cubic feet. At a divisor of 139 the dimensional weight is 69,120 divided by 139, or 497 pounds. If the skid actually weighs 120 pounds, it is billed as 497 pounds. The extra 377 pounds is the price of the air inside the box.

The divisor is the only part of the calculation that varies, and it is the part people get wrong. It is published by the carrier, it differs by service and rate type, it changes, and a negotiated agreement can carry a different one from the public figure. Larger divisor, smaller dimensional weight, lower bill.

Note that the imperial and metric divisors carriers publish are not exact conversions of one another. A divisor of 139 cubic inches per pound works out to about 5022 cubic centimetres per kilogram, but carriers publish 5000. Measuring the same box in centimetres therefore produces a very slightly higher dimensional weight than measuring it in inches. The gap is under half a percent and nobody should lose sleep over it, but it explains small discrepancies that otherwise look like errors.

There is one figure that tells you in advance whether a shipment will be billed on its size or its weight, and almost nobody uses it. It is the break-even density: the density at which dimensional weight and actual weight come out equal.

Work it out by dividing the units in a cubic foot, 1728 cubic inches, by the divisor. At 139 the answer is 12.4 pounds per cubic foot. At 166 it is 10.4. For air cargo at 6000 cubic centimetres per kilogram, one cubic metre is 1,000,000 cubic centimetres, so the break-even is 166.7 kilograms per cubic metre.

Freight denser than the break-even is always billed on actual weight. Freight lighter than it is always billed on the cube. That single comparison replaces the entire calculation for planning purposes, and it tells you which lever to pull.

This is also why taking weight out of a light shipment saves nothing. Below the break-even the bill is being computed from the box, so removing mass changes the actual weight, which was never the number being charged. The only thing that helps is making the shipment smaller or denser.

Dimensional weight is a parcel and air freight concept. The other modes solve the same problem, that light bulky freight underpays, but they do it with different machinery, and using the wrong method for the mode produces confident nonsense.

Air freight uses the IATA volumetric standard: cubic centimetres divided by 6000 gives kilograms, and the chargeable weight is the greater of that and the gross weight. This is the cleanest version of the rule and the one most of the internet describes.

Parcel and express use a divisor too, but a tighter one. FedEx and UPS at 139 charge more for the same cube than the air cargo standard at 166 does, because the parcel network is far more space-constrained per unit of revenue.

Ocean LCL abandons divisors entirely and quotes per revenue ton, written W/M for weight or measure. One cubic metre is treated as equivalent to one metric tonne, and the chargeable quantity is whichever is greater. A shipment of 1.20 cubic metres weighing 650 kilograms is billed as 1.20 W/M, because 1.20 beats 0.65.

North American LTL uses NMFC freight class, where density is the main input but the output is a class number rather than a weight. A low density pushes the class up and the class drives the rate. It reaches the same destination by a different road, which is why the freight class guide is a separate piece of arithmetic from this one.

Almost every dimensional weight dispute comes down to measuring a different object from the one the carrier measured. The carrier measures what it physically handles. That is the packaged, palletised, wrapped shipment at its widest points, including anything that sticks out.

Carriers also re-measure in the network, with automated dimensioners on the sortation line, and issue an adjustment when their figures differ from the declared ones. Those adjustments are usually correct, which is the uncomfortable part.