Week 4: Paying by Air - The Cubic Weight Formula

The formula a courier actually bills against, and the number that turns week 1's oversized lululemon box into a real line item

15 March 2027

Week 2 costed a box by its material. Week 3 costed it by what it has to survive. Neither of those numbers is what actually appears on a shipping invoice. This week is the formula that does: Australia Post’s cubic weight calculation, which is close to what every major carrier runs in some form.

The formula#

Cubic weight (kg) = length (m) x width (m) x height (m) x 250

A courier doesn’t just weigh your parcel — it also converts the parcel’s outer dimensions into a “cubic weight” using the formula above, then bills against whichever number is bigger, the cubic weight or the actual weight. This comparison typically kicks in once a parcel’s actual weight passes about 1kg; below that, small parcels are usually billed on actual weight regardless of size. The 250 constant isn’t arbitrary physics — it’s a density assumption the carrier has decided is “normal” for a parcel of a given volume, and anything less dense than that assumption gets billed as if it weren’t.

A worked example (illustrative dimensions, not a measured parcel)#

To see how sharply this bites, work through a hypothetical parcel — the dimensions and weight below are invented for this exercise, not a real quoted or measured figure:

A pair of shoes and their retail box weigh 1.2kg actual, and ship inside an outer box measuring 0.40m x 0.35m x 0.25m. Its cubic weight is:

0.40 x 0.35 x 0.25 x 250 = 8.75kg

Chargeable weight is the greater of the two: 8.75kg, not 1.2kg. The courier bills this parcel as if it weighs more than seven times what it actually does — and the entire difference is air, created by the box’s outer dimensions rather than the product inside it.

What this does to week 1’s oversized-box case#

This is the number behind week 1’s lululemon and Patagonia case: an oversized standard box that saves the warehouse a packing decision doesn’t just cost more cardboard (week 2) — every centimetre of unnecessary length, width or height gets multiplied straight into the freight bill, for every unit shipped, forever. A retailer running this formula against their own box sizes can put an exact dollar figure on “how much does standardising box sizes cost us in freight,” and weigh it against the labour and tooling savings on the other side of that trade. That’s not a guess anymore — it’s the calculation logistics-cost logic is actually running.

This week’s lab#

Cubic Weight Lab — measure your own week 1 package and find out whether its box size was quietly inflating its shipping cost.