Freight is quoted per kilo, but the kilo is not always the one your scale shows. Carriers work on chargeable weight — whichever is larger, the actual weight or the volume divided by a number. That number differs between agents, and inside one agent it often differs between lines. On a bulky but light parcel it is easily the biggest single lever on what you pay, and unlike the service fee it is invisible until the parcel is already at the warehouse.
Where the divisor comes from
Volume in cubic centimetres divided by a divisor gives a notional weight in kilograms. A 40×30×30cm carton is 36,000 cm³. Divide by 5000 and it weighs 7.2kg for billing; divide by 9000 and it weighs 4.0kg. Neither is wrong — the divisor is a convention each carrier sets, roughly reflecting how much space a kilo of typical freight occupies in their network. A carrier moving dense goods uses a small divisor and charges more for bulky ones; a carrier whose network has spare volume uses a larger one. There is no industry standard, which is the whole problem.
The values the five agents use
From the rate tables each agent publishes: Superbuy 5000, Supplyia 6000, BuckyDrop 8000 and Oopbuy 8000, CSSBuy 9000. Read that as a ranking rather than a fact about the world — a smaller divisor means bulky parcels are charged more, so on a light bulky box the agent with the largest divisor tends to win, all else equal. Two caveats. First, these are the platform defaults, and several agents override them per line. Second, one line in our data — BuckyDrop’s EMS — carries a divisor of zero, which is not a missing value: it means that line is billed on actual weight and ignores volume entirely.
One box, six possible chargeable weights
Take that 40×30×30cm carton and suppose it weighs 1.2kg on the scale. Depending on the divisor of the line carrying it, it bills as: 7.2kg at 5000, 6.55kg at 5500, 6.0kg at 6000, 4.5kg at 8000, 4.0kg at 9000, or 1.2kg on a line that ignores volume. That is a six-fold spread on the identical box, before any per-kilo rate is applied. Now put German rates against it: at 4.5kg BuckyDrop’s duty-prepaid line comes to about ¥234, while the same agent’s EMS — which charges actual weight — comes to about ¥175 on 1.2kg. Same agent, same parcel, ¥59 apart.
The trap: one agent, several divisors
This is where most comparisons go wrong, including one we built ourselves. The tempting shortcut is to compute chargeable weight once using the agent’s default divisor, rank the agents by that, and then bill using whatever the individual line actually specifies. We did exactly that, and the result was that the line we highlighted as cheapest was not the cheapest line — on the German parcel above we were pointing at ¥234 while ¥175 was sitting one row down. The fix was unglamorous: one function computes chargeable weight per line, and filtering, ranking and pricing all call that same function. The lesson generalises — if two parts of a comparison compute the same number differently, the ranking is decoration.
What to do before you order
Weigh and measure one unit before you commit to a quantity, not after. Ask the agent for the dimensional breakdown on the quote — actual, volumetric and chargeable — and check which of the three they multiplied. Where you have a choice of packaging, a carton 10cm smaller on the longest side can move you a whole chargeable kilo. Treat the per-kilo rate as half a number: it is meaningless without the divisor attached. And when you compare, compare the total for your own dimensions and destination, because the ordering genuinely changes between a dense 2kg parcel and a light 6kg one.
FAQ
Is the divisor fixed for each agent?
No. There is a platform default — 5000 at Superbuy, 6000 at Supplyia, 8000 at BuckyDrop and Oopbuy, 9000 at CSSBuy — but several agents override it per line, and at least one line we track is billed on actual weight with volume ignored. So the divisor belongs to the line, not the agent. Any comparison that applies one divisor to an entire agent will misprice bulky parcels, which is exactly what ours did until we fixed it.
Does sea freight use volumetric weight too?
Yes, the same way — chargeable weight is the larger of actual and volume divided by the divisor, and the sea lines in our data use the platform default rather than a special one. Two things make sea different in practice though. The per-kilo rate is roughly a third of air, so a volumetric penalty stings less in absolute terms; and every sea line we track carries a 15kg minimum, so a small parcel is not eligible at all regardless of how it is measured.
Can I reduce the chargeable weight by compressing the parcel?
Sometimes, and it is worth asking for, because vacuum compression is offered as a paid extra by several agents and can take a soft-goods parcel down a chargeable kilo or two. The limit is that by the time you see the number, the carton has usually already been sized by the warehouse, and re-packing is billed. The cheaper version of the same idea happens before you order: pick products that ship flat or dense, and treat any listing whose selling point is volume — plush toys, lampshades, inflatables — as carrying freight you have not priced yet.
Why does the quote differ from the calculator on the agent’s own site?
Three usual reasons, in order of how much they matter. The divisor: a site calculator may apply the platform default while the line you actually book uses its own. Rounding: carriers round chargeable weight up to a step, commonly half a kilo, and a parcel at 4.5kg can be billed as 5. Line-level extras: customs charges on some lines, sensitive-goods handling, and minimum weights. None of these are hidden, but none of them are visible in the headline per-kilo rate either, which is why we price the whole line rather than the rate.
Does the extension handle volumetric weight for me?
Yes, and it is the part we got wrong longest. Chargeable weight is computed per line using that line’s own divisor, including the lines that ignore volume entirely, and the same computation drives filtering, sorting and the price you see — there is one function doing it, so the ranking cannot disagree with the bill. Fill in dimensions and it re-ranks; leave them blank and it falls back to actual weight, which is honest about being a floor rather than a quote. The line list is ordered by what your parcel actually costs on each line, with the cheapest marked.