Your carton weighs 12 kg and you are billed for 16
A carrier does not sell you weight. It sells you space on a vehicle, and charges you for whichever of your shipment's two weights is larger: what it weighs on the scales, and what it would weigh if it were as dense as the carrier assumes cargo to be.
Take a 60 × 40 × 40 cm carton weighing 12 kg, 8 of them.
| Divisor 6000 (IATA air cargo standard) | Divisor 5000 (typical express courier) | |
|---|---|---|
| Volumetric weight per carton | 16 kg | 19.2 kg |
| Actual weight per carton | 12 kg | 12 kg |
| Chargeable per carton | 16 kg | 19.2 kg |
| Chargeable, 8 cartons | 128 kg | 153.6 kg |
| Actual, 8 cartons | 96 kg | 96 kg |
Two things fall out of that table, and the second is the one people miss.
The first is that you are paying for 128 kg of a 96 kg shipment — 33.33% above the scales, before anyone has quoted you a rate per kilo.
The second is that the divisor is not a detail. Moving from 6000 to 5000 adds 25.6 kg to the same physical shipment, 20% more, with nothing in the cartons changed. When you compare two freight quotes, comparing the rate per kilo without comparing the divisor compares nothing.
The density that makes it stop mattering
Volumetric weight only bites while your cargo is lighter than the carrier's assumed density. For this carton the crossover is at 8,000 — that is, a divisor above that number leaves you billed on actual weight, and below it you are billed on volume.
Practically, that gives you the question to ask before redesigning a box: is this carton above or below the line? If it is comfortably above, shaving a few centimetres off the height saves nothing at all, because you were being billed on the scales anyway. If it is below, every centimetre in all three dimensions is money, and the savings compound because volume is a product of three numbers rather than a sum.
Per piece, or per shipment?
The other thing worth reading in your rate agreement is which level the comparison happens at, because it changes the bill on a mixed load. Take four of the 12 kg cartons above and four identical boxes packed to 30 kg.
| Basis | How it is worked | Chargeable |
|---|---|---|
| Per piece | 64 kg (light, billed on volume) + 120 kg (dense, billed on the scales) | 184 kg |
| Per shipment | greater of 168 kg actual and 128 kg volumetric | 168 kg |
16 kg of difference on identical cargo. Per piece, each light carton is billed on its volume and each dense carton on its weight — you lose on both. Per shipment, the dense cartons' surplus weight absorbs the light ones' surplus volume, and only the larger total is charged.
Which follows a practical rule: if your carrier compares per shipment, mixing dense and bulky cargo into one consignment is worth real money, and splitting them into separate bookings throws it away. If it compares per piece, the mix makes no difference and you should optimise each carton on its own.
Making a shipment denser
This carton packs 8,000 cm³ into every kilo of product, and a 6000 divisor assumes 6,000. While a carton carries more space per kilo than the divisor allows, you are paying for air; once it carries less, you are paying for weight. Everything you can do about the bill is an attempt to move the carton across that line.
Right-size the carton. The commonest waste is a box chosen because it was in the warehouse. Trimming 60 × 40 × 40 cm by five centimetres on the longest side removes 1.33 kg of chargeable weight per carton at a 6000 divisor, on cargo that has not changed at all.
Remove the void fill. Air inside the carton is charged at exactly the same rate as product.
Mix dense and light in the same carton rather than segregating them. A carton of light goods and a carton of dense goods are charged separately; one carton of both is charged once.
The divisor is not a law of nature
Which divisor applies is a commercial term. Carriers publish 5000 or 6000 depending on service and account; consolidators and forwarders negotiate it. It is one of the few line items on a freight quote that is genuinely negotiable, and almost nobody asks, because it looks like a technical constant rather than a price.
When comparing two quotes, compare the divisor before the rate. On this shipment the move from 6000 to 5000 is 25.6 kg, so a quote on 5000 has to be about 16.67% cheaper per kilo just to draw level with one on 6000.
What the tool does not know
Minimum chargeable weights, per-piece minimums, and the rounding convention each carrier applies — half a kilo up, a full kilo up, or to the nearest 0.5 — all sit outside this arithmetic and all move the bill.
So treat the chargeable weight as the floor of what you will be billed, and ask the carrier for its rounding rule and its minimum. On small, light shipments the minimum frequently decides the whole invoice and the volumetric calculation never comes into it.
Work it for your own shipment
The figures above come from the same code the calculators run, so typing these inputs in reproduces them exactly. Your own numbers are the point though — run your own carton and divisor through the volumetric weight calculator.
These are estimates, not assessments. Duty rates, the taxable base, and the valuation rules that decide them are set by national law and by how your goods are classified. A customs broker or your national customs authority gives you the binding answer; this site gives you the arithmetic so you know roughly what is coming and which question to ask.