Dimensional weight: why a light box can cost more to ship
Carriers charge for space as well as weight. Here is how volumetric weight is calculated, worked through with real numbers — and how to reduce it.
If you have ever been charged for 12 kilograms on a parcel that weighed 4, you have met dimensional weight. Aircraft holds and delivery vans fill up by volume long before they reach their weight limit, so carriers charge for the space a shipment takes as well as what it weighs.
The rule is simple: you pay for whichever is greater — actual weight or volumetric weight. That figure is the chargeable weight.
The formula
Measure the outside of the package in centimetres, multiply the three dimensions, and divide by the carrier's divisor:
Always measure at the widest point, including any bulge, handle or protruding label. Carriers re-measure on automated equipment, and a corrected measurement can arrive as an adjustment charge after delivery.
Worked examples
| Package | Dimensions (cm) | Actual | Volumetric | You pay for |
|---|---|---|---|---|
| Dense box of tools | 40 × 30 × 20 | 18 kg | 4 kg | 18 kg — actual |
| Pillows | 60 × 50 × 40 | 3 kg | 20 kg | 20 kg — volumetric |
| Lampshades | 70 × 70 × 50 | 5 kg | 40.8 kg | 40.8 kg — volumetric |
| Machine part | 100 × 60 × 30 | 50 kg | 30 kg | 50 kg — actual |
The pillows show the effect clearly: 3 kilograms of real weight, billed as 20, because the box occupies the space of something far heavier. The same pillows at a 5,000 divisor would bill at 24 kilograms.
Carriers also round the chargeable weight up, commonly to the next half or whole kilogram depending on the service, so check the rule on your rate sheet.
How to reduce what you pay
- Right-size the box. The largest saving almost always comes from removing empty space. A box a few centimetres smaller on each side can drop the billed weight substantially.
- Avoid shipping air. Use void fill sparingly, and consider whether the product can be packed in a shape that suits its form.
- Consolidate. Several small packages are often billed more than one well-packed carton.
- Flat-pack where you can. Furniture and fixtures that can be shipped unassembled are dramatically cheaper.
- Compress soft goods. Textiles, bedding and cushions can often be vacuum-packed.
- Ask which divisor applies. For bulky, light goods, a freight service using 6,000 can be cheaper than express using 5,000 even if the base rate is higher.
What about sea freight?
Shared container loads use the same principle with a different ratio, typically treating one cubic metre as equivalent to 1,000 kilograms. You are charged per "revenue ton" — the greater of the volume in cubic metres or the weight in tonnes. Full container loads are priced per container instead.
Sources
Rules change. This guide is general information, not legal or customs advice — check the current requirements for your shipment before you send it.
