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For a short trip the parcel is usually small, which is exactly when volumetric charging catches people out. Shoes are dense, knitwear is compressible, and a bag that keeps its shape occupies space whether it is full or empty. Two parcels with the same contents can be charged differently because one was packed in a taller box.
Two weights, one charge
Actual weight is what the parcel weighs on a scale, including the box and any filling. Volumetric weight is a conversion of the parcel's outside dimensions into a notional weight, using a divisor that the carrier publishes. Whichever figure is higher becomes the chargeable weight for that service.
This is why a listed item weight is not enough. The item weight tells you nothing about the box it will travel in, and the box is what gets measured.
The calculation, with assumptions
Everything below is a labelled assumption, written to demonstrate the arithmetic. It is not a quote, and the divisor shown is not universal.
| Input | Value used | Note |
|---|---|---|
| Length | 40 cm | Outside measurement of the box |
| Width | 30 cm | Outside measurement of the box |
| Height | 15 cm | Outside measurement of the box |
| Volume | 18,000 cm³ | 40 × 30 × 15 |
| Divisor used | 5,000 (assumption) | Some services use 4,000, 5,000 or 6,000 |
| Volumetric weight | 3.6 kg | 18,000 ÷ 5,000 |
| Actual weight | 1.8 kg | Assumed scale reading |
| Chargeable weight | 3.6 kg | The higher of the two |
Under those assumptions the parcel is charged at twice its real mass. If the service used a divisor of 6,000 instead, the volumetric figure would be 3.0 kg, and the chargeable weight would still be 3.0 kg rather than 1.8 kg. The direction of the result is the same; the size of it depends on the rule.
Where the divisor comes from
Only the carrier or platform providing the service can tell you which divisor applies, and it can differ between services on the same route. Look for the phrase "volumetric", "dimensional" or "chargeable weight" in the service's own documentation, and note the figure you find. Do not carry a divisor across from a different carrier, a different service tier or an old blog post — including this one, whose divisor is explicitly an assumption.
Record dimensions properly
- Measure the outside of the box, not the item
- Round up to the nearest centimetre rather than down
- Keep one unit throughout — mixing inches and centimetres breaks the sum
- Measure once the parcel is packed and closed, not while it is being filled
- Note the divisor and where you read it, with the date
- Record the actual weight separately, so both figures are visible
In the planner on the home page there is no weight column, because the tool is deliberately limited to the items you control. Keep dimensions and weights in the notes instead, or in the shipping record described in the guide on estimating shipping from a quote.
Reduce volume, not just mass
If the chargeable weight is volumetric, then empty space is what you are paying for. Rolling knitwear into a pouch rather than folding it into a box, choosing a flatter box for a pair of shoes, and avoiding a rigid gift box inside a larger parcel can all reduce the measured dimensions. Whether that reduces the charge depends on which band the new dimensions fall into, so treat any saving as a possibility to check rather than a certainty.
Verify against the chosen service
Once you have packed, re-measure and re-check the divisor against the service you intend to use. If the figure moves, update the shipping column and the date beside it. The guide on choosing packing options covers the protection side of the same trade-off, and separating the parts of your budget shows where the estimate belongs once you have one.