Why a box of pillows costs as much to ship as a box of books
Couriers charge on whichever is greater: what the parcel weighs, or what its volume implies. One formula explains every surprising quote.
The formula
Multiply length by width by height in centimetres, then divide by the carrier's divisor — usually 5,000 — to get a weight in kilograms. In inches, divide by 139 for pounds.
A 40 × 30 × 30 cm box is 36,000 cm³. Divided by 5,000 that is 7.2 kg of dimensional weight. If the contents weigh three kilograms, you are billed for 7.2.
Why carriers do this
A van fills up before it reaches its payload limit. Charging by mass alone would mean light, bulky freight travelling almost free while occupying the space that pays for the trip.
The divisor is the lever
Divisors are not universal, and the difference is money:
| Divisor | Typical service | Same 36,000 cm³ box |
|---|---|---|
| 4,000 | Some domestic express | 9.0 kg |
| 5,000 | Standard international parcels | 7.2 kg |
| 6,000 | Air freight | 6.0 kg |
Three ways to pay less
- Cut the box down. Removing five centimetres from each side of a 40 cm cube cuts the volume by roughly a third.
- Choose padding by volume, not habit. Bubble wrap that adds three centimetres per side can push you into the next band.
- Compare divisors, not just rates. A slightly higher rate on a 6,000 divisor often beats a cheap rate on 4,000.
Measure what you ship
Measure the sealed parcel, not the product. Carriers round up to the next whole centimetre, so a 30.2 cm side is billed as 31.
How the practice started
Air freight introduced volumetric charging decades before parcel carriers adopted it, for an obvious reason: an aircraft hold fills long before its payload limit is reached, and a pallet of cushions displaces the same revenue-earning space as a pallet of machine parts. Road and parcel networks followed once e-commerce filled vans with lightweight boxes.
The divisor encodes an assumed density. A divisor of 5,000 assumes 200 kg per cubic metre; 6,000 assumes about 167. Anything less dense than the assumption is billed on volume, anything denser on actual mass.
Worked comparison
Take three parcels, each 40 × 30 × 30 cm — 36,000 cm³ — with a dimensional weight of 7.2 kg at a 5,000 divisor:
| Contents | Actual weight | Dimensional weight | Billed on |
|---|---|---|---|
| Cushions | 2.0 kg | 7.2 kg | 7.2 kg |
| Clothing | 6.5 kg | 7.2 kg | 7.2 kg |
| Books | 14.0 kg | 7.2 kg | 14.0 kg |
Only the third parcel escapes volumetric billing, and it does so by being denser than the carrier's assumption.
Where packaging decisions pay off
Because volume is the product of three dimensions, small reductions compound. Trimming a 40 × 30 × 30 cm box to 35 × 25 × 25 cm cuts the volume from 36,000 to 21,875 cm³ — a 39 per cent reduction, which takes the dimensional weight from 7.2 kg to 4.4 kg. That is often a whole pricing band.
- Right-size the carton. Void fill costs money twice: once to buy, once to ship.
- Watch the rounding. Most carriers round each dimension up to the next whole centimetre before multiplying, so 30.2 cm is billed as 31.
- Reconsider protective packaging. Moulded inserts often protect better in less space than loose fill.
- Split heavy from bulky. Two well-matched parcels can cost less than one poorly matched one.
Reading a quote properly
When comparing carriers, three figures determine your bill: the rate per kilogram, the divisor, and any minimum billable weight. A cheap rate on a 4,000 divisor with a 2 kg minimum can easily beat — or lose to — a higher rate on 6,000, depending entirely on what you ship. Model it against a real week of orders rather than a single example.
The takeaway
Measure the sealed parcel, apply the carrier's divisor, and compare the result with actual mass. If volume wins, the cheapest optimisation available is a smaller box.
Filed under Shipping · Tags: dimensional weight, courier, parcel size. Figures verified against the official definitions — see our editorial policy.