Two ideas in one rule
Parcel pricing depends on weight, but the weight on the invoice is not simply the number on a scale. Two separate mechanisms shape it. The first is the weight break: carriers price in bands, and a parcel is charged at the band it falls into. The second is the billable weight: the weight the carrier actually uses to find that band, which may differ from the scale weight. Understanding both explains most “why was I charged for that?” questions about the base rate.
Weight breaks and rounding
A rate table does not list a price for every possible weight. It lists prices for steps, such as up to 0.5 kg, up to 1 kg, up to 2 kg and so on, or in pounds and ounces in other markets. A parcel is charged at the price for the step it reaches.
Rounding rules decide what happens between steps. Common patterns include:
- Round up to the next whole unit. A parcel of 1.2 kg is treated as 2 kg if the carrier rounds up to the next kilogram.
- Round up to the next half unit or other fraction. Smaller steps reduce the effect of rounding.
- Banded ranges. The table lists a range, such as 5 to 10 kg, with one price for the whole range.
The specific steps and the rule differ between carriers, services and countries. For lighter parcels the steps are often small. For heavier ones they may be wider, and the price jumps at each break can be large relative to the extra weight.
This makes the weight breaks in your rate table useful information. A parcel only just over a break pays the price of the next band, so a small saving in materials or product can sometimes move a parcel down a band. Packaging lighter materials may be worth testing where parcels sit close to a break.
Volumetric weight
Carriers move parcels in vehicles and aircraft that fill up in space before they reach weight limits, so they charge for space as well as mass. To do this, they convert a parcel’s dimensions into a volumetric weight, also called dimensional weight. The conversion divides the volume by a divisor set by the carrier. The details, and the common divisors, are explained in the article on how dimensional weight is calculated.
For a metric example with a divisor of 5,000 cubic centimetres per kilogram:
- A box measuring 40 x 30 x 20 cm has a volume of 24,000 cubic centimetres.
- The volumetric weight is 24,000 / 5,000 = 4.8 kg.
The divisor varies by carrier and service, and some use different units, so use the figure in your own agreement.
The greater of actual and volumetric weight
The billable weight is generally the larger of the actual weight and the volumetric weight. Take the same box, filled with items weighing 3.2 kg in total:
- Actual weight: 3.2 kg.
- Volumetric weight: 4.8 kg.
- Billable weight: 4.8 kg, the greater of the two.
If the carrier then rounds up to the next whole kilogram, the parcel is rated as 5 kg. The customer’s item weighs about two thirds of the weight actually charged (3.2 / 5 = 0.64). Now compare a denser parcel: a smaller box of 20 x 20 x 15 cm has a volume of 6,000 cubic centimetres, giving 6,000 / 5,000 = 1.2 kg volumetric, and if the contents weigh 3.2 kg, the actual weight is greater and sets the charge.
This is why a sturdy box that is too large for its contents can cost more than a lighter, smaller one. Space is paid for even when it is filled with air. Right-sized packaging addresses exactly that, and the effects of choosing a mailer over a box are covered under poly mailers.
Where measurement happens
Carriers weigh and measure parcels in their network, often with automated scanners at sorting hubs. The result can differ from your own figures if:
- your scale or tape is slightly different from the carrier’s equipment
- the box was bulging, soft or deformed in transit
- the label declared dimensions that were wrong or out of date
- a rounding rule was applied differently from how you expected
When the billed weight is higher than your own, an adjustment appears on the invoice. Checking such adjustments is part of invoice audits, and a record of your measurements and box sizes strengthens any query.
Practical steps
- Keep accurate product and package data. Weight and dimensions in your shipping software should include the packaging, not just the product.
- Know your divisor and rounding rules. They should be in your service guide or carrier contract.
- Map your common parcels. For each standard box, calculate the actual weight, volumetric weight and billable weight. Highlight ones paying for air.
- Look for near-break parcels. If a common parcel sits just above a break, small changes may help.
- Compare services. A different service may use another divisor or different bands, and a rate shopping tool can show the effect for your actual parcels.
- Use air freight rules as a reference only. Air freight rates have their own chargeable weight calculation, which is similar in spirit but not identical.
A note on variation
Divisors, rounding rules and band widths are not the same everywhere, and they may change when a carrier updates its terms. Treat the examples in this article as illustrations of the method, and rely on your own agreement for the figures.