Volumetric weight explained

01The formula, and the divisor

Volumetric weight is length times width times height in centimetres, divided by a divisor. The divisor is usually 6000. Some lines use 5000, and a smaller divisor makes every bulky parcel more expensive, because dividing by a smaller number gives a bigger result.

A parcel 40 by 30 by 28 centimetres is 33,600 cubic centimetres. Divided by 6000, that is 5.6 kilograms of volumetric weight. Divided by 5000, it is 6.72 kilograms. Same box, same contents, materially different charge. That is why two quotes that look similar can land far apart, and why the divisor belongs on your checklist when you compare lines.

02Why carriers charge for space

A cargo aircraft and a container both fill up by volume before they reach their weight limit when the cargo is light and bulky. A parcel of pillows and a parcel of laptop batteries can occupy the same pallet position and weigh very different amounts. Charging only by weight would make bulky light cargo unprofitable to carry, so the industry compares the two measures and bills on the greater one.

This is not specific to this corner of the market. Postal services, express couriers and freight forwarders all use some version of it. What changes between them is the divisor and the rounding rule, which is why a line that looks cheap on a rate card can be expensive for a shoebox shaped parcel and cheap for a dense one.

01A worked comparison

Here is the same set of goods packed three ways, with sample rates from our own calculator so you can follow the arithmetic. These are not any agent's published prices. Rates used: first 0.5 kg at $8.50, then $4.20 per additional 0.5 kg.

As packed, a carton of 40 by 30 by 28 centimetres gives 5.6 kg of volumetric weight and a billable weight of 6.0 kg after rounding, which comes to $54.70. Remove the shoe boxes and the height drops to 24 centimetres, giving 4.8 kg, a billable 5.0 kg and a total of $46.30. Split into two parcels of 30 by 25 by 20 centimetres, each is 2.5 kg volumetric against an actual of 1.8 kg, so each bills at 2.5 kg for $16.90, or $33.80 for the pair.

Notice the shape of the result. Removing packaging saved $8.40 with no change to the contents. Splitting saved more in this example, but splitting can cost more on other lines because the first weight charge is paid twice, and it can also divide your insurance and make customs paperwork messier. The arithmetic is a guide, not a verdict.

02How to reduce it

There are only two levers: make the parcel smaller, or make it denser. Most savings come from the first. Removing shoe boxes is the classic move, worth 5 to 8 centimetres of height on a parcel containing two pairs. Flattening boxes, folding garments instead of rolling them into a ball, and asking the warehouse to repack into the smallest carton that fits are all the same idea.

Packing denser helps only when the parcel is below the volumetric figure and therefore paying by actual weight. If volumetric already exceeds actual, adding weight does nothing until you pass the volumetric number, which is usually not a sensible strategy. The exception is when you are close to a rounding boundary and the next 0.5 kg would push you up a step.

• Ask for shoe boxes to be removed if the shoes do not need them
• Ask for repacking into the smallest carton that fits
• Fold garments flat rather than stuffing them into gaps
• Check the divisor on the line before you compare quotes
• Measure the finished parcel, do not estimate from the goods

03When it does not apply cleanly

Long thin parcels, tubes and anything with one dimension far larger than the others can be billed on a length and girth rule instead, which the simple divisor formula understates. Oversized or irregular shapes may attract a handling surcharge on top. Sea freight sometimes bills by cubic metre rather than by a converted weight, which is a different calculation with the same idea behind it.

Restricted and battery containing items are another case, because the available lines narrow and the price difference between them is driven by what will accept the item rather than by weight. When a parcel is unusual in shape or contents, treat every calculator estimate, including ours, as a lower bound.