Volumetric weight explained: how carriers charge for space, and how to pay for less of it

Volumetric weight is a calculated figure, not something you can read off a scale. A carrier takes the outside dimensions of a parcel, works out its volume and converts that volume into a weight using a set ratio. The result tells the carrier how heavy the parcel would be if it were packed at an assumed density.

You will see it called dimensional weight, dim weight or volume weight depending on the carrier and the country, but the idea is the same. It matters most for light, bulky items such as pillows, lampshades, plastic storage boxes, toys in large retail packaging and anything shipped with a lot of protective padding.

A van, a lorry trailer or an aircraft hold fills up in two ways: by weight and by space. A cage full of duvets may be well under its weight limit and still be completely full. If carriers charged only by actual weight, light but bulky parcels would take up capacity without paying for it, so volumetric weight closes that gap.

Warehouses face the same constraint. Racking, shelving and floor space are finite, so warehouse storage is commonly billed by the space goods occupy rather than by how heavy they are. A pallet of lightweight cushions uses as much racking as a pallet of tinned food. At MuggleShip, for example, storage is billed by space and duration, whether that is box, pallet, shelf or container storage, not by the number of SKUs.

The common formula for volumetric weight in metric units is: length × width × height in centimetres, divided by a divisor, giving a result in kilograms. The divisor is set by each carrier and can differ between services, such as domestic and international or express and economy. Some carriers work in inches and pounds with their own divisor instead.

Because the divisor changes the answer so much, never assume one figure applies everywhere. Check your carrier's current terms, rate card or account documentation for the divisor, the units and any rounding rules before you price products or compare quotes.

The numbers below are illustrative only and use a divisor of 5,000 to keep the arithmetic simple. Your carrier's divisor may be different, and so may its rounding rules, so substitute your own figures before drawing any conclusions about cost or comparing one carrier against another.

Say you ship a set of three cushions in a carton measuring 50 cm × 40 cm × 30 cm. The volume is 60,000 cubic centimetres. Divided by 5,000, that gives a volumetric weight of 12 kg. On the scale, the packed carton weighs 7 kg. Because 12 kg is greater than 7 kg, you would be billed as if the parcel weighed 12 kg.

Now repack the same cushions, compressed slightly, into a 40 cm × 30 cm × 30 cm carton. The volume drops to 36,000 cubic centimetres, and the volumetric weight falls to 7.2 kg. The actual weight is still about 7 kg, so the chargeable weight is now 7.2 kg instead of 12 kg, purely from a smaller box.

Chargeable weight is the weight a carrier uses to price a shipment. In most cases it is the greater of the actual weight and the volumetric weight. Dense items, such as books or metal tools, are usually charged on actual weight, while light, bulky items are usually charged on volumetric weight.

The same logic applies across a whole consignment. On a multi-carton or palletised shipment, carriers may compare total actual weight with total volumetric weight, or assess each piece separately, depending on the service. Rounding also matters: if your carrier rounds chargeable weight up to a set increment, a parcel just over a boundary costs the same as one near the top of the next band. Check how your carrier applies both rules.

Most of the saving comes from packaging decisions made once and repeated on every order. Small reductions in each dimension compound, because volume multiplies all three. Trimming a few centimetres off each side of a carton can make a noticeable difference to the volumetric weight, as the worked example above shows.

Start with the products that ship most often or most bulkily, since that is where a change pays back fastest. Pack a few units of each into the cartons you currently use, note how much empty space is left, and compare the volumetric weight against the actual weight. Wherever the volumetric figure is clearly higher, there is usually room to tighten the packaging.

Volumetric weight is not only a parcel-carrier issue. It can affect almost every leg of a product's journey, and the same oversized carton can cost you more than once. Look at where your goods travel and where they sit, and check which of these legs are priced on chargeable weight or on space.

Carriers often measure parcels themselves as they pass through their network. If the dimensions you declared are smaller than what they record, the shipment can be re-billed at the higher figure, sometimes with an added charge. Bulging cartons are a common cause, because an overfilled box measures larger than its nominal size.

Keep a tape measure and a calibrated scale at the packing bench, record standard dimensions and weights for each SKU and carton combination, and update them whenever packaging changes. If you use a fulfillment provider, ask how they measure and record parcels. With MuggleShip's eCommerce Fulfillment service, for instance, orders are picked, packed and quality checked before dispatch, and custom packaging options can help match carton size to the product.