What is Cubic Weight? Dimensional Weight Explained
Cubic weight, also called dimensional or volumetric weight, is a freight charge based on the space a parcel takes up rather than what it actually weighs. In Australia the standard cubic conversion factor is 250, so cubic weight in kilograms is length times width times height in metres, multiplied by 250.
How cubic weight is calculated
Measure the parcel in metres, multiply the three dimensions together to get the volume in cubic metres, then multiply that volume by 250. Measure the parcel as it will actually travel, including the packaging, any bulge in the sides and the widest point of an irregular shape, not the bare item inside.
The carrier then compares the two figures and bills the larger one. That is the rule that catches people out: you are charged on the greater of actual weight and cubic weight, so a light parcel in a big box is priced as though it were heavy.
A worked example
Take a carton measuring 400 x 300 x 200mm that weighs 2kg once it is packed and taped.
In metres that is 0.4 x 0.3 x 0.2, which is 0.024 cubic metres. Multiply 0.024 by 250 and the cubic weight is 6kg. Actual weight is 2kg, cubic weight is 6kg, so the parcel is charged on 6kg. You pay for three times what the scales say.
Why carriers charge for space
A delivery van or a line haul trailer runs out of room long before it runs out of payload. A truck rated to carry several tonnes will be full of pillows or empty boxes at a fraction of that weight, and every cubic metre of air being carried is revenue the carrier cannot earn. Cubic weight is how carriers price the resource that actually constrains them, which is space on the vehicle.
Shop satchels, mailers and cartons
Right-sizing the pack is the cheapest freight saving available. These are the ranges to choose from.
How packaging choice changes the freight bill
This is where cubic weight stops being trivia. The table below sends the same order, 1.5kg of soft goods, in seven different packs QIS stocks. Actual weight is held at 1.5kg in every row so the only thing changing is the pack. Cubic weights are rounded to two decimal places.
| Pack (as it travels) | Actual weight | Cubic weight | Charged on |
|---|---|---|---|
| A4 courier satchel, 240 x 320mm, packed 30mm thick | 1.5kg | 0.58kg | 1.5kg (actual) |
| Honeycomb padded mailer size 2, 235 x 280mm, packed 40mm thick | 1.5kg | 0.66kg | 1.5kg (actual) |
| A3 courier satchel, 310 x 400mm, packed 40mm thick | 1.5kg | 1.24kg | 1.5kg (actual) |
| Carton 380 x 228 x 84mm | 1.5kg | 1.82kg | 1.82kg (cubic) |
| Carton 305 x 215 x 125mm | 1.5kg | 2.05kg | 2.05kg (cubic) |
| Carton 400 x 280 x 250mm | 1.5kg | 7.00kg | 7.00kg (cubic) |
| Carton 440 x 305 x 280mm | 1.5kg | 9.39kg | 9.39kg (cubic) |
The same 1.5kg of stock is billed as 1.5kg in an A3 satchel and as 9.39kg in a 440 x 305 x 280mm carton. That is more than six times the chargeable weight for an identical order, on every parcel, every day.
A satchel squashes down, a carton does not. A courier bag takes the shape of what is inside it, so its third dimension is whatever the packed parcel measures. Squeeze the air out of an A3 satchel and 40mm becomes 30mm, and the cubic weight drops with it. A carton holds its outside dimensions whether it is full or half empty, so an oversized box charges you for the empty space.
Right-size the carton. Compare the two flattest rows in the table. The same goods in a 380 x 228 x 84mm carton are charged on 1.82kg, and in a 400 x 280 x 250mm carton on 7kg. Nothing changed except the box. Holding two or three carton sizes and picking the smallest one that safely fits is usually the fastest freight saving a business can make.
Do not buy a bigger box to fit the void fill. Void fill does not add cubic weight of its own, because the outside dimensions of the carton have already set that figure. The cost comes from choosing a larger carton so there is room to pack it out. Use void fill to stop the contents moving, not to fill space you did not need in the first place.
Cubic weight in Australia
Australia Post and StarTrack both use a cubic conversion factor of 250 and charge on the greater of actual or cubic weight. The 250 factor is effectively the Australian default, which is why so many local rate cards and freight calculators quote it without explanation.
Australia Post generally does not apply cubic weight to parcels under 1kg actual weight, unless the parcel is large, irregular or cylindrical. Poster tubes and awkward shapes are the usual exceptions. Flat rate satchels are not subject to cubic weight either: with a satchel you are buying a fixed price for whatever fits inside, which is part of why satchels are so popular with Australian online sellers.
Other carriers may use a different conversion factor, and some apply cubic weight per consignment rather than per item. Check the rate card before you assume 250 applies.
Packs that keep cubic weight down
In stock in Brisbane, despatched same day on stock orders placed before 12pm.
Related guides
Satchels are the main tool for keeping cubic weight down, so start with what are courier bags. Then read what is void fill for what to put in the gaps without upsizing the carton, and what is bubble wrap for protection that adds very little thickness.
If you are choosing between board grades for a right-sized box, what is corrugated cardboard covers flute types and where each one is used.
Want help matching your product to the smallest safe pack? Contact our Brisbane team on 1800 555 343 and we will work through your sizes with you.
