Guide / Packaging basics / Chapter 1.5
Courier sizes and cubic weight.
A big, light parcel can cost more to send than a small, heavy one. Here's how cubic weight works, how to calculate it, and how the right box keeps your postage down.
The short answer
Cubic weight is a weight worked out from a parcel's size. For domestic parcels in your own packaging, carriers usually charge on whichever is greater: the actual weight or the cubic weight. Flat-rate and prepaid options, and some account arrangements, work differently. To calculate it, multiply length × width × height in metres, then multiply by the carrier's conversion factor. Australia Post uses 250 kg per cubic metre. The smaller and better fitting your box, the lower your cubic weight.
Why carriers charge by size
A courier van or truck fills up by space as well as by weight. A large box of pillows weighs very little but takes up as much room as a box of books. Cubic weight lets carriers charge fairly for the space a parcel uses.
That's why box size matters so much for online businesses. An oversized box with lots of filler can push a light order into a higher price band, on every single order.
How to calculate cubic weight
1. Measure the outside of the parcel
Use the external dimensions, at the widest points, after it's packed and sealed. For satchels, tubes and odd shapes, measure the maximum length, width and height.
2. Convert to metres and multiply
Convert each measurement to metres (500 mm is 0.5 m), then multiply length × width × height. This gives you the volume in cubic metres.
3. Multiply by the conversion factor
Multiply the volume by the carrier's conversion factor. Australia Post uses 250 kg per cubic metre. Other couriers and services may use a different factor, so check your carrier's rate card.
4. Compare with the actual weight
Weigh the parcel. You'll usually be charged on whichever is greater, the actual weight or the cubic weight.
Cubic weight calculator
Enter the outside size of your packed parcel in millimetres and its actual weight. We'll work out the cubic weight and which weight you're likely to be charged on.
A guide only, using Australia Post's 250 kg per cubic metre factor by default. Change the factor to match your carrier, and check your carrier's rules, flat-rate options and size limits.
Worked examples
Calculated with Australia Post's conversion factor of 250 kg per cubic metre. Your carrier's factor and rules may differ.
| Parcel size (outside) | Volume | Cubic weight | If it actually weighs |
|---|---|---|---|
| 500 × 400 × 300 mm | 0.06 m³ | 15 kg | 10 kg, you're charged on 15 kg |
| 310 × 225 × 102 mm | about 0.0071 m³ | about 1.8 kg | 2.5 kg, you're charged on 2.5 kg |
| 400 × 300 × 200 mm | 0.024 m³ | 6 kg | 2 kg, you're charged on 6 kg |
Australia Post: what to know
Australia Post explains that it typically charges on the greater of a parcel's cubic weight or actual weight, using a conversion factor of 250 kg per cubic metre. For parcels sent without a contract, such as through MyPost Business, it says cubic charging generally isn't applied to parcels under 1 kg actual weight, although it reserves the right to apply it, particularly for large, irregular or cylindrical parcels.
Australia Post also notes that its prepaid satchels and flat rate packaging avoid the uncertainty of cubic weight. Rules and prices change, so always check the current details on Australia Post's cubic weight page and your own account terms.
What it means for you
The same question, answered for where your business is today.
Choose your stage at the top of the page to open the advice for you.
Start-up · under 100 units
Weigh and measure a typical order, then use your carrier's calculator before you choose a box. Light products in big boxes are where postage costs creep up.
For soft, light products, a courier satchel is often cheaper to post than a box.
Growing · 100 to 2,499 units
Work out the cubic weight of your most common orders. If a smaller box or a better-fitting mailer drops an order into a lower weight band, that saving repeats on every order.
Try this: compare your three most common orders in your current boxes and in the next size down.
Enterprise · 2,500 units and more
Cubic weight is one of the biggest freight levers you have. Right-size your boxes, check them against your carrier contract's conversion factor, and track the saving per shipment.
Changing board thickness changes the outside size. Re-check cubic weight before switching board.
Common mistakes
- 1Measuring the inside of the box. Carriers measure the outside, after sealing. Use external dimensions.
- 2Assuming every carrier uses the same factor. Conversion factors vary by carrier and service. Check your rate card.
- 3Using a box that's too big. Extra space and filler can push a light order into a higher price band.
- 4Forgetting the bulge. An overfilled box or satchel measures bigger at its widest point than its listed size.
Cubic weight questions
What is cubic weight?
How do I calculate cubic weight?
Does Australia Post charge cubic weight?
How can I reduce cubic weight?
Do you sell Australia Post sized boxes?
Keep reading
Postage-friendly packaging
Want to bring your postage down?
Send us your most common orders and the sizes you ship in. Our Melbourne packaging team will suggest better-fitting boxes, usually within 1 business day.