Australia Post cubic weight: why light parcels cost more

Cubic weight is Australia Post's way of charging for the space a parcel takes up, not just how heavy it is. It works out a weight from the parcel's dimensions, compares it with the real weight, and typically charges you for whichever is greater. That's why a 1.2kg cushion in a big box can be priced like a 15kg parcel.

If you sell anything light and bulky (bedding, cushions, lampshades, plush toys, apparel in boxes), cubic weight is probably the biggest single thing driving your postage. The good news is that it's predictable, and most of it is in your control.

How Australia Post calculates cubic weight

At the time of writing, Australia Post uses a cubic conversion factor of 250kg per cubic metre. The formula from its cubic weight guide is:

Cubic weight (kg) = length (m) × width (m) × height (m) × 250

Most people measure boxes in centimetres, so this version is easier:

Cubic weight (kg) = length (cm) × width (cm) × height (cm) ÷ 4,000

Both give the same answer, because one cubic metre is 1,000,000 cubic centimetres and 1,000,000 ÷ 250 = 4,000.

A few measuring rules matter:

• Measure the packed parcel, not the product. The box, padding and any bulge from overfilling all count.
• Use the maximum points. Australia Post says to measure cylinders, satchels and odd shapes at their widest, longest and tallest points. A tube or a lumpy bag is priced as the box it would fit into.
• Round honestly. A box that measures 30.5cm is not a 30cm box.

Dead weight vs cubic weight: which one you pay

Your parcel has two weights:

• Dead weight (actual weight): what it weighs on the scale.
• Cubic weight: what its size works out to using the formula above.

Australia Post typically charges on the greater of the two. That figure is often called the chargeable weight, and it's what sets your postage, along with the service and the delivery zone.

At the time of writing there's one important exception for standard non-contract sending, including MyPost Business: Australia Post says it generally doesn't apply cubic charging to parcels under 1kg actual weight. It reserves the right to apply it anyway when a parcel is large, irregular or cylindrical in shape, so don't treat it as a loophole for big, light boxes.

Contract accounts can have different terms, so if you're on one, check your agreement rather than relying on the published rules.

A quick test: will this parcel be cubic weighted?

Here's a decision rule you can use without a calculator. Turn the formula around:

Charged on dead weight if:
volume (cm³) ≤ actual weight (kg) × 4,000

Some examples:

Actual weight	Largest box charged on dead weight
1kg	4,000cm³ (about 20 × 20 × 10cm)
2kg	8,000cm³ (about 25 × 20 × 16cm)
5kg	20,000cm³ (about 40 × 25 × 20cm)

If your packed box is bigger than that for its weight, you're paying for air. The further over you are, the more you're paying.

Dense products like books, tools or bottles rarely hit cubic weight. Light, fluffy products almost always do.

Worked example: one cushion, three boxes

Take a made-up homewares store sending a single cushion. Packed, it weighs 1.2kg on the scale. All postage figures below are illustrative and exclude GST. Australia Post prices change, so check current rates for your account.

Box A: the stock box (50 × 40 × 30cm)

50 × 40 × 30 = 60,000cm³
60,000 ÷ 4,000 = 15kg cubic weight
Chargeable weight = greater of 1.2kg and 15kg = 15kg

Box B: a better-fitting box (45 × 35 × 20cm)

45 × 35 × 20 = 31,500cm³
31,500 ÷ 4,000 = 7.875kg cubic weight
Chargeable weight = greater of 1.2kg and 7.875kg = 7.875kg

Box C: cushion vacuum-compressed (40 × 30 × 12cm)

40 × 30 × 12 = 14,400cm³
14,400 ÷ 4,000 = 3.6kg cubic weight
Chargeable weight = greater of 1.2kg and 3.6kg = 3.6kg

Same cushion, same weight on the scale, and the chargeable weight ranges from 3.6kg to 15kg. Now say Parcel Post for these chargeable weights costs, for this store's usual zone:

Box	Chargeable weight	Say postage is	Checkout charge	Margin on postage
A	15kg	$38.00	$12.00	$12.00 − $38.00 = −$26.00
B	7.875kg	$24.50	$12.00	$12.00 − $24.50 = −$12.50
C	3.6kg	$16.80	$12.00	$12.00 − $16.80 = −$4.80

Moving from Box A to Box C saves $38.00 − $16.80 = $21.20 on every cushion order. At 150 cushion orders a month, that's 150 × $21.20 = $3,180 a month, without changing the price, the product or the carrier.

The store still loses money on postage in Box C. That's a pricing problem, and it's much easier to fix from −$4.80 than from −$26.00. And postage is only one line: packaging, handling and fees come on top, as our guide to working out your true shipping cost per order shows.

The 1kg trap

The under-1kg rule creates a common surprise. Say a product weighs 0.85kg and goes in a large, light box that weighs 0.2kg:

Actual weight = 0.85kg + 0.2kg = 1.05kg

That parcel is now over 1kg, so it's no longer in the general under-1kg exemption, and cubic weight can apply in full. A heavier box, an extra insert or a bit more void fill can tip a parcel from cheap to expensive. If you have products that sit near 1kg packed, weigh them with the real packaging, not the product alone.

Size and weight limits to keep in mind

At the time of writing, the MyPost Business sending guide lists the domestic parcel limits as:

• maximum weight: 22kg
• maximum length: 105cm
• maximum volume: 0.25 cubic metres

The same guide notes that parcels up to 5kg (using the greater of physical or cubic weight) can be sent at national flat rates, while heavier parcels are charged on whichever weight is greater. That 5kg line is worth knowing: if cubic weight pushes a light parcel over it, the price can jump. Check the current guide before you design packaging around any of these figures.

How to cut cubic weight

1. List your bulkiest sellers. Run the quick test above on your top 10 products by order volume. Anything charged on cubic weight goes to the top of the list.
2. Right-size the box. Keep two or three box sizes that fit your common orders closely, instead of one big box for everything. Every centimetre counts on all three sides.
3. Compress soft goods. Vacuum bags or compression packaging can halve the volume of cushions, bedding and clothing.
4. Avoid awkward shapes. A tube is measured as the box around it. Flat-packing or folding can beat a cylinder.
5. Consider satchels for soft items. Australia Post points to its pre-paid satchels and flat rate packaging as a way to avoid uncertainty about chargeable weight. Compare the cost with a right-sized box for your actual products; our free satchel vs box price checker puts the two side by side for a given parcel.
6. Fix your product data. If your checkout calculates postage from product weights and dimensions, wrong data means wrong quotes. A cushion entered as 1.2kg with no box size can be quoted like a light parcel and then cost you like a 15kg one.

Check what checkout actually quotes

Once you've changed packaging, check the quote your customers see for your bulky products, not just the light ones. If you're still deciding how to price postage at checkout, our guide to Australia Post rates at Shopify checkout compares flat, weight-based and live rates. TrkShp lets Australian Shopify stores preview the domestic Australia Post services and prices their checkout will return, with any markup and minimum or maximum price applied, before customers see them. The setup docs explain how it works. A preview is an estimate of what checkout will charge, not your Australia Post invoice, so compare it with what you're actually billed.

For how postage fits into the profit on a whole order, see our guide to shipping profitability.

Key takeaways

• Cubic weight (kg) = length × width × height in centimetres ÷ 4,000, using the packed parcel's maximum points.
• Australia Post typically charges on the greater of dead weight and cubic weight.
• A parcel is charged on dead weight only if its volume in cm³ is no more than its weight in kg × 4,000.
• In the illustrative example, a better box and compression cut the chargeable weight of the same cushion from 15kg to 3.6kg.
• Rules and limits can change and contract terms differ, so check Australia Post's current guidance for your account.

Want to see what your checkout quotes for your bulkiest products? Find out how TrkShp works.