How to choose a box and calculate cubic weight

Start with the item, then its protection, then the sealed parcel. These are three different measurements. Mixing them up can give a misleading postage estimate.

Which calculator mode should I choose?

Find a box starts with the unpacked item. You can estimate the final weight of item, box and packing materials, or leave it blank: box fit and cubic weight still work, but the higher-weight comparison and physical weight limit cannot be checked. Already packed uses the outside measurements and actual weight of the complete sealed parcel. Its entries are kept separate so an estimate is not mistaken for a measurement.

1. Find the usable inside space

Measure one item at its widest points. Add clearance for cushioning on every side. For a 20 × 14 × 8 cm item with 1 cm clearance per side, look for at least 22 × 16 × 10 cm of usable internal space.

ParcelFit checks all six axis-aligned orientations. “Keep upright” allows only two orientations: rotating the base while preserving the item’s height. It does not solve diagonal fits, flexible packaging, multiple items or irregular shapes.

2. Measure the sealed outside

Internal dimensions help with fit; the external dimensions determine parcel volume. Our planning model adds twice the chosen wall allowance to each internal dimension. Real folds, seams, tape and bulges can make the parcel larger. Measure the sealed box rather than relying on that estimate.

3. Compare physical and cubic weight

Physical weight is what the complete parcel weighs on the scales. Cubic weight is a weight based on how much space its box takes up. Two parcels can weigh the same on the scales but have different cubic weights because their boxes are different sizes.

The equivalent formula is volume in cubic metres × 250. A 40 × 30 × 20 cm parcel has a volume of 0.024 m³ and a cubic weight of 6 kg. If its packed physical weight is 2 kg, the higher-weight estimate is 6 kg. If it weighs 8 kg, it is 8 kg.

For domestic own packaging, current Australia Post guidance uses the greater of physical and cubic weight. ParcelFit shows the mathematical result, rounded only for display to three decimal places. It does not apply a tariff band or promise a billed weight.

What about flat rate packaging?

Eligible Australia Post flat rate packaging at 5 kg or under uses size-based postage rather than this cubic comparison. A generic box of the same dimensions is not automatically eligible. Check the product, packed weight and carrier conditions. Flexible satchels need a real fit check; their flat dimensions are not a usable cuboid.

Size checks are only a first check

We check a maximum physical weight of 22 kg, a longest side of 105 cm and volume of 0.25 m³. The linked domestic guide also specifies at least 5 cm length and width for a box. Passing these checks does not confirm acceptance: contents, packaging, service and destination can add restrictions.

Sources and review date

Checked 21 October 2026. Follow the carrier’s current guidance if it changes.

• Australia Post: how postage is calculated (PDF) — formula, packed measurements and flat rate distinction.
• Australia Post: size and weight guidelines — current domestic limits.
• Domestic parcels guide (PDF, October 2019) — minimum box dimensions. Its older cubing wording differs from current web guidance; our formula follows the current guidance above.
• Parcel Post options and the official postage calculator — check actual services and estimates.

A practical final check

Pack the item securely, seal the box, measure its widest external points and weigh everything together. Use those measurements, your origin and destination, and any required extras to obtain the official estimate.