Picking a mailing box sounds simple until you're standing over a product with three different boxes, wondering which one won't cost a fortune to post or leave your item rattling around in transit. Box size is one of the few packaging decisions that hits both your shipping bill and your breakage rate, so it's worth getting right.
Here's how to choose a box size that protects your product and keeps postage costs down.
Why box size affects your shipping cost more than you think
In Australia, couriers and Australia Post don't just charge by weight. They charge by whichever is greater: the actual weight, or the cubic (volumetric) weight of the parcel. Cubic weight is calculated from the box's dimensions, so an oversized box full of air can cost you more to send than a snug one holding the same item.
The standard formula most carriers use is:
Length (cm) × Width (cm) × Height (cm) ÷ 5000 = cubic weight in kg
So a 40 × 30 × 20 cm box has a cubic weight of 4.8 kg, even if what's inside weighs 500 grams. Choose a box closer to the size of your product and that number drops fast. Over hundreds of orders, right-sizing your boxes is one of the easiest ways to cut freight costs without changing carriers.
Measure the internal dimensions, not the external
Boxes are almost always listed by their internal measurements, because that's the space you actually have for your product. When you measure your item, measure its longest length, width and height, then add a little clearance on each side for protective wrapping.
A good rule of thumb: leave 2–5 cm of space around the product for cushioning. Fragile or heavy items sit at the higher end of that range; flat, sturdy items need less. If you're posting something delicate, factor in room for bubble wrap or padding before you settle on a box size.
Match the box type to the product
Size isn't just one number. The shape of the box matters too:
Flat, low-profile items (books, apparel, prints) suit shallow die-cut mailing boxes that fold flat and post cheaply.
Bulkier or cube-shaped items need a squarer mailing box that won't force you to leave awkward gaps.
Soft, non-fragile goods (clothing, textiles) often don't need a box at all - a mailing satchel is lighter, cheaper to post, and takes up less storage.
Choosing the right format first usually narrows your size options down to one or two obvious choices.
Common mailing box sizes and what they fit
While every product is different, these size bands cover most small-business shipping:
Small (roughly 15-22 cm longest side): jewellery, cosmetics, accessories, small electronics.
Medium (around 24-31 cm): apparel, books, homewares, boxed goods.
Large (35 cm and up): multiple items, bulky homewares, gift sets.
Buy the right sizes in bulk and keep restocking simple
Once you know your two or three go-to sizes, buying them in bulk brings the per-unit cost down and means you're never scrambling mid-order. Sourcing from a local supplier also keeps freight on your inbound stock cheap and delivery fast, which matters when you're restocking often.
It also pays to keep a trusted supplier on hand for restocking, Packee also stocks a full range of mailing boxes and protective packaging alongside Ozpack's range.
