A sweater in a huge box can cost more to ship than a small box full of books. Carriers price light, bulky parcels by the space they take, not just their weight. Get the box right, and you save on postage. Get it wrong, and you're donating your margin to the carrier.

Here's how to lock in the right size.

Measure your items

Measure the length, width, and height. For clothes, measure them folded exactly how you'll ship them. For weirdly shaped items, find the smallest safe rectangular volume they fit into.

Weigh items after including retail packaging. For bundles, measure every item separately—two medium items side-by-side might need a wider base than one large item.

Add padding before choosing the box

Padding isn't something you squeeze in at the end. For clothes and soft goods, 1-2 cm per side is plenty. For fragile items (glass, electronics), give it 3-5 cm per side.

You want to stop the item from hitting the box walls, but you also don't want massive empty voids. Bubble wrap protects corners, Kraft paper fills gaps. Measure the wrapped item, then pick the box.

Test orientations

The same items need different boxes depending on how they sit. Books ship flat. Mugs are safer upright. Shoes might fit heel-to-toe better than side-by-side.

Test different rotations. A simple flip can save you an entire size tier. The best orientation is the one that creates the smallest safe outer box.

Try the tool →

3 mistakes that cost money

  • Measuring the outside of an empty box: Carriers price by outside dimensions, but your product needs usable inner space. Thick cardboard eats into that space.
  • Forgetting the packed state: Bubble wrap, tape folds, and product boxes add size. A 9 cm mug easily becomes 14 cm wide after you pad the handle.
  • Assuming the largest item sets the box size: The base footprint often matters more than height. Test the whole order, not just the biggest piece.

Hit the carrier size cliffs

Every carrier has size tiers (like InPost's A, B, C). A compact parcel in tier A is much cheaper than one that barely touches tier C, even if they weigh the same.

Decide which carrier tiers make sense for your business, then make sure your packed items stay inside those limits. One extra centimeter on height—or a bulging taped corner—can bump you into a pricier bracket.

Use the 3D calculator

If you're unsure, run your dimensions through the free 3D Packing Calculator before buying a label. It builds a 3D preview, suggests the optimal box, and shows which carrier tiers fit your packed items perfectly.