Shipping is usually priced by weight, or by whichever of weight and volume comes out higher. Both numbers can be worked out before the parcel ever reaches a counter — the box just has to be counted along with everything that goes inside it.
The box is not free
An empty cardboard shipping box, single-wall corrugated and around 20 to 30 centimetres on its longest side, typically weighs 70 to 150 grams on its own. That sounds small until the parcel itself is light: on a 700-gram parcel, a 100-gram box is not a rounding error, it is close to a seventh of the total weight being paid for. The lighter the contents, the larger a share of the final number the box itself turns out to be — which is why guessing a parcel’s weight from its contents alone tends to run low.
Packing material adds up too
Bubble wrap, crumpled paper and tape are light individually and easy to forget entirely. A metre of small-bubble wrap runs around 25 grams, a sheet of kraft packing paper around 10 grams, and a metre of packing tape only a few grams — but a well-padded parcel can use several of each, and the total across all of it commonly lands somewhere between 30 and 80 grams. None of that is the item being shipped, and all of it is on the scale at the post office counter.
Add up what you actually know
Anything going into the box that has a page on this site already has a number attached, and adding those numbers is more reliable than guessing at the finished parcel. A ceramic mug (264 grams), a Swiss Army knife (217 grams) and four AA batteries (23 grams each, 92 grams together) come to 573 grams of actual contents. Add a 110-gram box and 40 grams of wrapping and tape, and the parcel comes to 723 grams — a number built from five known weights rather than one guess at the whole.
The same approach works for anything with a listed weight: a cordless drill at 1208 grams or an umbrella at 651 grams can each be added straight into a running total, and the only guesswork left is the box and the packing around it.
Carriers often charge by volume, not by scale weight
This is the part that actually surprises people: past a certain size, many carriers bill on volumetric weight, a figure calculated from the box’s dimensions rather than read off a scale, and charge whichever number — actual or volumetric — is higher. A common formula multiplies length, width and height in centimetres and divides by 5000 to get a weight in kilograms; some carriers use 6000 instead, particularly for air freight, so the exact divisor depends on the carrier being used.
Take an umbrella boxed up for shipping: 651 grams for the umbrella itself, say 150 grams for a box long enough to hold it unfolded and 50 grams of padding, for an actual weight of 851 grams. If that box measures 60 by 20 by 20 centimetres, its volume is 24,000 cubic centimetres; divided by 5000, that comes to 4.8 kilograms of volumetric weight — more than five times the actual weight on a scale. The umbrella is barely over the weight of a bag of sugar, and the carrier bills it as if it were most of the way to five kilograms, because it takes up the space of something that heavy. Anything long, hollow or oddly shaped runs into this before anything dense does; a compact, heavy item like a cordless drill rarely triggers it at all.
Before sealing the box
Round the box and packing material up rather than down — both are easier to underestimate than the contents — and check the specific carrier’s volumetric divisor if the parcel is bulky for its weight rather than dense. A number built by adding up a box, its packing and every item going inside it will be closer to what the scale at the counter actually shows than any single guess at the finished parcel.