Work out what a carrier will actually bill you for a parcel. Enter the box dimensions and the real weight, and this returns the dimensional weight, the billable weight, and how much of what you are paying is air rather than product.
Carriers do not charge for weight. They charge for the greater of weight and space, because a van fills up by volume long before it runs out of payload. Dimensional weight is how that space gets converted into something they can put on an invoice: length times width times height, divided by a number the carrier publishes. If that figure comes out above the scale weight, that is the number you pay for.
This is the single most common way an ecommerce shipping estimate goes wrong. A cushion weighs almost nothing and costs a fortune to send. A box with two inches of spare room on every side can cost a third more than the same goods packed tight. Sellers who set a flat shipping rate from a kitchen scale discover the gap at the end of the month, in the carrier invoice, on every order at once.
Divisors differ by carrier, by service and by whether the shipment crosses a border, and they change. The common ones are pre-filled below and every one is editable. Your own carrier contract is the authority, and if you have negotiated rates your divisor may be better than any default here.
Commonly 139 for domestic parcels priced in inches and pounds, and 5000 or 6000 for parcels priced in centimetres and kilograms. Your carrier contract is the authority; a negotiated divisor is often better than the published one.
How to make the number smaller
Take the air out of the box. Dimensional weight is volume, so every spare inch on every side is billed. Dropping a 12 by 10 by 8 box to 11 by 9 by 6 cuts the volume by almost 40 per cent, and the billable weight with it.
Use a mailer where a mailer will do. Soft packaging has almost no volume of its own and it is the single biggest saving available to anyone shipping clothing, prints or anything else that does not need protecting from crushing.
Check the threshold, not the average. Carriers often apply dimensional pricing only above a size threshold, so a small parcel may be billed on scale weight and the next size up on volume. The jump is not gradual, which is why a slightly bigger box sometimes costs disproportionately more.
Measure the outside, after packing. The carrier measures the box as presented, bulges included. A tape measure across the widest point is the number they will use, not the flat pack dimension printed on the box.