← Dimensional weight calculator
Every parcel carrier bills the greater of two numbers: what your package weighs, and what the space it occupies is priced at. The second is dimensional weight, and for anyone shipping light or bulky goods it is almost always the one that wins. The arithmetic is simple, it is entirely in your control, and most sellers meet it for the first time on an invoice rather than before choosing a carton.
Open the free dimensional weight calculator
A four pound item in a sixteen by twelve by ten inch carton is 1,920 cubic inches. At a 139 divisor that is a dimensional weight of 13.81 pounds, so the package bills at 14 pounds to move four pounds of goods. Ten pounds of that invoice is empty space. The break even cube is the actual weight times the divisor, or 556 cubic inches here, so any carton smaller than roughly eight by eight by eight would have billed on the goods instead. On this footprint the same ceiling is a packed height of 2.9 inches, which tells you plainly that the fix is a different box rather than better packing.
Multiply the length, width and height of the packed carton in inches to get cubic inches, then divide by the DIM divisor in your carrier agreement. That result is the dimensional weight in pounds. The carrier bills the greater of that figure and the package's actual weight, rounded up to the next whole pound. Most carriers round each side to the nearest whole inch before multiplying, so a 12.5 inch side is cubed as 13.
A higher one, because it divides the same cubic inches into fewer pounds. The published US domestic figure for the major parcel carriers has been 139. Negotiated accounts commonly carry 166, and 194 or 250 appear on higher volume agreements. If you ship light bulky goods, a better divisor is often worth more than a headline rate discount, because it changes the weight every single package is rated at rather than the price of a weight you were never going to hit.
Because the box is what gets priced, not the item. A four pound item in a sixteen by twelve by ten inch carton is 1,920 cubic inches, which at a 139 divisor is a dimensional weight of 13.81 pounds and a billable weight of 14. Ten of those pounds are air. Nothing about the item changed; the carton decided the bill, and a smaller carton would have ended the penalty entirely.
Get the cube below your actual weight multiplied by the divisor. That product is the break even: any carton under it bills on the goods instead of the space. For a four pound item at a 139 divisor the ceiling is 556 cubic inches, which is roughly an eight by eight by eight box. Right sizing cartons, using fewer sizes with less void fill, and shipping flat where the product allows are the three changes that move it.
Girth is the longest side plus twice each of the other two, which approximates the strap around measurement of the package. Parcel surcharge tables are written in it because it captures awkwardness in a single number. Past certain lengths and girths a package picks up an additional handling charge, then a large package surcharge with its own minimum billable weight, and past the maximum it is refused as a parcel and has to move as freight.
No, though they solve the same problem. Parcel carriers convert volume to weight with a divisor and bill the greater of that and the scale weight. Less than truckload freight instead sorts a shipment into a class band from its density in pounds per cubic foot, and handling, stowability and liability can move that class regardless of density. Use a divisor for anything that ships as a parcel and a density band for anything on a pallet.
The point of working it out before you list a product rather than after you ship one is that both levers are cheap in advance and expensive later. Carton sizes are a purchasing decision you make once and live with for thousands of shipments, and the DIM divisor is a line in an agreement you can ask about at renewal. Neither is available to you on the day the invoice arrives.
Shipping on pallets instead? Less than truckload freight prices on density bands rather than a divisor — the LTL freight class estimator covers that case.