Every online shop eventually asks how many carton sizes it should keep. Too few and small orders ship in big boxes, billed by the carrier for the air inside them. Too many and the packing bench fills with boxes nobody can find. The useful way to see the trade-off is as a curve: the cost of shipping your month's orders with the best one size, the best two, the best three, and so on. It always falls, and it almost always flattens quickly.
Open the free box size optimizer
Parcel carriers bill the greater of a package's actual weight and its dimensional weight, which is the box's outside cubic inches divided by a DIM divisor such as 139. A light product in a box sized for your largest product is billed as if it weighed what the empty box would hold. The first extra size you add takes your most common small orders out of the big box, and that is where most of the dimensional weight was. Each size after that rescues fewer orders, and fewer pounds per order.
An invented shop ships three products: a ceramic mug (5 × 4 × 4.5 inches, 0.9 lb, 300 orders a month), a candle tin (3.5 × 3.5 × 3 inches, 0.7 lb, 420 a month) and a folded linen throw (12 × 10 × 3 inches, 2.2 lb, 90 a month). It can buy a 6 × 6 × 6 box at $0.55, an 8 × 6 × 4 at $0.50, a 14 × 12 × 6 at $1.30 and a 12 × 12 × 12 at $1.45. Postage is $9.50 for the first billable pound and $0.85 for each one after it, at a divisor of 139, with half an inch of padding on every side, an eighth-inch wall and $0.35 of void fill per empty cubic foot. All of these figures are invented for the example.
| Sizes | Best set | Per month | Saved by this size |
|---|---|---|---|
| 1 | 14 × 12 × 6 | $13,718 | — |
| 2 | 6 × 6 × 6, 14 × 12 × 6 | $9,390 | $4,328 |
| 3 | 8 × 6 × 4, 6 × 6 × 6, 14 × 12 × 6 | $9,367 | $23 |
With one size, the only box that holds the throw is the 14 × 12 × 6, so the mug and the candle ship in it too and every order bills at 8 pounds. The 12 × 12 × 12 looks roomier but cannot take a 12-inch throw once padding is added. A second size, the 6-inch cube, takes 720 of the 810 orders down to 2 billable pounds and saves $4,328 a month. A third size moves the candle into a slightly cheaper box and saves $23 a month, which is less than most shops would put on keeping another carton on the shelf. The answer for this shop is two sizes, and the reason is visible in the table rather than argued.
The optimizer does steps 4 and 5 for you, and it checks every combination of sizes when the list is small enough, so the best set is the best set rather than a rule of thumb.
Only when your products are all close to the same size or heavy enough that the scale weight beats the dimensional weight anyway. For light goods of different sizes, one box big enough for the largest product bills every small order for the air around it.
Shelf or pallet space, a minimum order quantity that ties up cash, a second reorder to remember, and packers grabbing the wrong box. Put a monthly figure on that, even a rough one, and stop adding sizes when the next one saves less than it.
Compare the best set of stock sizes against the best set that includes custom sizes on the same order mix. If custom boxes only save a little more than well chosen stock sizes, the minimum order and lead time of a custom run usually is not worth it.
Checking a single carton rather than a whole assortment? The dimensional weight calculator shows the billable weight of one box and the size at which the penalty stops.