How to choose a shipping box size
Turn product measurements into a useful internal carton size, test packing arrangements, and estimate the outside size used for shipping.
Have your measurements ready? Put the method to work.
Box Size CalculatorMeasure the product in its permitted orientation
Begin with the product's longest external dimensions, including handles, protrusions, and any primary packaging that remains in the carton. Decide whether the item must stay upright. A smaller geometric enclosure is not useful if it damages the contents or contradicts the product's handling requirements.
For multiple products, decide whether they need separation. Product gap is the space between neighbors; outer padding is the space between the whole arrangement and each carton wall. They represent different allowances and should not be counted as the same thing.
Build an arrangement, then add protection
For a rectangular grid, each internal axis is: number of products on that axis × product dimension + number of gaps × gap size + padding on both outer sides. There is one fewer gap than products along an axis.
The compact arrangement option searches a bounded set of rectangular grids. It is useful for comparing shapes but does not prove the smallest possible packing of irregular or nested products. For an existing carton, use the box packing calculator to inspect actual item placements.
Worked example: four products in one layer
Each product measures 20 × 15 × 10 cm. Place four in a 2 × 2 × 1 grid, with 1 cm between neighbors and 2 cm padding at every outside face. Internal length is 2 × 20 + 1 + 4 = 45 cm; width is 2 × 15 + 1 + 4 = 35 cm; height is 10 + 4 = 14 cm.
With 3 mm board and the simple two-wall allowance, estimated outside dimensions are 45.6 × 35.6 × 14.6 cm. Their volume is 0.023701056 m³. At a 5,000 cm³/kg divisor, the dimensional weight estimate is about 4.74 kg before any carrier rounding.
Internal and external dimensions serve different jobs
Internal dimensions determine whether products and cushioning fit. External dimensions determine occupied shipping space. The board-thickness model is an estimate: folded flaps, seams, compression, and manufacturing tolerances can change the measured outside size.
Ask the supplier how it specifies dimensions and tolerances. Check a closed sample with the actual contents, inserts, and closure. The box must close without forcing panels outward or crushing the product.
Compare the full packaging decision
A smaller carton may reduce dimensional weight and void fill, but an awkward arrangement may increase packing time or damage risk. Compare carton price, cushioning, labor, shipping assumptions, and product protection together.
Use the dieline generator to communicate a dimensional prototype once the arrangement is settled. A packaging manufacturer still needs to confirm board grade, score allowances, joint design, and the distribution tests appropriate to your product.
Technical references
Service rules and equipment can change. Verify the specifications for your shipment.