Start with usable space, not the box label
A carton can look large enough and still be a poor fit. What matters for packing is the usable space inside the carton and whether the item can be placed there in an allowed orientation.
That is why Packmetry treats internal carton dimensions as the geometry used for placement. External package dimensions have a different purpose: they matter when dimensional-weight analysis is enabled.
Internal dimensions answer “will it fit?” External dimensions answer questions about the outside package, including dimensional-weight calculations.
Do not optimize empty space in isolation
A lower empty-volume number is useful, but it is not the only thing that can make one plan better than another. The number of cartons, available inventory, supported handling rules and your selected packing objective can all change the preferred result.
For an everyday shipment, a practical decision often starts with a simpler question: can all requested items be packed into fewer cartons without violating the dimensions and constraints you entered?
When two boxes both fit
If two candidate cartons both produce verified plans, empty space becomes a useful tie-breaker. A tighter plan usually means less unused internal volume, but you should still leave any real-world cushioning or spacing needs in the inputs rather than assuming a perfectly tight pack.
Rotation can change the answer
The same item can fit in very different ways depending on its rotation policy. An unrestricted item may use any solver-supported orientation. An upright or fixed item has fewer valid placements, which can require a larger carton even when raw volume suggests otherwise.
Volume alone cannot prove that an item fits. Geometry matters because the length, width and height still need to occupy valid positions inside the carton.
Use the verified arrangement as the source of truth
Packmetry does not treat the 3D view as a decorative reconstruction. The visualization follows the canonical verified placement plan. That means the result summary and the 3D arrangement refer to the same packing result.
A good box decision is therefore not simply “pick the smallest dimensions.” It is: choose a carton that produces a verified plan under the constraints that actually matter for the job.