The smallest possible carton is not automatically the right shipping carton. A useful packaging decision starts with the packed product group, then adds protective clearance, board thickness and the tolerances needed to pack consistently.
Start with the product orientation
Measure the product in the orientation it will actually use in the carton. If several identical products share one box, the arrangement can change the required carton dramatically. The Box Size Calculator tests whole-number rectangular grids and base rotations so the recommendation is based on a packable group rather than a simple volume ratio.
Internal and external dimensions are different
Internal dimensions describe usable space. External dimensions include the board wall and are the dimensions that matter for freight cube and dimensional weight. A carton supplier may specify dimensions using a particular convention, so confirm which set is being quoted.
Padding belongs around the group
Protective clearance should reflect the actual cushioning system, not a random safety number. Adding two centimetres per side means adding four centimetres to the full dimension on that axis.
Check the freight consequence
Once you have an external size, test it with the Dimensional Weight Calculator and Packaging Efficiency Calculator. A box that protects the product but leaves excessive void can create avoidable shipping cost.
Finish with a physical sample
Software can narrow the size, but it cannot certify crush strength, product movement, closure performance or warehouse packability. Build and test a physical sample before committing to production quantities.