Measure the packed item
Use a ruler or tape in inches. Record the longest, widest, and tallest points after any product packaging you will actually ship.
Add protection
Fragile goods typically need cushioning on all six sides. The box size calculator adds per-side cushioning twice on each axis.
Watch outside vs inside
Retail box sizes are often outside dimensions. Carriers bill the package they measure.
After you have inches
Check ECT vs burst for strength, then void fill for leftover space.