Two common US markings
Edge Crush Test (ECT) relates to stacking strength. Burst or Mullen tests relate more to puncture and rough handling. A 32 ECT single-wall box is a common small-parcel starting point; it is not automatically equivalent to a 200 lb burst box.
How to read a box maker’s certificate
Look for the stamp on the box bottom. Confirm flute and wall construction from the manufacturer—not a marketplace title.
Limitations
Desk research, not a lab test of a SKU. Method labels: Editorial standards.
Size is separate
Inside dimensions still come from the box size calculator. Sometimes a box is the wrong format: mailers vs boxes.