Turn over almost any corrugated shipper and somewhere on a bottom flap there is a circle of small print. It is called the Box Maker's Certificate, it has been more or less standardised since the early twentieth century, and in our experience roughly one packaging buyer in twenty has actually read one.
What is in the circle
- The manufacturer's identification — who made it and where.
- The construction: single wall, double wall or triple wall.
- A strength rating: either a Mullen bursting test value in pounds per square inch, or an Edge Crush Test value in pounds per inch.
- Size limit: the maximum sum of length, width and depth in inches the box is certified for.
- Gross weight limit: the maximum weight of box plus contents.
The two tests, and why it matters which one you have
Mullen measures bursting strength. A rubber diaphragm is forced through the board until it ruptures. It tells you about puncture resistance — drops, corner impacts, being dragged across a dock plate. It was the standard of an era when freight was handled roughly and individually.
ECT measures edge crush. A short strip of board is stood on end and crushed, reporting pounds per linear inch. It predicts stacking performance: how high a pallet goes before the bottom box fails. In any palletised supply chain, this is almost always the number that matters.
| Mullen | ECT | Wall | Typical max gross weight |
|---|---|---|---|
| 200 lb | 32 ECT | Single | 65 lb |
| 275 lb | 44 ECT | Single | 95 lb |
| 275 lb | 48 ECT | Double | 120 lb |
| 350 lb | 51 ECT | Double | 180 lb |
| 500 lb | 61 ECT | Double | 260 lb |
| 1,100 lb | 82 ECT | Triple | 2,400 lb static |
Treat that table as a rule of thumb. The equivalences exist because carriers historically specified Mullen and the industry needed a translation; they are not physics and they are not a contract.
The size limit is the one people break
The certificate states a maximum sum of the three dimensions. Exceeding it voids the certification even if the box is physically capable, which matters in a freight claim. If your 32 ECT box is certified to 75 inches combined and you order one at 80, the stamp is no longer accurate and a carrier can decline a damage claim on that basis alone.
Why a used box's certificate is history, not specification
This is the point we make most often and the one that lands least easily. A used container's certificate describes what the board was when it left the converter. It has since been loaded, stacked, trucked, possibly exposed to humidity swings, and folded and unfolded some number of times.
On a used box, the Box Maker's Certificate is a historical document. We grade on observed condition instead, and we say so on every quote.
Flutes take a set. Score lines soften. Liner absorbs and releases moisture. None of that is visible in the stamp, and a supplier quoting you the original rating on a used container is either being careless or hoping you will not notice.
What to actually ask for
- For new board: the test type on the certificate, stated explicitly. Not 'heavy duty'.
- For new board: the certified size limit, checked against the size you are ordering.
- For used board: a named grade from a published scale, with its definition attached.
- For used board: photographs of the actual lot you are buying.
- For both: your real stack height, including the pallet somebody adds at 4pm on a Friday.
One practical consequence
If you are specifying new boxes for a palletised operation and your supplier offers Mullen, ask why. There are good reasons — rough individual handling, export, parcel networks — but if your product goes on a pallet and stacks, you are buying puncture resistance and then failing on compression. That is an expensive way to be over-specified and under-protected at the same time.
Our pieces are written by the people who do the work rather than by a content agency, which is why they contain more numbers and fewer adjectives than is fashionable. If a figure here does not match your experience, we would genuinely like to hear about it — two of our published coefficients have changed that way.