Key takeaways
- The round stamp states burst or edge crush rating, size limit and gross weight limit — as manufactured, not as it is now.
- ECT predicts stacking strength; Mullen predicts puncture and rough-handling resistance. They are not interchangeable.
- On used boxes the stamp is frequently illegible, overprinted or missing, so we rate from construction, caliper and condition.
- A five-year-old box with a legible 1,300 psi stamp and a crushed bottom edge is a 4A box.
Turn a bulk box over and there is usually a circle printed on a bottom flap. It is the box certificate — sometimes called the box maker’s certificate — and it is one of the few pieces of genuinely objective information printed on a container.
What it tells you
Four things, in most formats:
- Whether the box is rated by bursting test (Mullen, in psi) or by edge crush test (ECT, in lb/in). These are different measurements and not interchangeable.
- The value of that rating.
- The maximum size in combined inside dimensions — length plus width plus depth.
- The gross weight limit the construction was certified for.
Some also carry the construction — single, double, triple wall — and the manufacturer’s identification.
Burst versus edge crush, briefly
Mullen burst measures the pressure needed to rupture the board. It predicts puncture and rough-handling resistance. Edge crush measures the force a one-inch strip withstands on edge before buckling, and it predicts stacking strength.
For bulk containers that stack, ECT is the more useful number. For containers that get dragged, dropped and speared by forks, Mullen matters more. A box certified to 1,300 psi Mullen and one certified to 112 lb/in ECT are not directly comparable, and anybody converting between them is estimating.
| Mullen | Approx ECT | Typical construction | Typical bulk use |
|---|---|---|---|
| 200 psi | 32 lb/in | Single wall | Shipping cartons |
| 275 psi | 44 lb/in | Single / light double | Heavy cartons |
| 500 psi | 51 lb/in | Double wall | Light bulk, small bins |
| 700 psi | 67 lb/in | Triple wall | Octabins, shallow trays |
| 1,100 psi | 90 lb/in | 4-wall | Standard used gaylords |
| 1,300 psi | 112 lb/in | 5-wall | Heavy industrial bulk |
Why we treat it as evidence, not authority
Three reasons.
First, on used boxes it is frequently scuffed, overprinted, covered by a label or missing entirely. Second, it describes the box as manufactured, which may have been five years and four trips ago. Third, and most importantly, it says nothing at all about the condition the box is in now — and condition is the entire variable in used packaging.
A five-year-old box with a legible 1,300 psi stamp and a crushed bottom edge is a 4A box. The stamp is accurate and irrelevant.
A supplier quoting purely from a stamp is quoting a number that nobody has verified since the day it was printed.
How we rate instead
- Record the stamp where legible, as a starting point.
- Measure board thickness and identify the construction physically.
- Assess condition: moisture, edge integrity, delamination, squareness.
- Apply a derating margin below the original figure, always.
- Publish the resulting payload as our own number, which we stand behind.
What to do with this as a buyer
Ask for a photograph of the certificate where one is legible — it costs a supplier nothing. Then ask what payload they are prepared to state for the box in its current condition. The gap between those two answers tells you how seriously a supplier takes grading.
Questions