Key takeaways
- The round stamp states a burst or edge crush rating, a size limit and a gross weight limit — as manufactured, not as the box 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 condition and construction matter more.
- A years-old box with a legible stamp and a crushed bottom edge is a heavily downgraded box. The stamp is accurate and irrelevant.
Turn a bulk box over and there is usually a circle printed on a bottom flap. It is the box maker’s stamp, 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 rated 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 rated by burst and one rated by edge crush are not directly comparable, and anybody converting between them is estimating.
| Rating | What it measures | What it predicts | Matters most when |
|---|---|---|---|
| Mullen (burst), psi | Pressure to rupture the board | Puncture and rough-handling resistance | Boxes are dragged, dropped or struck by forks |
| ECT, lb/in | Edge load before a strip buckles | Stacking strength | Boxes are stacked, in storage or in transit |
| Wall count (single to five-wall) | Number of board layers | Overall stiffness and payload class | Choosing a box for a heavier load |
Why the stamp is 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 years and several 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 years-old box with a legible stamp and a crushed bottom edge is a heavily downgraded box. The stamp is accurate and irrelevant.
How a supplier can rate a used box instead
- Record the stamp where legible, as a starting point.
- Identify the construction physically and measure the board thickness.
- Assess condition: moisture, edge integrity, delamination, squareness.
- Apply a margin below the original figure to allow for wear.
- State the resulting payload as the supplier’s own figure, which it should be willing to stand behind.
What to do with this as a buyer
Ask for a photograph of the stamp where one is legible. Then ask what payload the supplier is prepared to state for the box in its current condition. The gap between those two answers tells you how seriously a supplier takes grading. When in doubt, confirm load ratings directly with the supplier before you rely on them.
Questions