Look inside almost any corrugated box and you will find a round stamp — the Box Maker's Certificate. It carries either a Mullen (bursting) rating or an ECT (Edge Crush Test) rating. They are not interchangeable, and choosing the wrong one costs real money.
Mullen measures puncture
The Mullen test pushes a rubber diaphragm through the board until it bursts, reporting pounds per square inch. It tells you how well a box resists being poked, dropped on a corner, or dragged across a dock plate. It was designed for an era of rough single-trip handling.
ECT measures stacking
The Edge Crush Test stands a short strip of board on its end and crushes it, reporting pounds per linear inch. It predicts how high you can stack a pallet before the bottom box fails. In a modern palletised supply chain, this is almost always the number that matters.
Rough equivalences
- 200 lb Mullen ≈ 32 ECT — the standard single-wall shipper.
- 275 lb Mullen ≈ 44 ECT — heavier single wall.
- 275 lb double wall ≈ 48 ECT.
- 350 lb double wall ≈ 51 ECT.
- 1,100 lb triple wall ≈ 67 – 82 ECT — gaylord territory.
Treat these as conversational, not contractual. The equivalence tables exist because carriers historically specified Mullen; they are rules of thumb, not physics.
Why it matters for used boxes
A used box no longer meets its printed certificate. Flutes have taken a set, the liner has absorbed and released moisture, and the score lines are softened. We grade on observed condition rather than quoting the original stamp, because the stamp is a historical document at that point.
The practical rule
If your product is dense and your pallets stack, ask for ECT and grade A or B used board. If your product is light and your handling is rough, Mullen and a double-wall build will serve you better. If a supplier will not tell you which test their certificate carries, that is itself useful information.
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.