Look at the bottom flap of almost any shipping carton and there is a printed circle: the box maker's certificate. It carries a strength rating, and that rating is given one of two ways. Which one it uses tells you what the box was engineered to survive.
What each test actually does
The Mullen burst test clamps a sample of board and pushes a rubber diaphragm through it until it ruptures. The result is the pressure needed, in pounds per square inch. It is a measure of how hard you can hit or puncture the board before it fails.
The edge crush test stands a short section of board on its edge and presses down until it collapses. The result is pounds of force per inch of edge. It is a measure of how much weight the box carries from above, which is the load that matters when boxes are stacked on each other or on a pallet.
| Mullen (burst) | ECT (edge crush) | |
|---|---|---|
| Measures | Puncture and rupture resistance | Top-to-bottom stacking load |
| Units | Pounds per square inch | Pounds per inch of edge |
| Predicts | Survival of rough handling | Survival of being stacked |
| Favours | Heavier, stronger liners | Efficient board construction |
Why the industry moved toward ECT
Mullen came first and dominated for decades. The problem is that it measures the wrong failure for most modern supply chains. Boxes are far more likely to be crushed under a stack in a warehouse than punctured by something sharp, and a high burst rating does not guarantee a box will hold its shape under load.
ECT measures that failure directly. It also rewards good board construction rather than simply heavy paper, so an ECT-rated box frequently uses less fibre to achieve equivalent stacking performance. Less fibre means a lighter box, which means lower shipping weight.
The common equivalences are 32 ECT to 200# Mullen and 44 ECT to 275# Mullen, both single wall. Treat these as rules of thumb for specifying a replacement, not as statements that the two boards behave identically. They fail in different ways.
Which to specify
- Palletised, warehoused or stacked in transit: specify ECT.
- Heavy or sharp contents, or rough single-parcel handling: Mullen still has a case.
- Standard ecommerce parcels: 32 ECT single wall covers the large majority.
- Heavy or tall stacks: step up to 44 ECT or double wall rather than guessing.
One caveat worth knowing: both ratings describe the board as it was tested, flat and dry, in a laboratory. Humidity degrades corrugated significantly, and a box that has sat in a damp warehouse carries materially less than its printed rating. Die-cut handles, ventilation holes and large print windows all remove material from load-bearing faces too. If your stack is near the limit on paper, it is already over it in practice.
Specifying it on your order
Tell us the weight of a filled box, how high they will be stacked and whether they are palletised, and we will specify the rating rather than defaulting to whatever is cheapest. Over-specifying board is as wasteful as under-specifying it, in cost and in shipping weight.