Two Numbers, Two Philosophies
Open any box certificate stamp and you will usually see one of two ratings. A Mullen rating reads as something like 200# or 275# test, describing burst resistance in pounds per square inch. An ECT rating reads as 32 ECT or 44 ECT, describing edge crush strength in pounds per linear inch. They are not interchangeable numbers, and comparing them directly is a common purchasing mistake.
The confusion is understandable because both once appeared on the same manufacturer certificate. For decades the industry leaned on Mullen because burst testing was simple and familiar. Over the past several decades ECT gained ground as shippers realized that most boxes fail from being stacked, not from being punched. Knowing which failure mode threatens your product tells you which number to trust.
How Mullen Testing Works
The Mullen test clamps a sample of board over a rubber diaphragm and inflates it until the paper bursts. The pressure at rupture, measured in pounds per square inch, becomes the burst strength. A 200# box wall resists roughly 200 psi before it tears. This models puncture and rough handling, a container getting jabbed by a forklift tine or a sharp corner during a fall.
Because Mullen rewards heavy, high-strength fiber, boxes built to a burst spec tend to use more paper by weight. That made sense when handling was rough and stacking was low. The downside is cost and material use. A box can pass a demanding burst test yet still crush under its own stacked weight, because bursting and crushing stress the board in completely different directions.
How Edge Crush Testing Works
The Edge Crush Test stands a small section of corrugated board on its edge and presses down until the flutes buckle. The result, in pounds per linear inch of edge, reflects how much vertical load the walls can carry. This is precisely the force at play when boxes are stacked on a pallet three or four cartons high in a warehouse or shipping trailer.
ECT correlates closely with box compression strength, which is what actually keeps a stack standing. That relationship lets engineers predict how a filled carton will behave under warehouse conditions. Because ECT rewards efficient flute geometry rather than raw fiber weight, a well-designed ECT box can match the stacking performance of a heavier Mullen box while using noticeably less paper.
Rough Equivalents and Their Limits
Conversion charts float around the industry pairing, for example, 200# Mullen with 32 ECT, or 275# with 44 ECT. These are guidelines rooted in common single-wall constructions, not exact translations. A supplier can hit a target ECT with different liner and medium combinations, so two 32 ECT boxes may not have identical burst behavior.
Treat any equivalence table as a starting point for conversation, never as a contract term. If your specification historically called for a burst rating, ask your supplier what ECT value they recommend for the same product weight and stack height. Requiring both ratings on one order usually forces heavier board and higher cost with no real performance gain.
Choosing the Right Spec for Your Product
Start by asking how your box actually fails. If cartons sit stacked in a warehouse, ride pallets, or wait in humid storage, stacking strength dominates and ECT is the smarter target. If your goods face sharp objects, drops onto corners, or aggressive manual handling, burst resistance still matters and a Mullen spec may earn its keep.
Weight and density of contents matter too. Dense products like tools, bottled liquids, or hardware push down hard on the box below them, favoring a higher ECT. Light bulky goods rarely stress the walls at all. Humidity is the quiet variable: corrugated loses a meaningful share of its compression strength as it absorbs moisture, so specify with a safety margin for damp Chicagoland warehouses.
Buying and Reusing With Confidence
When you evaluate used boxes for reuse, look for the box maker certificate printed on a bottom flap. It records the rating type, the value, and the maximum size and gross weight the box was built for. A used carton that still shows crisp, uncrushed flutes usually retains most of its original edge strength, making it a sound reuse candidate for lighter loads.
Sourcing reclaimed boxes with known ratings is one of the easiest ways to cut packaging cost without gambling on performance. Suppliers like Boxes Chicago sort and grade used corrugated so you can match a rating to your job rather than guessing. When in doubt, run a short stacking trial with your real product before committing a full shipping program.
Key takeaways
- Mullen measures burst resistance in psi; ECT measures edge crush in pounds per linear inch.
- ECT predicts stacking strength, the way most boxes actually fail in warehouses.
- Mullen rewards heavy fiber and suits puncture-prone rough handling.
- Conversion charts are guidelines, not exact translations; requiring both ratings raises cost.
- Corrugated loses compression strength in humidity, so specify with a safety margin.
- Check the box maker certificate on a bottom flap to confirm a used box's rating.
Marcus Webb
Operations Lead — Runs the recovery floor at Boxes Chicago and has graded more Gaylords than he can count.