ECT vs Mullen: choosing corrugated box strength
Every corrugated box usually carries a small printed stamp on a bottom flap called the box maker's certificate. It tells you how strong the board is. There are two ways that strength is rated: Edge Crush Test (ECT) and Mullen, also called burst strength. Knowing which one matters helps you avoid both crushed boxes and paying for board you don't need.
What ECT measures
The Edge Crush Test measures how much force the board can take on its edge before it crushes. That's the force a box feels when others are stacked on top of it. ECT is given in pounds per inch, so a 32 ECT board withstands 32 pounds of force per inch of edge.
ECT is the most common rating for modern shipping boxes, because stacking strength is what matters most in warehouses and parcel networks.
What Mullen measures
The Mullen or burst test measures how much pressure it takes to punch through the side of the board. That's the kind of stress a box faces from rough handling, sharp corners and heavy contents pushing outwards. Mullen is given in pounds per square inch, for example 200#.
Mullen-rated board is often heavier and used where puncture resistance matters more than stacking, such as heavy, dense products or boxes handled roughly.
Typical ratings and weight guidelines
These are commonly cited guidelines for single-box weight. Always check your supplier's certificate and your carrier's packaging rules.
| Board | ECT | Mullen equivalent | Guideline max contents |
|---|---|---|---|
| Single wall | 32 ECT | 200# | about 40 to 65 lb |
| Single wall, heavy | 44 ECT | 275# | about 65 to 95 lb |
| Double wall | 48 ECT | 275# | about 80 to 95 lb |
| Double wall, heavy | 61 ECT | 350# | about 100 to 120 lb |
ECT and Mullen are different tests, so the "equivalent" column is an approximate pairing used by suppliers, not a conversion.
How to choose
Start with product weight
For most e-commerce shipments under about 40 pounds, 32 ECT single wall is the standard choice and what most stock boxes use.
Think about stacking
Boxes stored in warehouses, stacked on pallets or shipped in mixed freight need stacking strength. Choose ECT and step up a grade if boxes will sit under heavy loads for long periods, or in humid conditions, which weaken corrugated board.
Think about puncture and rough handling
Dense, heavy or sharp-cornered products push outwards on the walls. A Mullen-rated or double wall board resists punctures better.
Consider fragility
Stronger board protects the box, not necessarily the product. Fragile items still need enough cushioning space. A double wall box with no cushioning can still deliver a broken product.
Board strength and box size
Stronger board is usually thicker. Moving from single wall to double wall can add about ⅛ inch per wall, which increases the outside dimensions and can raise dimensional weight. Pick the board type in the box size calculator to see the effect on outside size.