ECT is short for Edge Crush Test. It is a number attached to corrugated board that describes how much compressive force a short strip of that board will take when it is stood on edge, expressed in pounds per linear inch. Because a stacked box carries its load through its vertical walls, that edgewise figure is the one that predicts whether the bottom box in a pallet stack holds up.
ECT and Mullen describe different failures
For decades corrugated was specified by bursting strength, usually called Mullen, measured in pounds per square inch. Mullen tells you how hard it is to push something through the board. ECT tells you how much weight the board carries on edge. They answer different questions, and a box can be strong by one measure and weak by the other.
The rough rule is that ECT matters when boxes stack and Mullen matters when boxes get handled roughly, punctured or dropped. Most buyers today specify ECT, because stacking in a warehouse or on a pallet is the load that actually breaks boxes.
Grades you will see quoted
| Construction | Grades commonly quoted | Where they usually appear |
|---|---|---|
| Single wall | 32 ECT | General purpose shipping for light to moderate weights |
| Single wall | 44 ECT | Heavier contents or more stacking than a 32 grade handles |
| Double wall | 48 ECT and 51 ECT | Heavy goods, tall stacks, long or multi-leg freight lanes |
These are grades the industry quotes, listed so you can recognise them on a supplier document. They are not iBox stock specifications, and the right grade for your job depends on your weight, stack height and storage conditions.
The things that quietly reduce it
A board grade is measured on fresh, flat, dry material. Three things in the real world work against it. Moisture is the biggest: corrugated absorbs humidity and loses stiffness, which is why cold rooms and humid warehouses are hard on stacked boxes. Time is the second: a stack that holds on day one can creep and fail after weeks under constant load. Handling is the third, since a crushed corner or a bulged wall no longer carries load down the edge where the strength lives. Design in headroom rather than specifying to the exact calculated figure.
Reading it on a box
Many corrugated shippers carry a round printed stamp on one flap that records the board description. If your customer or your carrier expects that stamp, ask for it during the quote conversation so it is designed in rather than added later. We print what is agreed on the quote and we do not make claims beyond that.
How to ask for the right board
Rather than naming a grade, describe the job: packed weight, how many boxes stack on each other, how long they sit, whether storage is cold or humid, and how many freight legs the box takes. A supplier can turn that into a board grade. If you want the sizing method that goes with it, how to size a single wall box covers the dimensional side, and full overlap box dimensions covers the style used for heavier contents.
Minimum order is 100 boxes, from about $0.48 per unit at bulk volumes, confirmed on your written quote, with die, plates, design support, US shipping and a digital proof included. Lanes run Rush, Standard and Cost saver, from about 7 business days, plus 2 to 5 for delivery. Send your weights and stack plan to /quote/ and we will recommend a board rather than leaving you to guess a grade.
