There are currently two tests in the industry that determine cardboard strength, represented by a round or sometimes square stamp on a flap of the box that shows the manufacturer follows the rules and regulations of the United States Commerce Commission. For a long time the accurate test for cardboard, or corrugated fiberboard, was the Mullens Burst Test. The Bursting Test is a measurement of the force required to puncture or rupture the face of the cardboard. Once a result is found the board gets a rating. A very common rating is 200#. The other test is relatively newer and in some cases is the preferable test.
The ECT or Edge Crush Test is a laboratory test that compresses a small piece of board edgewise between two plates until a peak force is reached. This is measured in pounds per inch (lb/in), but usually shown as an ECT value, for example 32 ECT.
A Mullens 200# and ECT 32 are widely viewed as being interchangeable. However they both have their advantages. Every fiberboard has a specific “Recipe” that effects the strength, weight, thickness, and other special properties. The Mullens test however requires that the stock meet certain weight requirements to achieve a higher rating. This gives you a more rugged box but at the cost of weighing more. The ECT has no such weight requirements but is held strictly to the crushing strength of the cardboard. This means that a manufacturer with a great “Recipe” and design can produce an ECT box as strong or stronger as a similar Mullens box, but also cutting the overall weight.