Corrugated material is made up of two outer liners and a medium material in the center. This medium gives corrugated board its strength. Flutes come in various sizes-A, B, C, E and F. There are four standard thicknesses of corrugated: Single face, Single wall, Double wall and Triple wall.

Most boxes used for shipping today are either Single wall In C flute or Double wall in a combination of B and C flute.

This stamp indicates the strength of the material used to construct your box, as well as its size and weight limits.

Within the seal, or freight stamp, you may also see an Edge Crush Test (ECT) or Burst Test. Both are procedures evaluating the strength of a cardboard box in pounds per square inch.

ECT measures the stacking strength of corrugated boxes. Determining the compression strength is defined by the amount of force that is needed to crush the material by standing it on its edge. Think about the transport of your cartons-they may be palletized or loaded in delivery trucks, so stacking strength becomes critical.

The Burst Test or Mullen Test measures the force of pounds per square inch required to rupture the sidewalls of your box. This test indicates the materials’ ability to withstand external or internal forces during rough handling.

Single Wall  
Mullen Combination
125 26/23/26
150 33/23/33
171 35/23/35
175 35/23/35
200 42/23/42
243 56/23/56
250 56/23/56
252 69/23/42
275 69/23/69
350 90/23/90
Flute Caliper  
ECT Combination
32 ECT 35/23/35
40 ECT 56/23/56
44 ECT 56/23/56

Double Wall  
Mullen Combination
200 35/23/26/23/35
275 42/23/42/23/42
350 56/23/42/23/42
400 69/23/42/23/69
450 69/23/69/23/69
500 69/23/69/23/69
600 90/26/90/26/90
Flute Caliper  
42 ECT 33/23/33/23/33
48 ECT 35/23/35/23/35
51 ECT 35/23/35/23/35
61 ECT 42/23/33/23/33
71 ECT 56/23/35/23/35


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