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The small circular stamp on the bottom flap of a corrugated box tells you more than most buyers realize. Here is how to read it.
Most corrugated shipping boxes carry a printed certificate, usually on a bottom flap. It identifies the manufacturer and states either an edge crush test value or a bursting test value, along with size and weight limits that apply to the rating. When a listing and the certificate disagree, trust the certificate on the box you actually receive and ask the seller about the difference.
Edge crush test, written as ECT, measures how much force the corrugated board withstands when pushed on its edge, expressed in pounds per inch of edge. That edgewise strength is what keeps a box from collapsing when other boxes are stacked on top of it, which is why ECT is closely tied to stacking performance.
Common ratings include 32 ECT for everyday single wall cartons, 44 ECT for heavier single wall board and 48 ECT or higher for double wall construction. A higher number means stiffer walls under a stacked load, but it also usually means more weight and cost per box.
The bursting test, often shown as Mullen or as a pound value such as 200#, measures how much pressure the board withstands before it ruptures. It reflects resistance to punctures and rough handling more than stacking. Burst-rated boxes are common where cartons are thrown, dropped or loaded with dense items that press outward against the walls.
Choose based on how the box will be treated, not just how much it holds.
Palletized or stacked storage: prioritize ECT, because stacking is the main stress.
Parcel networks with many touches: consider burst strength or a heavier ECT, because the box will be dropped and conveyed.
Heavy or dense contents: check the maximum gross weight printed on the certificate and stay under it.
Long storage in humid areas: allow extra margin, because corrugated loses strength as it absorbs moisture.
Single wall board has one fluted layer between two liners. It covers most shipping. Double wall has two fluted layers and three liners, which adds stacking strength and puncture resistance for heavier items or longer journeys. Triple wall is used for very heavy or bulk shipments. Each step up adds thickness, so check outside dimensions if dimensional weight matters.
The most frequent mistake is buying the strongest box for every order. That adds weight and cost without helping light shipments. The second is reusing boxes whose corners are already crushed or whose flaps have been taped and cut several times, which removes much of the original strength. A third is overfilling or underfilling, both of which weaken a stack regardless of rating.
Finally, a strong box closed with weak tape still fails. Pair the board with a tape suited to its weight and surface.
How boxes are arranged on a pallet changes how much of their rated strength they can use. Column stacking, with each box directly above the one below, keeps the corners aligned and carries load down the strongest part of the carton. Interlocked or brick patterns make a more stable load but can reduce compression strength because corners rest on the softer middle of panels below.
Overhang is another hidden weakness. A box that hangs over the edge of a pallet loses support along that edge and can buckle under load. Keep cartons within the pallet footprint, use corrugated pads between layers where needed, add edge protectors under strapping, and finish with stretch wrap to hold the load together without crushing the top tier.
Corrugated board is paper, and paper absorbs moisture. Boxes stored for weeks in a humid warehouse or shipped through wet conditions can lose a large share of their dry strength. Time under load also matters: a stack that holds for a day can slowly settle and lean over weeks. If your cartons sit in storage, choose a rating with comfortable headroom instead of one sized only for the trip.
Check the certificate on boxes you already use and note the rating and weight limit.
Compare listings with the same inside dimensions and different board grades.
Use edge protectors and stretch wrap when stacking boxes on pallets.
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