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The ECT number stamped on a shipping box is its stacking strength rating: the higher the number, the more weight the box can bear when other boxes pile on top of it. Single-wall boxes carry the lightest ratings; double-wall and triple-wall go higher. Match the rating to what you are shipping so your parcel is not the one that collapses at the bottom of a truck.
| What the stamp says | What it means | Typical use |
|---|---|---|
| ECT 32 | Standard single-wall box | Light household goods, clothes, documents |
| ECT 44 or 48 | Stronger single-wall | Heavier books, small appliances |
| ECT 55+ | Double-wall territory | Heavy items, stacking in storage, fragile goods |
| Burst strength (lb) | Older puncture-resistance rating, not stacking | Seen on older box specs; ECT replaced it for shipping |
The test crushes a sample of the corrugated board on its edge and records the force it takes to fail. That tells manufacturers how the box handles the dominant load in shipping: top-to-bottom compression from the stack above it. Carriers care about stacking because trailers and sorting facilities pile parcels high; a box that is strong against side punches but weak under vertical load still ends up crushed.
The ECT number is not a weight limit by itself. Box makers publish the rating alongside a recommended maximum load, and the two numbers travel together. The practical habit: look at the box maker's stamp on the bottom flap (it lists the ECT rating and max load), and pick a box whose max load exceeds your packed weight with room to spare.
Three situations call for double-wall. Heavy items are the obvious one: dense goods like books, canned food, or tool parts put enormous point loads on the bottom of the box, and single-wall buckles under concentrated weight. Stacking is the second: if your box will sit at the bottom of a storage stack or a pallet, the ECT rating has to account for everything above it, not just the parcel itself. Fragile items are the third: a stronger outer wall is part of protecting what is inside, alongside cushioning.
For light, compact shipments, single-wall is fine and cheaper. Canada Post Xpresspost and other carrier services do not require a specific ECT rating; the requirement is yours to get right, because the carrier's liability for damage drops if the box was clearly too weak for the contents.
Check the bottom-flap stamp before trusting a used box. Reused boxes have already survived one trip, which means they have already absorbed one cycle of compression and handling. A second use is usually fine for light items; for heavy or fragile shipments, a fresh box with a known rating is the safer call. If the stamp is torn off or illegible, treat the box as unknown and pack light.
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