One of the most common packaging failures I'm called in to investigate is boxes that were fine in testing, purchased based on supplier specifications, and then collapsed under load in storage or transit. It's usually not the supplier's fault and it's usually not a defective batch. It's almost always a specification problem.
The Box Compression Test (BCT)
Box compression test — BCT — measures how much force a box can withstand before it fails when loaded vertically. It's the standard measure of stacking strength for corrugated packaging.
Your supplier will quote BCT figures. They're tested in laboratory conditions: 50% relative humidity, 23°C, fresh manufactured board. These are ideal conditions. Most warehouses and distribution centres do not resemble ideal laboratory conditions.
Why Real-World Performance Falls Short
Humidity is the biggest variable. Corrugated board absorbs moisture from the atmosphere. At 85% relative humidity (a damp UK warehouse in autumn), a corrugated box can lose 50% or more of its BCT compared to its laboratory rating. This is not an edge case — it's normal.
Time matters. A box on a pallet at the bottom of a stack isn't just experiencing a one-off load — it's experiencing a sustained load over days or weeks. Creep compression (gradual deformation under sustained load) occurs at much lower forces than the BCT figure, which is measured instantaneously.
Board moisture at point of use matters. Board that has been stored in a cold warehouse and then brought into a warm environment will absorb moisture as condensation forms. If you're stacking heavy pallets immediately after goods arrive into a warm facility, you're loading boxes at their worst.
The McKee formula adjusts BCT for real-world conditions using a combination of ECT (edge crush test), board caliper and box perimeter. A simplified rule of thumb: your boxes need a BCT rating at least four times the maximum load they'll experience in the worst-case stack — not two or three times as many buyers assume.
Calculating What You Actually Need
Work out:
- The maximum weight per pallet (total goods weight plus pallet weight)
- The number of pallets that will be stacked on each other at maximum — in your warehouse or at your customers' warehouses
- The expected storage humidity
The load on the bottom box in a three-high pallet stack is roughly the weight of the two pallets above it divided by the number of boxes on the bottom layer. That gives you the per-box load. Apply a safety factor of four or more for humidity and time, and that's your minimum BCT requirement.
Most businesses have never done this calculation. They bought what the previous person bought, or they bought the cheapest corrugated that looked similar to what they needed.
What to Change
If your boxes are failing in storage, your first step is to measure the actual humidity conditions they're stored in. If it's above 65% regularly, you need to revise your BCT specification significantly.
Options include: heavier board grade, double wall, treating the box surface with a moisture-resistant coating, or redesigning the box to improve structural performance (an RSC with more overlapping base material, for example).
For very high humidity environments — chilled storage, damp warehouses — there are corrugated board grades with moisture-resistant liners that significantly reduce the humidity sensitivity of BCT. They cost more but can be cost-effective compared to the write-offs from collapsed pallets.