Box compression strength and the McKee formula
A box that crushes in the warehouse fails before it ever reaches a customer. Box compression strength tells you how much weight a box can carry on top, and the McKee formula lets you estimate it without a lab.
What box compression strength means
Box compression strength, or BCT (box compression test), is the load a box can take from above before it buckles. It's measured by squeezing an empty box between two plates until it fails. In a stack or on a pallet, the bottom box carries everything above it, so its BCT sets how high you can stack.
The McKee formula
In the 1960s, researchers at the Institute of Paper Chemistry found that BCT could be predicted from three things: the board's edge crush strength, its thickness and the box's perimeter. The simplified McKee formula is:
ECT is in pounds per inch, caliper (board thickness) and perimeter in inches, and the result is in pounds of force.
A worked example
A 16 × 12 inch box in 32 ECT C-flute board (about 0.16 inch thick):
- Perimeter: 2 × (16 + 12) = 56 inches.
- Caliper × perimeter: 0.16 × 56 = 8.96. The square root is about 2.99.
- BCT: 5.87 × 32 × 2.99 ≈ 562 lbf.
What the formula tells you
- Board grade matters most. ECT scales the result directly. Moving from 32 to 44 ECT raises strength by more than a third.
- Bigger boxes are stronger. A larger perimeter means more wall to carry the load, though the gain is under a square root.
- Height hardly matters. Within normal proportions, a tall box and a short box of the same footprint have similar BCT.
From lab strength to real life
BCT describes a new, dry box under a quick test. In storage, strength drops with humidity, time under load, pallet gaps and overhang, handholes and vents, and rough handling. That's why warehouses divide BCT by a safety factor, typically 3 to 5. At a safety factor of 4, the example box can safely support about 140 lb on top over time.
Limits of the formula
McKee works well for regular slotted containers with normal proportions, usually within about 10% of test results. It's less reliable for unusual shapes, very small boxes, die-cut styles or boxes with large cutouts. For critical packaging, test real boxes.