Box Compression Strength (McKee)
Estimates the vertical compression strength of a corrugated box with the simplified McKee formula, BCT = 5.87 × ECT × √(board caliper × box perimeter), where ECT — the edge crush resistance measured per TAPPI T 811 (ISO 3037) — is in newtons per millimetre and both dimensions are in millimetres. The result, in newtons, is the load an empty box withstands in the laboratory compression test, with the board conditioned at 23 °C and 50% relative humidity. Because strength grows with the square root of the perimeter and linearly with ECT, doubling the perimeter buys only 41% more, while switching to a flute with 30% higher ECT buys the full 30% — upgrading the board usually beats reshaping the box. For real pallet stacking, divide the BCT by a safety factor of 3 to 5, which covers the stiffness lost to ambient humidity, the creep of board under load over weeks and the misalignment between boxes in the column. Enter the ECT, the board caliper and the box perimeter.
Result
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How much load a corrugated box takes in a stack
The box that crushes is always the bottom one, and the damage surfaces three weeks later at the distribution centre, pallet already built and invoice already issued. Packaging engineers, board buyers and whoever signs off the pallet pattern want a BCT figure before the flute specification closes, because with a number on the table the argument over six layers versus four turns into arithmetic. Without it the choice becomes habit — C flute, as always — and the cost reappears as damage returns, usually well above the price gap between boards.
Simplified McKee: BCT = 5.87 × ECT × √(h × P). ECT is edge crush resistance, tested to ASTM D5013, in newtons per millimetre — B flute lands between 3 and 5, C flute between 5 and 7, and a BC double reaches 14. The h term is board caliper: near 3 mm for B, 4 mm for C, 7 mm for BC. P is the base perimeter, 2 × (length + width). Newtons and millimetres from start to finish, with no exception: ECT quoted in kgf/cm converts at 0.98, and in lb/in at 0.175.
The test runs on an empty, brand-new, squarely glued box conditioned at 23 °C and 50% relative humidity — three conditions the warehouse contradicts. At 85% humidity board sheds close to half its stiffness; under steady load, creep eats away strength over weeks; and a hand hole, a window or a tear strip removes material the formula knows nothing about. Hence the safety factor of 3 to 5 applied to BCT. The equation further assumes a regular slotted container of sane proportions: tall, narrow packs come out weaker than predicted.
Frequently asked questions
How many boxes can I stack from the default result?
Is it better to raise ECT or to thicken the board?
Does the perimeter mean the base or the whole box?
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