What box compression strength tells you
Box compression test (BCT) value is the top-to-bottom load a case can bear before it crushes. It matters because the box at the bottom of a stack — on a pallet, in a trailer, in a warehouse rack — carries everything above it. The McKee formula estimates BCT from two things you can specify: the board's edge crush (ECT) and the box's size. Bigger boxes of the same board are weaker, because a longer perimeter buckles more easily.
Why you divide by a safety factor
The McKee number is a fresh, lab-condition estimate. In the real world, corrugated loses strength the moment it leaves the plant: it absorbs moisture, it creeps under sustained load over days and weeks, and pallet overhang or misaligned stacking concentrates force on corners. That's why a safe working load divides the estimate by a factor — commonly around 3 in ideal, short-term conditions and 5 to 7 or more for humid environments or long storage.
Once you know a box holds its stack, see how many cases fit a layer and how high you can go with the pallet calculator.
Questions
What is the McKee formula?
The McKee formula estimates a box's compression strength from the board's edge crush (ECT), the board caliper, and the box perimeter. The common short form is BCT ≈ 5.87 × ECT × the square root of caliper times perimeter. It is an estimate, not a replacement for testing.
What is the difference between ECT and BCT?
ECT (edge crush test) measures the strength of the board itself, in pounds per inch. BCT (box compression test) measures the strength of the finished box, in pounds of top load. McKee estimates BCT from ECT and the box dimensions.
Why divide box compression by a safety factor?
Because the McKee value is a fresh, lab-condition estimate and real boxes lose strength to humidity, storage time and creep, pallet overhang, and off-center loads. Dividing by a safety factor — often 3 to 7 — gives a load the box can safely carry in the field.
Does humidity affect box strength?
Significantly. Corrugated absorbs moisture, and high humidity can reduce compression strength by roughly half. Use a higher safety factor for humid environments or long storage, and validate critical loads by test.
Spec board that survives the stack.
PackOS estimates box compression and safe stack load as you spec the pack — so the bottom case isn't the thing that fails in the field.