The McKee formula is a widely known engineering estimate, first published in the 1960s, for the top-to-bottom compression strength of a regular slotted corrugated box. In its common simplified form it relates a box's estimated compression strength to the edge crush of the board (ECT), the board thickness (caliper), and the perimeter of the box. It is one of the reasons edge crush test values matter in corrugated design: they feed directly into this estimate. The McKee formula is an estimate, not a guarantee — it was developed for a particular common box style and set of conditions, and the industry understands it as an engineering approximation. It remains one of the most recognized reference relationships in corrugated packaging engineering.