Stack Load on the Bottom Case

The bottom case in a stack carries everything above it and nothing of itself, so a stack $n$ cases high puts $n-1$ case masses on the bottom one. The minus one is the entire content of the equation and it is the term most often lost.

How much it matters depends on how tall the stack is. On a ten-high stack, counting all ten instead of nine overstates the load by 11% — annoying but survivable. On a two-high stack it overstates it by 100%, and short stacks of heavy cases are exactly where the error tends to be made, because the arithmetic feels too simple to check.

Two things this equation deliberately leaves out, and both can bite. The first is pallets. This gives the load on the bottom case of a stack of CASES. If loaded pallets are stacked on one another in a rack, the bottom case is carrying every case above it in its own pallet load AND every pallet and case above that, and the count multiplies rather than adds. A three-pallet-high rack of six-high stacks does not put five case masses on the bottom box; it puts seventeen, plus two pallet decks.

The second is load sharing through the product. This equation assumes the load reaches the board, which is what a column-stacked pattern on a flat deck does. Where the primary package is rigid and load-bearing — canned goods, glass bottles standing on their bases, a moulded pulp tray under compression — a share of the stack load travels down through the product instead of through the box walls, and the box is carrying materially less than this says. That is a real and useful effect. It is also the reason a change of primary package can quietly destroy a shipping case that has worked for a decade: switch from glass to a thin-walled PET bottle and the load that was going through the product is suddenly going through the corrugated.

A convenience of the imperial system worth knowing. A pound-force is defined as the weight of a pound mass under standard gravity, so if you work in pounds and pounds-force the two conversions cancel and the stack load is simply $n-1$ times the case weight, with no $g$ to carry. In metric there is no such cancellation — a 15 kg case exerts 147.1 N, not 15 of anything — and a stack load written in kilograms is a mass pretending to be a force. This site holds $g$ at exactly 9.80665 m/s² and does the conversion for you.

Use the GROSS mass throughout: case, product, inner packaging, dunnage and tape. And treat the mass as one that has to hold for EVERY case, not for the average case. A single overfilled unit at the bottom of a rack is a genuine failure mode, and averaging is no protection against it.