Stacking strength and safety factors
A box's tested or estimated compression strength is what it holds new, dry and for a short time. Real stacks sit for weeks, absorb humidity and rarely line up corner to corner, so the box needs a margin over the load: the safety factor.
From load to required strength
Start with the load on the bottom box: every box above it times its gross weight. Then divide by what the box keeps of its strength after each loss. If storage costs 40% and humidity another 40%, the box keeps 0.6 × 0.6 = 36% of its strength, so it needs 1 ÷ 0.36 = 2.8 times the load.
Losses multiply rather than add, which is why a few modest-looking factors can call for a much stronger box.
A worked stack
A 16 × 12 in box, 25 lb gross, stacked 6 high on a pallet for 30 days. Packgent's default loss assumptions are shown; they are not sourced values and every tool lets you change them.
| Load on bottom box | 5 × 25 = 125 lbf |
|---|---|
| 30 days in storage | −40% |
| Safety factor | 1.67 |
| Required BCT | 208 lbf |
| ECT needed in C flute (McKee) | 12 lb/in |
| 32 ECT C flute estimate | 556 lbf |
What changes the answer most
Time and humidity hit every box in the stack. Pattern and overhang hit the bottom tier hardest: interlocking puts corners over panel centres, and overhang leaves the strongest part of the wall hanging in air. Column stacking on a pallet the size of the load is the cheapest strength there is.
Use it
- Stacking strength
- Mckee bct
- Safety factor
- Astm d642
- Grades
- Right-sizing boxes to cut dimensional weight
- Reading a box maker's certificate
- Unit loads: boxes on a pallet
Last reviewed 2026-10-27. Formulas and sources: method, sources.