Edge Crush Test is the one that matters
ECT measures how much force, in pounds per linear inch, a board edge withstands before crushing. Since a stacked load is carried almost entirely by the vertical walls and corners, ECT predicts stacking performance far better than any other single figure.
Burst strength is a legacy number
Mullen burst test measures puncture resistance and was the historical basis for carrier rules. It still appears on certificates and still matters for sharp-edged contents, but it tells you little about whether you can stack two loaded gaylords.
Deriving a safe stack
Compression strength falls with time under load, humidity and handling. Industry practice applies substantial safety factors, and bulk handling deserves the conservative end.
- Storage over 30 days: divide laboratory compression by about 4.
- Humidity above 70% RH: reduce a further 25 – 35%.
- Any hand-stacking or repeated forklift contact: reduce a further 10 – 15%.
- Then stack to the result, not to what the corner will physically bear on day one.
A practical rule of thumb
Two high loaded for triple wall, one high loaded for double wall, six high nested empty, and never stack a loaded gaylord on a unit whose corners you have not personally looked at. Following those three rules prevents most stack collapses.