Reading a box maker's certificate
A box maker's certificate is the round stamp on a box flap in which the maker certifies the box meets a freight classification rule. It tells you which test the board was graded by and the grade it was built to.
What the stamp says
A certificate names the box maker and its location, and states that the box meets the construction rules of the truck classification (NMFC Item 222) and often the rail one (UFC Rule 41). It then gives the grade, either as an edge crush value or as a burst value with a combined weight of facings.
Some stamps also give a size limit and a gross weight limit, which come from the same classification table as the grade.
Edge crush or burst
An ECT stamp grades the board by how much load it carries on its edge, which is what matters for stacking. A burst (Mullen) stamp grades it by how much pressure it takes before it bursts, which tracks puncture and rough handling. The two are different properties; the classification sets equivalences between them for its own purposes.
What the stamp does not tell you
A certificate is a minimum construction claim, not a performance guarantee for your product. It says nothing about how long the box will last in a humid warehouse or whether it survives a parcel network. For that, estimate strength with storage and humidity losses and test to an ISTA procedure.
Packgent has not yet published the classification's grade limits: they could not be checked against the current tariff.
Use it
- Nmfc item 222
- Box makers certificate
- Grades
- Tappi t 811
- Tappi t 810
- Right-sizing boxes to cut dimensional weight
- Stacking strength and safety factors
- Unit loads: boxes on a pallet
Last reviewed 2026-10-27. Formulas and sources: method, sources.