The short answer
The stamp certifies the board, not the packed box. It says the combined board met a carrier rule’s minimum strength for a stated size and gross weight.
It is a carrier rule, not a law. So it binds where that classification binds, which is why parcel carriers do not stamp their own boxes.
The round stamp on a box flap looks like a guarantee. It is closer to a receipt.
It records that a box plant made the board to a minimum written into a freight classification rule, and that the box stays inside a size and weight ceiling. Those are real claims, just narrower than most readings.
The gap matters because buyers read the stamp as a strength spec. They see 32 ECT and treat it as a property of the shipment, when it is a measurement taken on a specimen in a controlled room.
This reference covers what the stamp certifies, the two rules behind it, and the two ways a box can qualify. It also covers why the most repeated claim about it is both true and false.
What the Stamp Certifies
Read the word literally. The mark is a box plant’s own certification that the box meets stated construction and performance standards.
Six things normally appear in the circle: board construction, strength rating, size limit, weight limit, the plant’s name or symbol, and the city of manufacture.
Each line is a claim about the empty box, not about what goes inside it, how high it stacks, or how damp the warehouse gets. A box compression test measures the packed box instead.
Four of the six lines are limits. The strength rating is the only one most buyers read.
The Two Rules, and the Two Ways to Qualify
Two freight classifications sit behind the mark. Rule 41 belongs to the rail classification, and Item 222 belongs to the motor one, where it sets size and weight limits, material strength and the stamp itself.
Neither rule names a single strength: the requirements vary by gross weight and by united dimensions, which is length plus width plus depth added together.
For single wall that runs from a 20 pound, 40 inch category up to 120 pounds at 105 inches. So the same board can pass in one size and fail in a larger one.
Inside the rule there are two routes, and a box needs only one. A board can meet a minimum burst strength, or it can meet a minimum edge crush value from the rule’s own table.
| Route | What gets measured | Unit on the stamp |
|---|---|---|
| Bursting test | Pressure to rupture the board, with liner basis weight behind it | psi (for example 200) |
| Edge crush | Force a board edge carries on end, closer to how a stack loads it | lb/in (for example 32) |
Edge crush came in as an option to the older corrugated basis weight and burst requirement. That is why two stamps can read nothing alike and still certify to the same rule, which is the ECT vs Mullen question in another form.
Why the Certified Number Is a Lab Number
The strength figure is a measurement, so it has conditions. ASTM D685 sets the standard atmospheres for conditioning paper, paperboard, fiberboard and containers made from them.
The standard is blunt about why that matters: temperature and relative humidity have significant effects on the physical properties of paper and board.
There is a subtler point in it too. Properties at 50 percent humidity depend on whether the sample reached 50 percent from a higher or a lower humidity, so the board’s moisture history changes the reading.
None of that is on the stamp. It prints a conditioned-room number as though that number travels with the box into a damp trailer, which it does not.
Required, or Dead?
Here the published answers split: one box plant states that carrier classification rules require the stamp.
A packaging consultant says close to the opposite. There is no agency rule demanding a stamp unless the product is hazardous or the government is the buyer. Firms have stopped stamping with no consequences, and neither FedEx, UPS nor DHL stamp their own boxes.
Both are right about different things. A classification rule is a condition of moving freight under that classification, not a statute. So it binds LTL and rail shipments, and has no hold over a parcel carrier running its own guidelines.
The rule also allows another route entirely. Shippers can qualify packaging on performance under Items 180 and 181, tested by a lab registered with the classification body, which replaces the stamp rather than adding to it.
Common Questions
Is a stamp legally required on a corrugated box?
Not by statute for general freight. It is a carrier classification requirement, so it applies where that classification governs the shipment, and a parcel carrier with its own packaging guidelines sits outside it.
Does a higher ECT number mean a stronger box?
It means a stronger board edge, measured dry and conditioned. Stacking strength also depends on box size, flute direction, stacking pattern, and how much moisture the board has picked up. The rating sets a ceiling rather than predicting performance.
Why do some stamps show psi and others show lb/in?
They record different qualification routes: a psi figure comes from the bursting test, and a lb/in figure comes from the edge crush test, and the rule accepts either one. Neither converts cleanly into the other.
What this covers and what it does not. This is a reference to published classification rules and test methods, not advice on whether to stamp a box. That depends on the carrier, the classification the freight moves under, and a shipper’s own contracts. Rules also change, so confirm current requirements with the carrier.
Key takeaways
- The stamp certifies the empty board against a carrier rule minimum, with size and weight ceilings attached.
- Rule 41 and Item 222 set requirements that vary by gross weight and united dimensions, not one fixed strength.
- Strength figures are measured on conditioned specimens, and the board’s moisture history changes the reading.
- A classification rule is not a law, which is why parcel carriers do not stamp their own boxes.