The short answer
An ECT grade chart lists board grades against a gross weight. That weight is the weight of the contents you put inside the box. It is not the load the box can carry on top of it, and the two numbers are not interchangeable.
The figure is also conditional. It holds only while the outside length, width and depth added together stay inside the size limit printed alongside it.
Most shipping boxes carry a stamp on a bottom flap with a number like 32 ECT. Somewhere there is a chart telling you what that number is worth in pounds, and nearly every version of it says the same thing.
Read closely, though, and the weight column gets described as the weight the box supports. That is a different quantity, and the gap is where cartons get specified wrongly.
This guide covers what an ECT number measures, which grades get specified, what the weight figure certifies, and the conditions attached to it.
What an ECT Number Measures
ECT stands for edge crush test. A lab stands a short column of board on edge between two plates and presses until the edge folds, then reports the result in pounds per lineal inch. A 32 ECT board takes 32 pounds on every inch of its edge before it gives way.
TAPPI T 811 defines the method. The value belongs to the combined board rather than to any finished box, which is the first thing the chart obscures.
Two boxes cut from one sheet carry the same ECT and different compression strength, because box strength also tracks perimeter, caliper and construction. A tall narrow carton and a wide shallow one behave differently on identical board.
The Grades in Common Use
Single wall runs from the low twenties into the fifties, double wall above that, triple wall higher again. Four grades cover most of what gets ordered.
| Grade | Construction | Where it is specified |
|---|---|---|
| 32 ECT | Single wall | The common retail and ecommerce grade. Certificates commonly read 65 lb gross weight against a 75 inch size limit |
| 44 ECT | Single wall | Heavier contents, or cartons that sit lower in a taller stack |
| 48 ECT and up | Double wall | Industrial parts, and goods that need a larger certified size |
| 71 ECT and up | Triple wall | Bulk bins and gaylords |
The full pairings live in the carrier classification tables, not in any chart online, and they are not reproduced here. What matters is that a grade never travels alone. A certificate pairs it with a size and a weight.
The certificate answers the question on the left. Box sellers routinely quote it as though it answered the question on the right.
What the Weight Column Actually Means
The gross weight limit is the heaviest contents the box may hold. Box makers describe it that way themselves, calling it the maximum weight you may place inside.
Stacking capacity is a separate question, and what a carton carries depends on how many boxes sit on it, how well they line up, and how damp the warehouse gets. No stamp answers that.
So a chart reading 32 ECT, 65 lb tells you what may go inside, not that the box will hold 65 pounds of other boxes.
The second condition gets left out entirely. That weight applies only within a stated size limit, measured as outside dimensions, length plus width plus depth added together. Build a larger box from the same board and it is weaker, so the certified weight no longer stands.
Both rules carry a trade-off provision: a box may exceed the normal size limit where the weight drops correspondingly, and the certificate has to say so below the stamp. The grade alone settles nothing.
The pairing is the specification. Item 222 of the National Motor Freight Classification and Rule 41 of the Uniform Freight Classification set minimum board grades against size and gross weight together. Quote a grade without its size limit and you have quoted half a specification.
Where the Chart Stops Applying
Published carrier guidance draws a hard boundary here. A certificate’s weight limits target palletized freight rather than small parcel shipments, and parcel carriers publish their own box strength guidance instead. Nearly every chart online speaks to parcel shippers.
Exceeding the stated gross weight carries consequences when something breaks, because the classification body notes that shippers who ignore the certificate see damage claims denied.
Three things sit outside the chart entirely. Board loses strength as moisture rises, and sustained load causes creep so a stack that held on day one can sag by week six. Pallet overhang cuts real stacking strength whatever the grade says.
One last point of status: the certificate is a maker’s declaration, not a government approval and not third-party certification. That makes it a sound starting point and a poor finishing point.
Common Questions
Does 32 ECT mean the box holds 65 pounds?
It means up to 65 pounds of contents may go inside, provided length plus width plus depth stay within the size limit on the same certificate. It says nothing about weight carried on top.
Is a higher ECT grade always better?
No. A higher grade costs more and adds material, and it only helps where compression is what fails. Where handlers move boxes singly and knock them about, puncture resistance matters more.
What is the size limit on a box maker’s certificate?
The maximum outside dimensions, measured as length plus width plus depth added together. A larger box from the same board is weaker, so the certified weight applies only within that limit.
Do these charts apply to parcel shipments?
Not directly. Carrier guidance puts certificate weight limits in palletized freight, and parcel carriers issue their own box strength guidance, so check the rule that covers how you actually ship.
Reading a Grade Chart Properly
This guide has covered what an ECT value measures and the conditions attached to the weight beside it. Three things matter: the column counts contents rather than stack load, it holds only inside a stated size limit, and carrier guidance places it in palletized freight rather than parcel.
Read that way, a grade chart is a useful shortlist. Read as a table of box capacities, it will let you specify a carton that passes on paper and fails on a pallet.
Key takeaways
- ECT counts pounds per lineal inch and belongs to the board, not to a finished box.
- The gross weight on a chart counts contents, not the load the box carries from above.
- It applies only while length plus width plus depth stay within the stated size limit.
- Item 222 and Rule 41 certify grade, size and weight together, so a grade alone is half a spec.
- Carrier guidance puts these limits in palletized freight rather than small parcel.