Short answer: ECT (edge crush test) rates stacking strength, and burst (Mullen) rates puncture resistance. Most everyday shipping uses 32 ECT single wall. Heavier or stacked loads move up to 44 ECT or double wall.
ECT vs. burst test: what's the difference?
- ECT (edge crush test) measures how much force the board's edge can take before crushing, in pounds per linear inch. It predicts how well a box survives being stacked, which is what matters most on pallets and in warehouses.
- Burst (Mullen) test measures how much pressure it takes to rupture the board's face, in pounds per square inch. It relates to rough handling and punctures.
Because ECT-rated board can often reach the same stacking strength with less fiber, ECT boxes are frequently lighter and less expensive.
Common box grades
Carrier rules and box manufacturer certificates commonly pair these grades with a maximum weight for the box and its contents. Treat them as a starting point, not a guarantee. Stacking, humidity and storage time all reduce real-world strength.
| Construction | ECT rating | Commonly paired burst grade | Typical max weight (box + contents) |
|---|---|---|---|
| Single wall | 23 ECT | 125# | 20 lb |
| Single wall | 26 ECT | 150# | 35 lb |
| Single wall | 29 ECT | 175# | 50 lb |
| Single wall | 32 ECT | 200# | 65 lb |
| Single wall | 40 ECT | 250# | 80 lb |
| Single wall | 44 ECT | 275# | 95 lb |
| Double wall | 42 ECT | 200# | 80 lb |
| Double wall | 48 ECT | 275# | 100 lb |
| Double wall | 51 ECT | 350# | 120 lb |
| Double wall | 61 ECT | 400# | 140 lb |
Weight limits also depend on box size. Carrier rules cap the combined length, width and depth for each grade. Check the box maker's certificate (the round stamp on the bottom flap) for the exact rating.
How to choose the right strength
- Start with how it ships. A single parcel and a box at the bottom of a pallet stack need very different strength.
- Add margin for Florida. High humidity can cut stacking strength significantly. See how humidity weakens boxes.
- Compare single and double wall. A higher-ECT single wall box sometimes matches double wall performance at lower cost.
- Test before switching. Run real shipments or a stacking test before changing grades across your whole line.
Not sure your boxes are the right grade? Our free packaging audit checks board grade against how you actually ship.