The grade printed on a fiberboard box tells you how much force the box can withstand before it fails. It’s either a burst strength rating (measured in pounds per square inch) or an edge crush rating (measured in pounds per inch of width), and you’ll find it stamped on the bottom flap of most corrugated boxes inside a circular certificate. That number, along with the box’s wall construction and weight limits, is the manufacturer’s guarantee of what the box can handle during shipping and storage.
The Two Grading Systems
Fiberboard boxes are graded using one of two tests, each measuring a different kind of strength. The grade you see on a box will reference one or the other.
The Mullen Burst Test measures puncture resistance. A hydraulic system pushes a flexible membrane against the face of the board until it ruptures, and the result is reported in pounds per square inch. A box stamped “200#” can withstand 200 pounds per square inch of pressure before its face gives way. Common burst strength grades include 200# and 275#, with higher values for heavier-duty boxes. This test reflects how well the box resists being poked through by sharp objects or burst open by heavy, oddly shaped contents.
The Edge Crush Test (ECT) measures compressive strength. A strip of corrugated board is placed upright in a press, with the internal flutes running vertically, and squeezed until it buckles. The result is reported in pounds per inch of width. A box stamped “32 ECT” can handle 32 pounds of force per linear inch along its edge before collapsing. This value directly predicts how much weight can be stacked on top of the box, making it the more relevant test for palletized goods sitting in a warehouse.
Which Grade Matters for Your Use
The two tests answer different questions about box performance. Burst strength tells you how well the box protects its contents from rough handling, punctures, and impacts during transit. Edge crush strength tells you how much stacking weight the box can support.
Mullen-rated boxes are typically preferred when shipping heavy products, items with sharp features, or anything likely to encounter rough handling. If you’re sending automotive parts with pointed edges or dense machinery components, burst strength is the priority. ECT-rated boxes are the standard choice for products that get palletized and stacked in warehouses or shipping containers. Most everyday e-commerce and retail shipments use ECT-rated boxes because stacking strength matters more than puncture resistance for those applications.
An ECT-rated box can often use slightly less material than a Mullen-rated box of comparable performance, which is one reason ECT grading has become more common in recent decades.
Reading the Box Maker’s Certificate
The round or oval stamp on the bottom flap of a corrugated box is called the Box Maker’s Certificate, and it contains everything you need to know about the box’s grade. A typical certificate includes:
- Board construction: whether the box is single wall, double wall, or triple wall
- Test method and value: either an ECT rating (e.g., 32 ECT) or a burst strength rating (e.g., 200#)
- Maximum gross weight: the heaviest load the box is certified to carry
- Maximum size limit: the combined length, width, and height the design supports
- Manufacturer name and location
These certifications aren’t optional branding. They exist because freight carriers require boxes to meet minimum standards. The railroad industry established these requirements through a specification known as Rule 41 in the Uniform Freight Classification, and trucking companies adopted a nearly identical standard called Item 222 in the National Motor Freight Classification. Any corrugated box accepted for shipment by rail or truck must comply with one of these standards, and the Box Maker’s Certificate is how the manufacturer proves it.
How Wall Construction Affects the Grade
A single-wall box has one layer of fluted (wavy) material sandwiched between two flat liner sheets. This is the most common structure for standard shipping cartons and covers the majority of everyday packaging needs. Single-wall boxes typically carry ECT ratings of 32 or higher, or burst ratings starting around 200#.
Double-wall boxes add a second layer of fluting and a third liner sheet, significantly increasing compression strength. They’re used for heavier products, bulk shipments, or items needing extra protection over long distances. Triple-wall boxes go further with three fluted layers and four liner sheets, forming the strongest corrugated structure available. Triple-wall is strong enough to replace wooden crates for export shipments, automotive components, and other high-weight applications.
The wall type is always printed on the Box Maker’s Certificate alongside the grade value, because a 32 ECT single-wall box performs very differently from a 32 ECT double-wall box in terms of overall durability and weight capacity.
How Flute Size Plays a Role
Inside the walls of a corrugated box, the wavy layer (the flute) comes in different sizes that affect both strength and thickness. Flute size isn’t printed on the Box Maker’s Certificate, but it directly influences the grade a box can achieve.
A-flute is the thickest option at roughly 5 mm, with about 33 waves per foot. Its large arches provide the best cushioning and stacking strength, making it the go-to for fragile or heavy items like glassware. C-flute, at about 3.5 to 4 mm, is the most widely used for general shipping cartons because it balances cushioning with compression strength. B-flute is thinner (about 2.5 to 3 mm) with more waves per foot, giving it high crush resistance and a smoother surface that prints well, which is why you see it on beverage trays and retail displays.
For lighter retail packaging, E-flute and F-flute are much thinner (roughly 1 to 1.8 mm and 0.8 to 1.2 mm respectively) with very fine corrugations. These provide a nearly flat surface for high-quality printed graphics while still offering more protection than plain paperboard. You’ll find them on cosmetics packaging, small electronics boxes, and other products where shelf appearance matters as much as protection.
A box’s final grade depends on the combination of flute size, liner weight, and wall construction. Two boxes with the same ECT rating might use completely different flute profiles to get there, with one trading cushioning for printability or thickness for cost savings.