Corrugated cardboard is a packaging material made from three layers of paper: two flat outer sheets (called linerboard) sandwiching a wavy, ridged inner sheet (called fluted medium). That wave-shaped middle layer is what gives corrugated its strength, cushioning ability, and rigidity, and it’s what separates corrugated from plain cardboard or paperboard. Nearly every shipping box you’ve ever received is made from corrugated board.
How the Three Layers Work Together
The standard corrugated board you encounter in shipping boxes is called single-wall or double-faced board. It has two flat linerboard facings glued to one fluted medium in between. The linerboard provides smooth, printable surfaces and resists punctures, while the fluted medium acts like a series of tiny arches running in parallel. Those arches distribute pressure across the board, absorb impacts, and create an insulating air gap between the two flat layers.
The layers are bonded with a starch-based adhesive, typically made from a mixture of starch, water, sodium hydroxide, and borax. This adhesive system accounts for only about 2% of the finished board’s weight but creates a bond strong enough to hold up under stacking, shipping, and rough handling. Starch glue is favored across the industry because it’s inexpensive, biodegradable, and made from renewable plant sources.
For heavier-duty applications, manufacturers can add more layers. Double-wall corrugated has two fluted mediums separated by a third linerboard in the center, creating a five-layer sandwich. Triple-wall corrugated goes further with three fluted layers and four linerboards. These multi-wall boards are used for heavy industrial shipping, pallets, and even temporary furniture.
Flute Sizes and What They’re Used For
The wave pattern inside corrugated board comes in several standardized sizes, each identified by a letter. The flute size determines thickness, cushioning ability, and how well the outer surface prints.
- C-flute: About 3/16 inch thick, this is the most widely used flute size. It offers a good balance of cushioning and stacking strength, making it the default for standard shipping boxes.
- B-flute: About 1/8 inch thick, originally developed for packaging canned goods. The flatter surface compared to C-flute makes it better for high-quality printing when a laminated top sheet is applied. It’s also popular on high-speed automated packing lines.
- E-flute: About 1/16 inch thick, commonly used for smaller cartons and die-cut packaging. It serves as a sturdier alternative to plain paperboard folding cartons, offering more protection without much added bulk.
- F-flute: About 1/32 inch thick, developed for small retail packaging and fast food clamshell containers. Its lower fiber content produces a rigid box with less material, which also means less waste going to landfills.
A-flute, the thickest common option at roughly 1/4 inch, provides the most cushioning and is used when maximum shock absorption matters more than a compact profile. In practice, many boxes use a combination: double-wall boards often pair two different flute sizes to balance cushioning with compression strength.
How Strength Is Measured
When you see ratings printed on shipping boxes, they typically reference one of two standardized tests that measure different kinds of durability.
The Edge Crush Test (ECT) measures how much vertical compression force a piece of corrugated can withstand along its edge before it buckles. This test was developed around 1990 specifically to gauge how well boxes hold up when stacked on pallets or shelved in warehouses. If your main concern is whether a box can support the weight of other boxes stacked on top of it during shipping or storage, ECT is the relevant rating. A higher ECT number means greater stacking strength.
The Mullen Burst Test, originally developed in 1887, measures bursting strength: the amount of force (in pounds per square inch) required to punch a hole through the corrugated material. This test reflects how well a box resists damage from outside pressure during individual handling, like being grabbed, dropped, or bumped while moving through a warehouse or onto a delivery truck.
Both ratings appear on the round certification stamp printed on the bottom of most shipping boxes. ECT-rated boxes are generally lighter because they can use less material to achieve the same stacking performance, which has made them increasingly popular for companies looking to reduce shipping weight and material costs.
What Corrugated Board Is Made From
Both linerboard and fluted medium are made from wood pulp fibers, either virgin kraft pulp (made from fresh wood chips) or recycled fiber sourced from recovered paper and old corrugated containers. Virgin kraft linerboard is stronger and more moisture-resistant, which is why it’s often used for the outer facings of boxes that need to withstand rough conditions. Recycled content is common in the fluted medium and in linerboard for lighter-duty applications.
The manufacturing process runs on a machine called a corrugator. The fluting medium is fed through heated, gear-like rollers that press the paper into a wave shape. Starch adhesive is applied to the tips of the flutes, and a linerboard sheet is pressed onto each side. The combined board travels through a heating section that cures the adhesive, then gets cut and scored to the required box dimensions. Modern corrugators run at high speeds, producing hundreds of feet of finished board per minute.
Why It’s So Widely Used
Corrugated cardboard dominates packaging for practical reasons. It’s lightweight relative to its strength, which keeps shipping costs down. The fluted structure provides built-in cushioning that protects contents without requiring separate padding in many cases. It’s easy to print on, can be die-cut into custom shapes, and folds flat for efficient storage before assembly.
From a sustainability standpoint, corrugated performs well compared to many alternatives. The starch adhesive is plant-based, the board itself is made from renewable wood fiber, and it’s one of the most widely recycled materials in the waste stream. The recycled fiber then goes back into producing new corrugated board, creating a relatively closed loop. Thinner flute profiles like F-flute further reduce fiber use for applications that don’t need heavy-duty protection.
Corrugated also takes well to coatings and treatments that expand what it can do. Wax coatings or water-resistant liners allow corrugated boxes to handle produce, frozen foods, and other moisture-heavy products. Specialty printing techniques turn corrugated into retail-ready displays and branded packaging that goes straight from the shipping pallet to the store shelf.
Corrugated vs. Cardboard vs. Paperboard
People use “cardboard” as a catch-all, but the packaging industry distinguishes between three materials. Paperboard is a single thick sheet of paper, like what cereal boxes and shoe boxes are made from. It has no fluted layer and offers much less strength and cushioning. Corrugated board, as described above, has at least one fluted medium between linerboard facings. The term “cardboard” isn’t a formal industry designation at all; it loosely refers to any stiff paper-based material, but when most people say cardboard box, they mean corrugated.
The distinction matters when you’re choosing packaging. Paperboard is fine for lightweight retail products that won’t face rough handling. Corrugated is necessary when the contents need protection during shipping, stacking, or storage. The fluted layer is the difference between a box that collapses under a few pounds of pressure and one rated to support 32 or more pounds per square inch of edge compression.