Corrugated board looks simple, but there is more going on inside it than many first-time buyers expect. The type of paper, the number of layers, and the shape of the flutes all change how the board feels and performs.
This is why terms such as E flute, 5-ply, or 140 gsm kraft only describe one part of the material. If you are buying or designing packaging, learning these basic terms will make it much easier to compare quotations and explain your needs to a supplier.
In this guide, we will explain what corrugated board is made of, what 3-ply and 5-ply mean, how common flute types differ, and why thicker board does not always mean stronger packaging.
What Is Corrugated Board?
Corrugated board is made by gluing flat paper sheets to a wave-shaped paper layer in the middle.
A typical single-wall board has three paper layers:
- an outer liner — the flat paper on the outside;
- a fluted medium — the wave-shaped paper in the middle;
- an inner liner — the flat paper on the inside.
The curved flute creates a series of small arches between the two liners. Once these layers are bonded together, they create a board that is light but much more rigid than flat paper alone. The flute also adds space inside the board, which helps with cushioning and protection.
Corrugated Board, Corrugated Cardboard, or Paperboard?
Corrugated cardboard is a common everyday name for corrugated board. In the packaging industry, corrugated board is the more precise term.
Paperboard is different. It is usually a solid sheet without a fluted middle layer. Materials such as SBS and FBB are often used for folding cartons, while corrugated board is commonly used when a package needs more rigidity, cushioning, or shipping protection.
So a cosmetic folding carton and a corrugated shipping box may both be called “cardboard boxes” in normal conversation, but their materials and construction are very different.
What Is Corrugated Board Made Of?
The papers used to make corrugated board can be divided into two basic groups:
- linerboard — the flat paper used on the faces of the board;
- corrugating medium — the paper that is formed into the wave-shaped flute.
This difference matters because two boards can both be called “B flute” but still use different paper grades and paper weights. As a result, they may not perform the same way.
Kraftliner
Kraftliner is usually made mainly from fresh wood fibre. It is often chosen when good paper strength and stable performance are important.
There are different kraftliner grades, including brown and white-top versions, so “kraftliner” does not describe one single paper specification.
Testliner
Testliner is usually made mainly from recycled fibre. It is widely used in corrugated packaging and is available in many grades.
It is too simple to say that kraftliner is always “good” and testliner is always “poor.” Modern recycled papers can perform very well. The actual result depends on the paper grade, paper weight, board construction, and what the box needs to do.
White-Top Liner
A white-top liner gives the outside of the corrugated board a white surface. This is useful when appearance and printing quality matter.
It is often used for branded e-commerce boxes, retail packaging, displays, and other boxes where a brown kraft surface is not preferred.
Simple way to remember it: “white top” mainly tells you what the visible surface looks like. It does not tell you the full paper grade by itself.
Corrugating Medium
The corrugating medium is the paper that is formed into the flute. Common types include recycled fluting and semi-chemical fluting.
This layer is not just empty filler. Once it is shaped and glued between the liners, it helps give the board rigidity, cushioning, and resistance to compression.
What Does GSM Mean?
GSM means grams per square metre. It tells you the paper weight per square metre.
A supplier may describe a simple board like this:
This tells you the GSM of the individual paper layers, but it does not tell you the complete strength of the finished box.
Two papers with the same GSM can still perform differently because the fibre quality and paper grade may be different. After the papers are made into corrugated board, the flute type, number of walls, box size, humidity, and production quality also affect performance.
Common Paper Components in Corrugated Board
| Paper Type | Where It Is Used | Typical Fibre Base | What a Buyer Should Know |
|---|---|---|---|
| Kraftliner | Outer or inner liner | Mainly fresh fibre | Often chosen when strength and stable performance are important. Many grades are available. |
| Testliner | Outer or inner liner | Mainly recycled fibre | Widely used and available in different performance levels. |
| White-top liner | Usually the outer liner | Depends on grade | Provides a cleaner white surface for branded or printed packaging. |
| Recycled fluting | Fluted middle layer | Mainly recycled fibre | A common fluting material for many general packaging applications. |
| Semi-chemical fluting | Fluted middle layer | Mainly fresh fibre | Used when higher medium performance may be needed. |
Buyer tip: A paper name or GSM number by itself is not a complete corrugated-board specification. Look at the full material combination.
How Is Corrugated Board Constructed?
Corrugated board can have different numbers of flat liners and fluted layers. This is where terms such as single wall, double wall, 3-ply, and 5-ply come from.
Single Face
Single-face corrugated board has:
The flute is exposed on one side. This material is commonly used for wrapping, cushioning, dividers, or other protective uses rather than as a normal box wall.
Single Wall / 3-Ply
Single-wall corrugated board has:
That means two flat liners and one fluted layer — three paper layers in total. This is why it is also commonly called 3-ply corrugated board.
Single wall is widely used for shipping cartons, mailer boxes, die-cut boxes, and many general packaging applications.
Double Wall / 5-Ply
Double-wall corrugated board has:
That means three liners and two fluted layers — five paper layers in total. It is therefore commonly called 5-ply corrugated board.
The two fluted layers do not have to be the same flute size. Combinations such as BC or EB flute are common.
Triple Wall / 7-Ply
Triple-wall board has four liners and three fluted layers, giving seven paper layers in total.
This type of board is usually used for heavier-duty packaging where more structural support is needed.
Buyer tip: “3-ply,” “5-ply,” and “7-ply” are common sourcing terms. “Single wall,” “double wall,” and “triple wall” describe the same idea more clearly. For a quotation, it is still better to include the flute combination and other material requirements as well.
Corrugated Flute Types Explained
The flute is the wave shape inside the corrugated board.
Different flute profiles have different heights and different numbers of waves over the same distance. This changes the thickness, cushioning, rigidity, and surface smoothness of the board.
Important: Flute letters are names for different profiles. They are not strength grades, and they are not arranged from thickest to thinnest in alphabetical order.
A Flute
A flute is one of the deeper common flute profiles. Its typical flute height is around 4.2–4.8 mm.
The deeper wave creates more space between the liners, which can help with cushioning and bending stiffness. The trade-off is that its surface is usually less smooth than a finer flute.
C Flute
C flute is typically around 3.4–3.8 mm in flute height.
It offers a practical balance of cushioning and structural performance, which is why it is widely associated with general shipping cartons.
B Flute
B flute is typically around 2.4–2.8 mm.
It is more compact than A or C flute and gives the board a firmer surface. It is commonly used for die-cut boxes, shipping boxes, and many retail or e-commerce packages.
E Flute
E flute is a fine microflute, usually around 1.2–1.6 mm in flute height.
Because the flute is fine and closely spaced, it creates a thinner board with a smoother surface. E flute is popular for smaller e-commerce mailers, retail boxes, displays, and packaging where appearance matters.
F Flute
F flute is even finer, usually around 0.7–1.1 mm.
It creates a very compact corrugated structure and is useful for small-format or presentation-focused packaging where a thick shipping-box look is not wanted.
Corrugated Flute Comparison
| Flute | Typical Flute Height* | General Character | Common Uses |
|---|---|---|---|
| A | ~4.2–4.8 mm | Deep profile, good cushioning | Protective applications and products needing more cushioning |
| C | ~3.4–3.8 mm | Balanced general-purpose profile | Shipping cartons and general transport packaging |
| B | ~2.4–2.8 mm | Compact and firm | Die-cut boxes, shipping and retail packaging |
| E | ~1.2–1.6 mm | Thin microflute with a smoother surface | Retail packaging, e-commerce mailers and displays |
| F | ~0.7–1.1 mm | Very fine microflute | Small-format and presentation packaging |
*These are typical flute-height ranges, not guaranteed finished-board thickness. Actual dimensions can vary depending on the corrugator, paper combination, and production conditions.
What Do BC, EB, and Other Double-Wall Flute Names Mean?
When two flute letters appear together, they usually describe a double-wall board that uses two different flute profiles.
- BC flute = B flute + C flute
- EB flute = E flute + B flute
- EC flute = E flute + C flute
The idea is not simply to make the board “twice as strong.” Different flute combinations allow a supplier to balance thickness, cushioning, rigidity, surface smoothness, and compression performance.
For example, a finer E flute can help create a more compact and smoother outer structure, while a larger B or C flute adds more depth.
Does Thicker Corrugated Board Mean Stronger Packaging?
Not always.
Thickness is easy to see, so buyers sometimes use it as a quick way to judge quality. But corrugated board performance depends on the complete material and box structure.
A thicker board made from lower-performance papers is not automatically stronger than a thinner board made from better-performing papers.
Finished box performance can be affected by:
- liner and fluting paper grades;
- paper GSM;
- flute profile;
- single-wall or multi-wall construction;
- box size and structure;
- flute direction;
- production quality;
- humidity and storage conditions.
This is why important shipping boxes may also be specified or tested using performance measurements such as ECT (Edge Crush Test) and finished-box compression testing rather than thickness alone.
We will cover corrugated-box strength and ECT/BCT in a separate guide.
Frequently Asked Questions
What is corrugated board made of?
Corrugated board is made from flat liner papers bonded to one or more layers of wave-shaped fluting paper. The paper grades, GSM, flute profile, and number of walls can all vary.
Is corrugated cardboard the same as cardboard?
“Corrugated cardboard” is a common everyday name for corrugated board. But “cardboard” can also refer to solid paperboard, so the terms are not always technically the same.
What is the difference between 3-ply and 5-ply corrugated board?
3-ply normally means single-wall board with two liners and one fluted layer. 5-ply normally means double-wall board with three liners and two fluted layers.
What is the difference between B flute and E flute?
B flute has a deeper profile, typically around 2.4–2.8 mm, while E flute is a finer microflute around 1.2–1.6 mm. E flute gives a thinner and smoother board, while B flute gives more structural depth.
Is 5-ply corrugated board always stronger than 3-ply?
No. Wall count is only one part of the specification. Paper grade, GSM, flute combination, box size, environment, and box design also affect performance.
Which corrugated flute is the strongest?
There is no single “strongest flute.” A flute profile describes the shape and size of the wave, not the complete strength of a box. The right flute depends on the product and the full packaging requirement.
What Comes Next?
Knowing the difference between liner papers, wall structures, and flute profiles gives you the basic language needed to understand corrugated board.
The next question is more practical: which board should you actually use for your packaging?
Product weight, box dimensions, shipping conditions, stacking, printing quality, and lamination method can all affect that decision. In our next guide, How to Choose Corrugated Board for Packaging, we will explain how these factors work together and what information to give a packaging supplier.
