The Ultimate Guide to Corrugated Board Flutes, Grades and Types

  • Last Updated: November 16, 2025
a cover image showing different types of corrugated board flutes and grades

Table of Contents

When it comes to packaging, the structure of your box matters.

Choosing the right material for e-commerce shipments, retail displays, or industrial crates is a critical decision. Get it right, and you perfectly balance protection, cost, and print quality. Get it wrong, and you risk product damage, poor branding, and wasted budget.

Corrugated board is not just "cardboard"—it's a sophisticated, engineered material. But the world of flutes, liners, and grades can be confusing.

In this guide from Dauxin, you'll learn everything you need to become an expert:

    • What corrugated board flutes are (and why it's so important).

    • The different types of corrugated board (Single vs. Double Wall).

    • How to read corrugated board grades (like ECT and Mullen).

    • A simple flow for choosing the perfect board for your product.

Let's dive in.

1. First, What Is Corrugated Board?

A simple clear diagram showing the 3 parts Outside Liner Medium Flute and Inside Liner

At its core, all corrugated board is made of three components, as detailed by industry leaders like the Fibre Box Association. When you look at the cut edge of a box, you'll see them:

  1. Inside & Outside Liners: These are the flat, smooth "faces" that make up the outside and inside of the board. This is what you print on.

  2. Medium: This is the wavy, "fluted" paper that is sandwiched between the liners.

  3. Flute: This refers to the shape and size of that wavy medium.

The flute is the "architecture" of the board. It's what gives the material its cushioning, stacking strength, and rigidity. Changing the height and shape of the flute (from thick to fine) completely changes the board's performance.

2. Understanding Corrugated Board Flutes (A, B, C, E, & F)

A to scale diagram showing the 5 main corrugated flute profiles A B C E F side by side for easy comparison

The "Flute Profile" is the single most important factor in your board's performance. Different profiles were developed to serve different needs—from heavy-duty stacking to high-end printing.

Here are the most common flute types, from largest to smallest.

  • A-Flute:

    • Approx. Height: 5 mm

    • The Original: This is one of the original flute profiles. Its tall flutes provide the best cushioning and stacking strength.

    • Best For: Packaging fragile items, heavy-duty industrial crates, and stacked pallets. Less ideal for high-quality printing.

  • C-Flute:

    • Approx. Height: 4 mm

    • The All-Rounder: C-Flute is the most common flute profile used for standard shipping boxes. It offers a good balance of cushioning, stacking strength, and print surface.

  • B-Flute:

    • Approx. Height: 3 mm

    • The Print-Friendly Staple: With more flutes per foot than C-Flute, B-Flute provides a smoother, flatter surface that's better for printing. It has good puncture resistance and is commonly used for mailers, subscription boxes, and retail displays.

  • E-Flute:

    • Approx. Height: 1.5 mm

    • The "Micro-Flute": E-Flute is very fine. It provides a superior surface for high-quality printing (including litho-laminating) and die-cutting. It's lightweight, uses less material, and is perfect for premium retail boxes (like for cosmetics or electronics).

  • F-Flute:

    • Approx. Height: 0.8 mm

    • The Premium Finish: This "micro-flute" is even finer, offering a "paperboard-like" feel with the structural integrity of corrugated. It's used for premium, small-format retail boxes (like for perfume) where branding is key.

Flute Profile

Approx. Height

Flutes per Foot

Key Characteristic

A-Flute

~5 mm

~33

Excellent Cushioning & Stacking

C-Flute

~4 mm

~39

Common All-Purpose Shipping

B-Flute

~3 mm

~47

Good Print Surface & Strength

E-Flute

~1.5 mm

~90

Superior Printing & Fine Boxes

F-Flute

~0.8 mm

~125+

Premium, Lightweight Retail

Key Takeaway: Large flutes (A, C) are for protection and stacking. Fine flutes (E, F) are for printing and premium branding. B-Flute is the balance between them.

3. Understanding Board Construction

clear diagram showing the layers of Single Wall Double Wall and Triple Wall board

Choosing a flute is only part of the story. You also need to decide on the board's construction or "wall" type.

  • Single Face: One liner + one medium (flute). This is flexible and often used as a protective wrap or internal cushioning.

  • Single Wall (SW): One flute layer sandwiched between two liners. This is the most common construction for standard shipping boxes (e.g., C-Flute Single Wall).

  • Double Wall (DW): Two flute layers and three liners (liner / flute / liner / flute / liner). This is used for heavier loads and extra stacking strength.

    • Note: The flutes can be combined. The most common Double Wall is BC-Flute (one layer of B-flute, one layer of C-flute) or EB-Flute (for premium print and strength).

  • Triple Wall (TW): Three flute layers. This is a heavy-duty "crate" material used for extremely large, heavy industrial items and palletized loads.

4. Understanding Corrugated Board Grades

"Grade" is a shorthand for the board's strength, which is determined by the liners used.

Kraft vs. Test Liners

  • Kraft Liner (K): Made from virgin (new) softwood fibers. It is the strongest, most durable liner and has the best puncture resistance. It's the premium choice, specified by a "K" (e.g., 125K).

  • Test Liner (T): Made from recycled fibers. It is more cost-effective and sustainable, but not quite as strong as Kraft. It is specified by a "T" (e.g., 125T).

A common specification might be 125K/B/125T. This means:

    • 125K: 125 gsm (grams per square meter) Kraft outer liner.

    • B: B-Flute medium.

    • 125T: 125 gsm Test inner liner.

How Strength is Measured: ECT vs. Mullen

When you order, you'll also see two main strength tests.

  1. Edge Crush Test (ECT):

    • What it is: This is the most important metric today. It measures how much force a board can withstand on its edge before crushing.

    • Why it matters: This test directly predicts stacking strength. A higher ECT (e.g., 32 ECT) means your boxes can be stacked higher in a warehouse or on a pallet without collapsing.

  2. Burst Strength (Mullen Test):

    • What it is: This measures how much pressure is needed to "burst" or puncture the face of the board.

    • Why it matters: This test is good for measuring a box's ability to withstand rough handling, impacts, and potential punctures.

5. How to Choose the Right Corrugated Board (A 3-Step Flow)

A photo clearly showing the flute effect on a printed E flute mailer box in different angles

Now that you're an expert, here is a simple flow to make the right choice.

Step 1: What is your product's weight and fragility?

    • Heavy, Fragile, or High-Value: You need protection. Start with C-Flute or A-Flute for cushioning. If it's very heavy, move to a Double Wall (BC-Flute) board.

    • Lightweight & Non-Fragile: You can optimize for cost and print. Start with B-Flute or E-Flute.

Step 2: How will it be shipped and stacked?

    • Stacked High on Pallets: Stacking strength is your #1 priority. You must use a board with a high ECT rating. This is more important than the Mullen test. A Double Wall board is often the best choice here.

    • Single E-commerce Shipments: The box will be handled individually and potentially thrown. Here, puncture resistance (Mullen test) is also important. A standard C-Flute or B-Flute Single Wall is often sufficient.

Step 3: What are your printing and branding needs?

    • Simple, 1-Color Logo: You can print this on almost any flute. A standard C-Flute or B-Flute is fine.

    • High-Quality, Full-Color Graphics: Print quality is paramount. You must use a fine "micro-flute" like E-Flute or F-Flute. Their smooth, flat surface ensures your graphics look sharp and premium, without any "fluting" lines showing through.

6. Common Mistakes to Avoid

  • Over-specifying: Using a Double Wall, 100% Kraft-liner box for a lightweight product is a common way to waste money. Run a test shipment with a lighter board first.

  • Under-specifying: Choosing E-flute for a heavy motor part. The box will fail.

  • Ignoring Flute Direction: The flutes should almost always run vertically (top-to-bottom) on a standard box. This is where the stacking strength (ECT) comes from. Orienting them sideways makes the box weak.

  • "Flute Show-Through": Trying to print high-res graphics on a C-Flute or A-Flute. You will see the "wavy" lines of the flute underneath your print, which looks unprofessional.

7. Sustainability & Recycling

Corrugated board is a sustainability success story, with recycling rates over 90% in many regions.

    • Recycled Content: Using Test liners (recycled) instead of Kraft liners (virgin) increases the recycled content of your box, though it may slightly reduce maximum strength.

    • Right-Sizing: The most sustainable box is the one that uses the least material. Finer flutes (E, F) use less paper fiber.

    • Light-weighting: Don't over-spec! Using a 32 ECT board when a 26 ECT board will do the job saves material, weight, and shipping-related carbon emissions.

8. How to Specify & Order (Your Cheat Sheet)

When you contact your supplier (like Dauxin!), here is a clear way to specify what you need:

    • Construction: Single Wall (SW) or Double Wall (DW)

    • Flute Profile: e.g., C-Flute, B-Flute, or BC-Flute

    • Liners: e.g., 125T / 125T (125 gsm Test inner and outer) or 150K / 125T (150 gsm Kraft outer, 125 gsm Test inner)

    • Strength (Optional but helpful): e.g., "Must meet 32 ECT"

    • Print: e.g., "1-color flexo print" or "4-color litho-laminated"

Example Order Snippet: "We need a Single Wall, E-Flute box, with 150K / 150T liners, printed 4-color on the outside. Must be suitable for high-end retail."

9. FAQs

Q1: What are corrugated flutes?

Flutes are the wave-shaped medium layer inside a corrugated board. Their size and shape (e.g., A, B, C, E, F) determine the board's cushioning, stacking strength, and printability.

Q2: Which flute is best for shipping fragile items?

A taller flute (A-Flute or C-Flute) provides the most cushioning. For very fragile items, consider a Double Wall (BC-Flute) board.

Q3: What is the difference between single wall and double wall corrugated?

Single Wall has one flute layer and is used for most standard shipments. Double Wall has two flute layers, offering significantly more strength for heavy items or high-stacking.

Q4: What is ECT and why does it matter?

Edge Crush Test (ECT) measures how much force a board can take on its edge before crushing. It is the single best predictor of a box's stacking strength.

Q5: Which flute is best for high-quality printing?

E-Flute and F-Flute. These "micro-flutes" are very fine and provide a smooth, flat surface essential for sharp, premium-quality graphics.

Why Your Packaging Supplier Matters

Understanding flutes and grades is the first step. The next is partnering with an expert who can help you execute.

A great packaging partner like Dauxin won't just take your order—they will:

  • Audit your needs: Do you really need that expensive Double Wall box?

  • Suggest alternatives: Could a lighter E-flute board save you money and improve your brand image?

  • Provide prototypes: Test the board with your actual product before you commit to a full run.

Choosing the right material is a balancing act of cost, performance, and branding.

Ready to find the perfect board for your product? Contact the packaging experts at Dauxin today for a free consultation and quote.

Need a Custom Packaging?

Get a Free Quote Today!