Materials · Corrugated
Corrugated board: liners, flutes and strength
Corrugated board is a wavy fluting glued between flat liners. That structure gives it stiffness, cushioning and stacking strength for very little weight, which is why mailer boxes, transit boxes and display units are made from it. The flute size, the liners and the number of walls decide how it performs and how well it prints. This guide explains E, B and C flute, double wall and board grades in plain terms.
Overview
How corrugated board is built
A sheet of corrugated board has three parts. The fluting, or medium, is paper formed into a wave. The liners are flat sheets glued to the tips of the flutes on each side. Together they work like a beam: the liners resist bending, and the flutes hold them apart and absorb impact. One fluting between two liners is single wall; two flutings with three liners is double wall.
Flute size is the first decision. Flutes are named by letter, and the letters do not run in order of size. E flute is fine, typically about 1.5 mm thick, with a smooth surface for print. B flute is about 3 mm and is the general-purpose choice for postal boxes. C flute is about 4 mm, with more cushioning and stacking strength. Exact thickness varies between corrugators, so treat these as typical figures.
Double wall combines two flutes. BC double wall, typically around 6–7 mm, is the usual heavy-duty shipping board. EB double wall, around 4–5 mm, gives double-wall strength with the fine E flute on the outside for better print. Double wall is chosen for heavy, fragile or high-value contents, for boxes stacked in storage, and for floor-standing displays.
The liners do as much work as the flute. Kraft liner is made mainly from virgin kraft fibre and is the strongest and most tolerant of moisture. Test liner is made from recycled fibre; it is weaker weight for weight but perfectly adequate for many boxes. White-top liners add a bleached surface layer for brighter print. Liners commonly range from about 115 to 200 gsm, and heavier liners add strength.
Board grades are a shorthand for those parts: outer liner, flute and inner liner. A grade might read as a 150 gsm kraft outer, B flute and a 125 gsm test inner. You do not need to learn the codes. Tell us the product weight, how many go in a box, how it travels and whether boxes are stacked, and your quotation will state the grade we recommend.
Specification
Typical specification
| Specification | Typical range | Notes |
|---|---|---|
| E flute | About 1.1–1.8 mm (commonly about 1.5 mm) | Fine flute with a smooth print surface. The usual board for printed mailer boxes, product boxes and counter display units. |
| B flute | About 2.5–3 mm | The general-purpose single wall, with good cushioning and crush resistance for postal and transit boxes. |
| C flute | About 3.5–4 mm | Taller flute with more cushioning and stacking strength; common for transit boxes and heavier contents. |
| F flute (micro flute) | About 0.8–1 mm | A very fine flute used where a box needs to be slimmer than E flute but stiffer than carton board. |
| EB double wall | About 4–5 mm | Double-wall strength with an E flute outer face that prints better than B or C. |
| BC double wall | About 6–7 mm | Heavy-duty board for heavy, fragile or stacked loads and floor-standing displays. |
| Liners | Commonly 115–200 gsm | Kraft (virgin fibre, strongest), test (recycled fibre) or white-top for brighter print. |
Best for
Where it works best
Printed mailer boxes
E flute for crisp print and neat folds; B flute when the contents are heavier.
E-commerce and subscription
Boxes that ship the order and present it on opening, often printed inside as well.
Outer shippers for rigid boxes
A plain B, C or double-wall outer protects a presentation box through courier networks.
Counter display units
E or B flute holds its shape on the counter and takes full-colour print.
Electronics
Corrugated boxes with fitted corrugated inserts stop products moving in transit.
Drinks bottles
Double wall, or single wall with fitted dividers, for heavy glass.
Inserts and dividers
Die-cut corrugated inserts hold products apart inside mailers and transit boxes.
Limitations
Where it falls short
- Corrugated loses much of its strength in damp or humid conditions, so storage conditions and the product's own moisture matter.
- Printing directly onto B and C flute can show faint ribs from the flutes (washboarding); fine artwork is better on E flute or a printed sheet laminated to the flute.
- Cut edges show the flute, which looks less refined than a carton or rigid edge; roll-end mailer constructions hide most edges.
- It does not take a clean emboss, and very fine foil detail needs a smooth printed surface.
- Heavier boards take more space when stored flat and are harder to fold neatly on small boxes.
- Board alone does not protect a loose product; an insert or void fill is usually needed for fragile items.
Pairs with
Works well with
Box style
Roll-end tuck front boxes
The classic mailer construction, usually in E or B flute.
Read moreProduct
Mailer boxes
Printed corrugated boxes that ship and present the order.
Read moreProduct
Corrugated boxes
Printed corrugated packaging for transit and delivery, in single and double wall.
Read moreBox style
Counter display units
Printed E or B flute displays that stand on the counter.
Read moreMaterial
Kraft board
Kraft liners give corrugated its natural brown finish.
Read moreFinish
Lamination
Laminating the printed sheet protects mailer print from scuffs in transit.
Read more
Compare
E, B and C flute and double wall compared
| Feature | E flute | B flute | C flute | BC double wall |
|---|---|---|---|---|
| Typical thickness | About 1.5 mm | About 3 mm | About 4 mm | About 6–7 mm |
| Print surface | Smoothest; best for detail | Good; slight ribbing possible | Fair; ribbing more likely | Coarse; choose EB for print |
| Cushioning | Light | Moderate | Good | High |
| Stacking strength | Lower | Moderate | Good | High |
| Typical uses | Printed mailers, retail, displays | Postal and transit boxes | Transit boxes, heavier goods | Heavy, fragile or stacked loads |
| Space when stored flat | Least | Moderate | More | Most |
How to choose
How to choose corrugated board
Start with weight and fragility
Light, robust products usually suit E flute. Heavier or fragile items move to B flute, C flute or double wall, often with a fitted insert to stop movement.
Match the board to the journey
A box that goes through courier sorting takes drops, knocks and compression; a box that sits on a shelf does not. Tell us how and how far it travels.
Think about stacking and storage
Boxes stacked in a warehouse need stacking strength, which comes from the flute, the liners and flutes running vertically in the box walls. Tell us how high boxes are stacked.
Decide how the artwork must look
Detailed full-colour artwork suits E flute, or a litho-printed sheet laminated to the flute. One- or two-colour logos print well directly onto kraft or white liners.
Choose kraft or test liners for the job
Kraft liners add strength and moisture tolerance for heavier loads; test liners keep the specification economical where the load is light.
Specify the insert with the box
Inserts change the internal space and do much of the protective work. Measure the product, not the box, and we will design the two together.
Finished work
Corrugated board on finished packaging
Roll-end tuck front mailer boxes · Corrugated board Corrugated board
Corrugated board questions
Questions buyers ask
Anything not covered here, put it in the notes on your quote and we will answer it directly.
Ask on your quoteWhat is corrugated board?
Corrugated board is a fluted paper layer glued between two flat paper liners. The flutes hold the liners apart, which makes the board stiff, cushioning and strong for its weight. It is used for mailer boxes, transit boxes, displays and inserts.
What is the difference between E, B and C flute?
They are flute sizes. E flute is about 1.5 mm thick with the smoothest print surface; B flute is about 3 mm and the general-purpose choice for postal boxes; C flute is about 4 mm with more cushioning and stacking strength. Exact figures vary by manufacturer.
Which flute should I choose for a mailer box?
E flute suits lighter products where print quality matters most. B flute is the practical choice for heavier contents or rougher journeys. We recommend the flute once we know the product, its weight and how it travels.
What does double wall mean?
Double wall board has two flutings and three liners. BC double wall, around 6–7 mm, is the usual heavy-duty grade; EB double wall, around 4–5 mm, puts the fine E flute outside for better print. It is used for heavy, fragile or stacked loads.
What is the difference between kraft liner and test liner?
Kraft liner is made mainly from virgin kraft fibre and is the strongest and most moisture-tolerant. Test liner is made from recycled fibre and is weaker weight for weight, but it is adequate for many lighter boxes. Many grades combine a kraft outer with a test inner.
How is corrugated strength measured?
Three tests come up most. Edge crush measures how much load the board takes on its edge, which relates to stacking strength. Box compression tests a whole made-up box. Burst strength measures resistance to puncture and rupture. Tell us the load and we choose the grade.
Why do flute letters not follow size order?
The letters reflect roughly the order in which the profiles were introduced, not their size. A flute came first, then B, then C, which sits between them in height. E and F are later, finer profiles. A flute is now rarely used for printed packaging.
Can corrugated boxes be printed in full colour?
Yes. For the best results a sheet is printed by litho and laminated to the flute, or the board is printed digitally. Direct flexographic printing onto the liner suits one- and two-colour designs and is common on kraft transit boxes.
What is washboarding?
Washboarding is a faint ribbed pattern that can appear when printing directly onto corrugated board, because the liner is supported only at the flute tips. It is more visible on B and C flute and on thin liners. E flute or a laminated printed sheet avoids it.
Can you print inside a corrugated mailer box?
Yes. The inside liner can be printed as well as the outside, which is popular for subscription and gift mailers. Inside print is part of the specification, so mention it when you request your quotation.
Does corrugated board cope with damp conditions?
Corrugated is paper, so it absorbs moisture and loses a significant part of its strength in damp or humid conditions. Store boxes dry and off the floor. If products are chilled or moist, tell us so we can consider heavier board or kraft liners.
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