The wavy layer inside corrugated board is called the flute. Its size affects cushioning, stacking strength, printability and thickness. Box makers name flutes by letters, which can be confusing because the letters do not follow size order. This guide explains the main flute types and when to use each, with our free Corrugated Board Calculator.
The main flutes
| Flute | Approx. thickness | Flutes per metre (approx.) | Typical use |
|---|---|---|---|
| A | 4.5 to 5 mm | 105 to 125 | Cushioning, fragile goods |
| C | 3.5 to 4 mm | 120 to 145 | General shipping cartons |
| B | 2.5 to 3 mm | 150 to 185 | Retail cartons, die-cut boxes |
| E | 1.1 to 1.8 mm | 290 to 320 | Mailers, retail packaging, good print |
| F | about 0.8 mm | 400+ | Small retail boxes, premium print |
Values vary slightly between manufacturers.
Bigger vs smaller flutes
- Larger flutes (A, C): better cushioning and stacking strength, thicker board.
- Smaller flutes (B, E, F): better crush resistance on the surface, smoother print and easier folding.
Combination boards
Double-wall (5-ply) board combines two flutes, commonly BC. Triple wall (7-ply) may use AAC or similar. Combinations balance strength and print quality. Read 3-ply vs 5-ply vs 7-ply.
Choosing a flute
| Need | Suggested flute |
|---|---|
| Standard shipping carton | C or B |
| Heavy export carton | BC double wall |
| E-commerce mailer box | E or B |
| Retail display or printed box | E or F |
Flute direction
Flutes usually run vertically on box sides to maximise stacking strength.
Thickness and box size
Thicker flutes increase outer dimensions; account for them when planning pallets and containers. See inner vs outer box dimensions.
For paper grades, read GSM and cardboard thickness. Estimate sheet sizes with the Cardboard Sheet Calculator.
Why the letters are out of order
The flute letters reflect the order in which they were introduced, not their size. A flute came first, then B, then C as a compromise between the two. E and F were developed later for retail packaging. That is why C sits between A and B in thickness.
Frequently asked questions
Which flute is most common for shipping boxes?
C flute, followed by B flute.
Which flute prints best?
Small flutes such as E and F.
What is BC flute?
Double-wall board combining B and C flutes.