Materials5 min read
Corrugated flute types explained: E, B, C and BC
What the flute in corrugated board does, how E, B, C and BC double wall compare, and how to choose the right board for your shipping box.
If you have ever asked a packaging supplier for "a strong box" and received a question back about flutes, this article is for you. The flute is the wavy layer of paper between the flat liners of corrugated board, and choosing the right one is the single biggest decision in specifying a shipping box.
Get it right and the box protects your product with the least material. Get it wrong and you either pay for board you do not need or see damaged goods at the other end.
What a flute actually does
Corrugated board works like a beam. Two flat liners are glued to a fluted medium, and the arches of the flute hold the liners apart. That separation gives the board its stiffness and its ability to resist crushing, while keeping it light.
The size of the flute, meaning its height and how many waves fit into a metre, changes how the board behaves:
- Taller flutes give more cushioning and better stacking strength.
- Smaller flutes give a flatter, smoother surface for printing and a crisper fold.
Every flute profile is a trade-off between those two qualities.
The common flute profiles
Flute sizes are not perfectly standardised. Exact dimensions vary slightly between corrugators, so treat the figures below as typical ranges, not guarantees.
| Flute | Typical board thickness | Strengths | Typical uses |
|---|---|---|---|
| E | about 1.2 to 2 mm | Smooth print surface, neat folds, slim profile | Mailer boxes, retail packs, food boxes |
| B | about 2.5 to 3 mm | Good puncture resistance, good printability | Shipping cartons for lighter goods, die-cut boxes |
| C | about 3.5 to 4 mm | Better stacking strength and cushioning than B | General shipping cartons, moving boxes |
| BC (double wall) | about 6 to 7 mm | High stacking strength and protection | Heavy or fragile goods, export cartons, archive boxes |
E flute
E flute is the thinnest of the common profiles. Because the waves are small and close together, the surface is flat enough for detailed printing, and the board folds cleanly. That makes it the usual choice for e-commerce mailers and branded retail packaging, where the box is part of the customer experience. It is not designed to carry heavy loads in a stack.
B flute
B flute is a versatile middle ground. It resists punctures well and still prints reasonably cleanly, so it is common for die-cut boxes and shipping cartons for products that are not especially heavy.
C flute
C flute is the workhorse of transit packaging. The taller arches give it more compression strength and cushioning than B flute, which is why it is so widely used for standard shipping cartons. The trade-off is a slightly rougher print surface.
BC double wall
BC is two layers of flute, B and C, combined with three liners. It is significantly stronger and stiffer than single wall board and is the usual answer for heavy products, long stacking times in warehouses or demanding export journeys.
Flute is only half the specification
Two boxes with the same flute can perform very differently. The other half of the specification is the paper itself:
- Liner grade. Kraft liners made mostly from virgin fibre are stronger, especially when damp. Liners made from recycled fibre are more economical and perfectly adequate for many uses.
- Paper weight. Heavier liners and mediums increase strength, but also weight and cost.
- Box style. A regular slotted carton, a folding mailer and a die-cut tray will perform differently even in the same board.
For this reason we recommend specifying a box by its job, not just by its board. Tell your supplier what the product weighs, how many boxes will be stacked on a pallet, how long they will be stored and how they will travel.
How to choose
A simple way to think about it:
- Is the box mainly for presentation? Start with E flute, especially if it will be printed.
- Is it a shipping carton for moderate weights? B or C flute is likely to fit.
- Is the product heavy, fragile or travelling a long way? Consider BC double wall.
- Will the boxes be stacked high or stored for long periods? Ask about stacking strength and liner grade, not just flute.
Less board can be the better choice
It is tempting to choose the heaviest board available "to be safe". In practice, over-specifying adds cost, weight and material without improving how your product arrives. A box sized correctly to the product, in the right flute and grade, often performs better than a larger box in heavier board.
If you are unsure which construction fits your product, send us the details. We will recommend a specification and explain why.