Materials
Corrugated Board Explained: Ply, Flute and Strength
Corrugated boxes look simple, but small changes in their construction make a large difference to strength, print quality and cost. Understanding a few key terms helps you specify a box that protects your product without paying for strength you do not need.
What corrugated board is made of
Corrugated board is made from flat paper sheets called liners bonded to a wavy paper layer called the fluting (or medium). The arches of the fluting act like a series of columns, giving the board stiffness and cushioning far greater than the paper alone.
3-ply, 5-ply and 7-ply
“Ply” counts the paper layers in the board:
- 3-ply (single wall): two liners and one fluted layer. Common for lighter products and retail shipping.
- 5-ply (double wall): three liners and two fluted layers. Used for heavier goods, higher stacking loads and longer journeys.
- 7-ply (triple wall): four liners and three fluted layers, for heavy-duty and industrial applications.
More plies generally mean more strength, but the paper grades used matter as much as the number of layers. A well-specified 3-ply board can outperform a poorly specified 5-ply one.
Flute profiles
Flutes are identified by letters. Larger flutes offer more cushioning and stacking strength; smaller flutes give a flatter surface for printing and a slimmer board. Approximate thicknesses (these vary between manufacturers):
| Flute | Approx. thickness | Typical characteristics |
|---|---|---|
| A | ~4.5–5 mm | High cushioning and stacking strength |
| C | ~3.5–4 mm | General-purpose shipping boxes |
| B | ~2.5–3 mm | Good puncture resistance, reasonable print surface |
| E | ~1–1.6 mm | Slim board with a smooth print surface — popular for mailers and retail packs |
Double-wall boards often combine two profiles, such as B and C flutes, to balance strength and surface quality.
How box strength is measured
Several tests are used to describe board and box performance:
- Bursting strength — the pressure needed to rupture the board. Often used to specify board for handling and puncture resistance.
- Edge Crush Test (ECT) — how much load the board can take on its edge, which relates closely to stacking strength.
- Box Compression Test (BCT) — the load a complete, assembled box can carry before it collapses.
Paper is commonly specified by weight (GSM) and, for kraft paper, by bursting factor (BF). Your supplier can translate these numbers into a recommendation for your load.
Don’t forget real-world conditions
Laboratory figures assume standard conditions. In practice, humidity, long storage times, poor pallet alignment and rough handling all reduce how much a box can carry. Corrugated board absorbs moisture, which lowers its compression strength — something to consider in humid climates or cold-chain storage.
That is why the best specification starts with how your product is stored and transported, not just its weight. When you request a quote, include stacking height, storage duration and transit conditions if you know them.
Keep reading
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