Understanding corrugated board grades
Flutes, liners, grammage and strength testing: a practical guide to what a board grade means and how to choose the right one.
How corrugated board is built
Every sheet of corrugated board is made from two types of paper. Liners are the flat outer and inner faces. The fluting (or medium) is the wavy layer glued between them. The arches of the flute work like a series of columns: they resist crushing, add cushioning and give the board its stiffness for very little weight.
- Single wall: one flute between two liners. Used for the majority of transit and e-commerce boxes.
- Double wall: two flutes separated by a middle liner. For heavier products, longer stacking and tougher distribution.
- Triple wall: three flutes. Heavy-duty applications such as bulk containers and industrial parts, often replacing timber crates.
Flute profiles
Flutes are identified by letter. The letters reflect the order they were introduced, not their size, which is why they can look confusing at first. Typical approximate thicknesses:
| Flute | Approx. thickness | Typical use |
|---|---|---|
| F | ~1 mm | Retail and presentation packs where print quality matters |
| E | ~1.5 mm | Mailers, die-cut boxes and printed retail packaging |
| B | ~3 mm | General-purpose boxes; good crush resistance and print surface |
| C | ~4 mm | Transit cases where stacking strength and cushioning matter |
| A | ~5 mm | Maximum cushioning; less common today |
| BC | ~7 mm | The standard double wall for heavy or stacked goods |
| EB | ~4.5 mm | Double wall with a finer print face |
Exact caliper varies between manufacturers, so always confirm dimensions for the specific board being quoted.
Liner papers and grammage
Liners are broadly divided into two families:
- Kraft liner: made mainly from virgin fibre. Strong for its weight, more resistant to moisture and a better surface for print.
- Test liner: made from recycled fibre. Lower cost and widely used, with performance that varies more between mills and grades.
Paper weight is expressed as grammage in grams per square metre (gsm). In the UK, board grades are often written as shorthand, for example 150K/C/125T: a 150gsm kraft outer liner, C flute, and a 125gsm test inner liner. The fluting paper is sometimes stated separately.
A heavier grade is not automatically a better one. The right combination of paper type, grammage and flute matters more than weight alone.
How strength is measured
Grade descriptions tell you what a board is made of. Test data tells you how it performs. The measures buyers will see most often are:
- ECT (edge crush test): how much load the board withstands on its edge. The best single predictor of stacking performance.
- BCT (box compression test): the load a finished, empty box withstands before collapsing. Depends on board, box style and dimensions.
- Burst strength: resistance to rupture from pressure on the face. Relevant to handling damage and heavy contents pushing outwards.
Corrugated is also sensitive to humidity. Board absorbs moisture from the air, and compression strength falls significantly in damp or chilled environments, something to factor in for cold chain, export and long warehouse storage.
Choosing the right grade
The right grade is the lightest construction that reliably protects the product through its whole journey. Start with the conditions, not the board:
- Product weight, fragility and whether the product itself carries any stacking load
- Pallet configuration, stacking height and how long pallets sit in storage
- Distribution route: parcel networks are harsher than palletised delivery
- Environment: humidity, chilled storage, outdoor or export shipping
- Print, presentation and any retail-ready requirements
What this means for buyers
Board grade is one of the largest drivers of corrugated cost and material use. Across large box ranges it is common to find grades that have drifted upwards over time, or several different grades doing the same job at different sites.
- Compare like for like. Make sure competing quotes are for the same grade, or ask for ECT and BCT data where grades differ.
- Specify performance, not just a name. Two test liners with the same grammage can perform differently.
- Standardise. Fewer grades across a range simplifies supply and strengthens buying power.
- Review regularly. Product, pallet and distribution changes can make a heavier grade unnecessary.