If you buy corrugated boxes and you're not sure what the grade codes on your specifications mean, you're not alone. Most buyers learn to specify what they had before rather than what they actually need. This is how businesses end up over-specifying (and overpaying) or under-specifying (and dealing with damaged goods).
Here's a straightforward guide to corrugated board grades.
The Flute: The Heart of the Box
Corrugated board is named for the fluted — wavy — sheet sandwiched between flat sheets called liners. The flute determines most of the board's properties.
A flute is the largest flute (around 5mm thick), with fewer flutes per metre. It offers excellent cushioning and stacking strength, but takes up more space and uses more material. Commonly used for fragile goods, fresh produce and some export packaging.
B flute (around 3mm) is a flat, compact flute with good resistance to crushing. It suits point-of-sale display work and smaller boxes where printability matters more than stacking strength.
C flute (around 4mm) is the most commonly used flute in the UK. It offers a good balance of stacking strength, cushioning and printability. Most standard transit boxes are C flute.
E flute (around 1.5mm) is very thin — more like a microflute. It prints exceptionally well and is often used for retail packaging and premium product boxes where appearance matters.
F flute is even thinner than E, used primarily for small retail packaging and cosmetics boxes.
Single Wall, Double Wall, Triple Wall
Standard corrugated board — one fluted layer between two liners — is called single wall. It accounts for the majority of boxes used in transit packaging.
Double wall (e.g. BC double wall) combines two fluted layers between three liners. It significantly increases stacking strength and compression resistance. Suitable for heavy goods, pallet-load applications and products stacked multiple high in storage or transit.
Triple wall is three fluted layers between four liners — approaching plywood-like rigidity. Used for very heavy industrial components, bulk bins and alternative-to-wood applications.
The Liner Grade
Board grade isn't just the flute. The liner grade — the paper used for the flat outer and inner layers — also matters significantly. Liner grades are expressed as a weight in grams per square metre (gsm).
Kraft liner (typically brown) gives better strength properties weight-for-weight than test liner (which uses recycled fibre). For the same box, a kraft liner board will generally outperform a test liner board on burst strength and edge crush test (ECT).
Your box specification should state both the flute type and liner grades — for example: 125B/125K means B flute with 125gsm test liner outer and 125gsm kraft inner.
Common Mistakes
The most common over-specification I see is double wall where single wall with a better liner would suffice. Double wall adds cost and weight. A 150gsm kraft liner C flute often outperforms a 125gsm test liner BC double wall on edge crush resistance.
The most common under-specification is using standard C flute on heavy goods stacked three high in a warehouse environment. If your stock sits in a high-humidity distribution centre or is palletised, you need to spec for that — not for a dry warehouse at ground level.
If you're uncertain whether your current board grade is right, ask your supplier for ECT data and compare it against the actual stacking loads your boxes face.