Corrugated board grades & GSM explained

What is GSM and why does it matter?

GSM stands for grams per square metre — the basis weight of a single sheet of paper. Every corrugated box is built from paper layers, and each layer has its own GSM. A 150 GSM Kraft liner weighs 150 grams for every square metre of its surface. As a rule of thumb, higher GSM paper is heavier and tends to be stronger, but GSM is only one ingredient. The finished board's performance depends just as much on the flute profile, the fibre (virgin Kraft versus recycled test liner) and how cleanly the board is glued and converted. Treat GSM as a useful comparison number, not the whole story.

What are liners and the fluted medium?

A sheet of corrugated board has three structural parts. The two flat outer sheets are the liners — the surfaces you print on and that take abrasion in transit. Sandwiched between them is the fluted medium, the wavy paper that gives the board its rigidity and cushioning. The medium acts like a row of tiny arches: it resists crushing and keeps the liners apart, which is what makes a thin sheet of paper behave like a stiff, protective wall. When a spec lists three weights, it is usually outer liner, medium, inner liner.

How do I read a board grade?

A typical callout looks like 150K / 120F / 150K, B flute. Reading from the outside in, that is a 150 GSM Kraft outer liner, a 120 GSM fluting medium, a 150 GSM Kraft inner liner, all formed into B-flute board. The "K" marks a Kraft (often virgin or high-strength) liner; "T" or "C" letters in some systems mark test liner or chip. The flute letter (such as A, B or E) tells you the wave profile and therefore the board thickness. Together those values define both the weight and the strength of your carton. If your buyer hands you a different notation, send it to us and we will translate it into a board we can actually run.

Single-wall vs double-wall board

Single-wall board — one medium between two liners — covers the vast majority of export master cartons. It is light, economical and strong enough for most garment, footwear and consumer-goods shipments. Double-wall board such as BC stacks two flute profiles and three liners for extra thickness, cushioning and stacking strength; it earns its place on heavy contents, tall pallet stacks and long sea voyages. Triple-wall exists for very heavy industrial loads, but most RMG exporters never need it. Choosing the lightest construction that still passes your buyer's tests is how you control both cost and freight weight.

Does a higher GSM mean a stronger box?

Not reliably. It is tempting to assume a heavier board is always a safer board, but a well-engineered lighter spec frequently outperforms a heavier, poorly matched one. Two boxes with identical total GSM can behave very differently depending on flute, fibre quality and conversion. That is why packaging is specified by performance tests — bursting strength and Edge Crush Test (ECT) — rather than weight alone. We measure those values so your carton is proven against your real load, not just heavy on paper. Our bursting strength vs ECT guide covers exactly how those two tests differ.

How board grade connects to flute and strength

Board grade, flute and box strength are three dials on the same machine. The board grade sets the paper weights and fibre. The flute profile sets thickness, cushioning and printability. Bursting strength and ECT confirm the finished result. Get all three in balance with your product weight, stack height and transit mode and you have a carton that protects the goods, passes the buyer's audit and does not waste a gram of board. When you are ready to put numbers to it, our free packaging calculators give you an indicative board area and grade in seconds.