Flutes, GSM and Burst Strength Explained
A plain-language guide to the specifications that decide how strong a corrugated box really is.
What is a flute?
The flute is the wavy middle layer of corrugated board. It works like a row of arches, giving the board its rigidity and cushioning. Flutes come in standard sizes — A (tallest), C, B and E (finest) — and each balances strength, cushioning and print quality differently.
Common flute types
C flute (about 4 mm) is the most common for shipping boxes, with good stacking strength. B flute (about 3 mm) is firmer and prints better. E flute (about 1.5 mm) is thin and smooth, used for mailers and retail boxes. Double-wall boards often combine B and C flutes.
What does GSM mean?
GSM is grams per square metre — the weight of the paper. Higher-GSM liners and flutes make a stronger, stiffer board. Typical kraft papers range from 100 to 200 GSM per layer. The total board weight is the sum of all layers, plus extra for the wave of each flute.
Bursting strength (BF and BS)
Bursting factor (BF) describes the paper; bursting strength (BS) is measured on the finished board in kg/cm². Higher BF papers resist punctures and pressure better. For e-commerce shipping, 16–18 BF papers are common; heavier goods call for 20–28 BF.
Edge crush and stacking
Edge Crush Test (ECT) measures how much load the board edge can carry before buckling — the key figure for boxes stacked in warehouses. Box compression strength depends on ECT, box size and humidity.
Putting it together
You do not need to specify every figure yourself. Tell us your product weight, how the boxes are stored and shipped, and the climate they travel through. We will propose a board build — ply, flute, GSM and BF — that meets the requirement at the lowest sensible cost.
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