Packaging guide
GSM and BF explained
The two numbers that decide how strong a carton really is — what they mean, how they relate, and what to ask a supplier for.
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In short
- GSM is the weight of the paper — grams per square metre. It tells you how heavy the paper is, not how strong.
- BF is burst factor: bursting strength divided by grammage. It measures strength independently of weight, so papers of different GSM can be compared.
- A useful specification quotes both, for example "150 GSM liner at 18 BF".
- Bursting strength of finished board is reported in kg/cm² and measured by rupturing a clamped sample under pressure.
- Paper grade is only half the answer. The ply count and flute profile decide how that paper carries load.
Why two numbers instead of one
Ask for a quotation on cartons anywhere in Pakistan and you will be quoted a paper grade written as a pair — something like 140 GSM, 18 BF. Buyers who have not met the shorthand often assume the bigger number is the better carton. It is not that simple, and the two numbers are answering different questions.
GSM answers “how much paper is there?” BF answers “how good is that paper?” You need both, because a carton can fail for either reason: too little paper, or paper of too poor a quality.
GSM: the weight of the paper
GSM stands for grams per square metre. It is the weight of a one-metre square of the paper. A 150 GSM liner weighs 150 grams per square metre; a 100 GSM liner weighs 100.
Higher GSM generally means thicker paper, a more rigid carton, and more cost — both in material and, because the carton is heavier, sometimes in freight. Lower GSM means a lighter and cheaper carton that will crush sooner.
A corrugated board is made of several papers, and they do not have to share a GSM. The outer liner, the inner liner and the fluted medium in between are often different weights, chosen for what each one has to do.
BF: the strength of the paper
BF stands for burst factor, and it is the number that actually describes quality. It is calculated by dividing the paper's bursting strength by its grammage:
Because weight has been divided out, burst factor is a fair comparison between a light paper and a heavy one. A 16 BF paper and a 22 BF paper are different qualities of fibre, whatever each one weighs. In practice, higher BF usually means more virgin kraft fibre and less recycled content in the furnish.
This is the single most useful thing to understand about buying cartons here: a heavier carton is not automatically a stronger one. A supplier can raise GSM, hand you a heavier box, and still deliver weaker board if the burst factor has dropped.
Bursting strength of the finished board
Burst factor describes the paper. Once the papers are combined into corrugated board, the finished board has its own bursting strength, reported in kilograms per square centimetre (kg/cm²).
It is measured by clamping a sample of the board and forcing a rubber diaphragm against it under rising pressure until the board ruptures. The pressure at the moment of failure is the bursting strength. It is the standard strength test in this market, and it is what a specification means when it names a strength figure for the board rather than the paper.
Reading a specification
Put together, a complete paper specification for a carton names the construction and the grade of each paper in it. In this market a specification will usually be written in this shape:
| Part of the spec | What it names | Example |
|---|---|---|
| Ply | How many layers of paper the board has | 5-ply |
| Outer liner | Weight and quality of the outside surface | 150 GSM, 18 BF |
| Fluting medium | The wave between the liners | 120 GSM |
| Inner liner | Weight and quality of the inside surface | 140 GSM, 16 BF |
| Board strength | Bursting strength of the finished board | kg/cm² |
| Size | Internal dimensions, length × width × height | 18 × 12 × 10 inches |
You do not need to arrive with all of this. If you can describe the product, its weight and the journey it makes, the specification is our job — this page exists so that the quotation you receive is readable rather than mysterious.
Rough guidance by application
The figures below are typical ranges seen in this market, not a standard. Mills vary, and the right grade always depends on the load, the stacking height and the journey.
| Application | Typical liner GSM | Typical BF |
|---|---|---|
| Light retail cartons, inner packs | 100–140 | 14–18 |
| E-commerce and courier boxes | 120–150 | 16–20 |
| Heavier goods, stacked storage | 150–180 | 18–22 |
| Export and industrial loads | 180+ | 20+ |
What GSM and BF cannot tell you
Paper grade is half of the answer. The other half is construction — how many plies the board has and how the flute is profiled. Good paper in the wrong construction still fails, and a modest paper in the right construction often performs better than expected.
That is the subject of the companion guide: 3-ply, 5-ply or 7-ply — how much weight each carries.
Common questions
What does GSM mean on a carton?
GSM is grams per square metre — the weight of one square metre of the paper used to make the board. A 150 GSM liner weighs 150 grams per square metre. Higher GSM generally means a thicker, heavier paper, and a heavier carton, but on its own it does not tell you how strong the paper is.
What is BF in kraft paper?
BF stands for burst factor. It is the bursting strength of the paper divided by its grammage, which makes it a measure of strength relative to weight. Because it removes weight from the comparison, two papers of different GSM can be compared directly on BF — a 16 BF paper and a 22 BF paper differ in quality regardless of how heavy each one is.
Is a higher GSM always stronger?
No. GSM only tells you the weight of the paper. A heavy paper made from poor-quality recycled fibre can have a lower burst factor than a lighter paper made from better fibre. That is exactly why the trade quotes both numbers together — GSM for the weight, BF for the strength.
What GSM and BF should I ask for?
It depends on the load and the journey, not on a single right answer. As a rough guide, lighter retail and e-commerce cartons commonly use liners around 120 to 150 GSM at 16 to 20 BF, while heavier goods and export cartons move up in both. Tell us the product weight, how it is stacked and how far it travels, and we will propose the grade.
How is bursting strength measured?
A sample of board is clamped and pressure applied through a rubber diaphragm until it ruptures. The pressure at which it bursts is the bursting strength, reported in kilograms per square centimetre for finished board. It is the standard strength test used across Pakistan and the region.
Does the flute affect strength as well as the paper?
Yes. The paper grade and the construction work together. GSM and BF describe the paper; the number of plies and the flute profile describe how that paper is arranged. A carton made from good paper in the wrong construction will still fail, which is why a specification names both.
