Paper Thickness Explained: GSM, Caliper, Paper Weight & Packaging Material Selection
Packaging material selection often involves multiple units or indicators such as GSM, points, caliper, micron (µm), pound (lb) and so on. Take 300 g/m² coated paper and kraft paper of the same grammage as an example, there are obvious differences in the feel and physical properties of the two, indicating that the applicability of the material cannot be fully characterized by the mass of the unit area alone.
This article aims to sort out the definition and measurement basis of GSM, the determination method of thickness (caliper), the main causes of thickness differences under the same GSM, and the material selection principles for different packaging forms .
- The meaning of GSM in Paper and Packaging Materials
- What Is Paper Thickness?
- Packaging Paper Thickness Chart: GSM, Caliper, mm, pt & Typical Applications
- GSM vs Paper Thickness: What's the Difference?
- Why Papers with Identical GSM Can Have Different Thicknesses？
- Fiber type and composition
- Density and compressibility
- Coating and surface treatment
- Comparing Paper, Cardstock, Paperboard, Cardboard, and Corrugated Packaging Materials
- Paper
- Cardstock
- Board
- Corrugated Board
- Selecting the Right Paper Thickness for Your Packaging Project
- Product demand
- Customer experience
- Production process
- Logistics factors
- Sample verification
- Conclusion
The meaning of GSM in Paper and Packaging Materials
GSM (grams per square meter) is defined as the grammage of paper in g/m², that is, the mass of a single sheet of paper of 1 meter × 1 meter. In the field of paper products and packaging materials, this indicator is used to indicate the weight grade of materials, not the thickness.
In practical application, the high or low GSM value directly corresponds to the physical properties and usage scenarios of the paper. High GSM paper (such as more than 200 g/m²) is usually heavier and has higher bending stiffness, which is commonly used in luxury packaging boxes, business cards or hardcover bookcases; low-GSM paper (such as 60–80 g/m²) is lighter and more flexible, suitable for inner pages, envelopes or promotional inserts. It should be clear that GSM is not equivalent to thickness – under the same grammage, the thickness of different types of paper (due to differences in fiber ratio, pressing process, etc.) may be significantly different.
The measurement of GSM is based on the ISO 536 standard, and the allowable error is about ±5%. The standardized measurement method enables packaging engineers and material purchasers in different regions to interpret the specification parameters uniformly, but this convenience should not cover up its limitations: GSM only reflects the quality of the unit area. If you want to evaluate the straightness, shading or printing suitability, it still needs to be supplemented by thickness (caliper), tightness, etc. Other indicators.
What Is Paper Thickness?
Paper thickness is often called a caliper in the printing and packaging industry, which is defined as the vertical distance between two surfaces of a single sheet of paper, which is a physical size parameter, not a weight indicator.
Thickness measurement adopts a variety of units, depending on regional practice: micron (µm, that is, one thousandth of a millimeter) is used for precision control; millimeter (mm) is the metric standard, which is suitable for thicker cardboard; the number of points (pt, that is, one thousandth of an inch) is commonly used in the field of printing and packaging in North America. The conversion relationship is: 1 inch = 25.4 mm = 1000 pt, so 1 pt = 0.001 inch ≈ 0.0254 mm, 1 mm = 1000 µm.
Take 12 pt paper as an example, its thickness is about 0.012 inches or 0.305 mm. The industry usually takes 0.3 mm as the boundary threshold between paper and cardboard.
Thickness measurement requires the use of special measuring instruments such as micrometer or caliper, and single-layer measurement is carried out under the specified pressure (usually 0.49-0.63 kg/in²). The measurement procedure is based on ISO 534 or TAPPI T 411 standards to unify the test results between different laboratories and production batches.
Packaging Paper Thickness Chart: GSM, Caliper, mm, pt & Typical Applications
| Material Type | GSM Range | Thickness Range | Point Range | Typical Applications |
|---|---|---|---|---|
| Coated art paper | 80–350 gsm | 80–350 microns | 3–14 pt | Wrap paper, sleeves, printed surfaces |
| Cardstock | 210–400 gsm | 250–600 microns | 10–24 pt | Folding cartons, paper cards |
| Kraft paper | 100–400+ gsm | 100–600 microns | 4–24 pt | Bags, sleeves, eco-style boxes |
| White cardboard | 250–450 gsm | 300–700 microns | 12–28 pt | Retail boxes, cosmetic cartons |
| Greyboard | 600–1,100+ gsm | 1.0–3.0 mm | 40–120 pt | Rigid boxes, book-style boxes |
| Corrugated board | Varies by flute | 0.8–7.0 mm | 32–280 pt | Shipping boxes, protective packaging |
GSM vs Paper Thickness: What's the Difference?
| GSM (Weight) | Thickness (Caliper) | |
|---|---|---|
| What it measures | Mass per unit area | Physical depth of the sheet |
| Units | grams per square meter | microns, mm, or points |
| What it affects | Opacity, tear resistance, freight cost, perceived quality | Rigidity, fold performance, die-cutting behavior |
| Key insight | Two papers with same GSM can have different thickness | Thickness is what you physically feel |
GSM is related to thickness, but it is not interchangeable. The former indicates the mass per unit area, and the latter indicates the vertical thickness of the paper.
Take two types of cardboard with the same 350 g/m² – coated board and kraft board – as an example. Although the grammage is the same, the actual thickness and feel of the two are often different. The reason is that variables such as fiber type, coating formula, slab density and moisture content will change the final thickness; the coated board is denser after coating, while kraft board is usually fluffy, and there is more air between the fibers, so the thickness is large.
Why Papers with Identical GSM Can Have Different Thicknesses？
GSM and thickness characterize the mass of the unit area and the vertical size respectively, and the two do not correspond linearly. Under the same grammage, paper thickness can be obviously discrete due to fiber characteristics, processing methods, coating treatment and production differences.
Fiber type and composition
The inherent density and morphology of different fibers are different. The cotton fiber tube cavity is large, the unit mass occupies more volume, and the paper is thicker; the kraft pulp fiber is longer and the cell wall is thicker, and there is a systematic difference between the thickness of the paper (caliper) and the bleached chemical pulp fiber.
Density and compressibility
The calendering process on the paper machine densifies the paper through mechanical pressure, reducing the thickness and improving the surface smoothness. Take 300 g/m² as an example, the thickness of the coated paper after heavy pressing can be reduced to about 270 µm, while the uncoated paper with the same grammage may reach about 360 µm.
Coating and surface treatment
The coating operation applies coating to the paper surface to increase the quality, but the thickness increment is limited. Under the same GSM, coated paper is smaller than uncoated paper, and feels thinner and smoother.
Differences in manufacturing processes
The furnish, pressing process, drying curve and on-line measurement standards of different production enterprises are different. Even if the nominal specifications are the same, the actual thickness will still be deviated by equipment and process variables.
Comparing Paper, Cardstock, Paperboard, Cardboard, and Corrugated Packaging Materials
Paper, cardstock, board, and corrugated board have their own definitions in terms of thickness, stiffness and structure. When selecting materials, they need to be distinguished according to specific performance indicators.
Paper
Paper is a light, flexible sheet material with a thickness usually less than 250 g/m² or 0.3 mm, which is suitable for leaflets, inserts and light lining. Coated art paper is about 80–350 µm thick and is often used for high-quality printing surface layers.
Cardstock
Cardstock is a single-layer smooth cardboard with a thickness range of 250–600 µm (about 10–24 pt), which is often used for retail packaging. Its surface is flat and suitable for printing, which is suitable for cosmetics, nutritional products, candles, light food and other products with high requirements for visual effects.
Board
Board is thicker and more rigid than cardstock, and the thickness and surface quality can be controlled. Typical products include white board (300–700 µm) and gray board (1.0–3.0 mm), which are widely used in folding cartons, hard boxes and high-end packaging structures.
Corrugated Board
Corrugated board is made of the middle flute layer and the liner on both sides. The corrugated structure gives it high compressive strength and cushioning performance. Depending on the flute type, the total thickness is about 0.8–7.0 mm, which is mainly used for transportation packaging, such as e-commerce boxes, subscription boxes and protective outer boxes for fragile goods.
Selecting the Right Paper Thickness for Your Packaging Project
When selecting the paper thickness for the packaging project, it is necessary to comprehensively weigh the product characteristics, usage scenarios, processing technology and logistics costs, not just based on grammage or single parameters. The following is a sub-discussion according to the decision-making dimension.
Product demand
The quality and fragileness of the packaging object directly affect the material selection: heavy products require high bending stiffness, and fragile products need to be protected with the help of buffer media such as corrugated structures. If the thickness of the large box is insufficient, the side panel is prone to bending and deformation.
Customer experience
In the retail scene, the surface of the card paper of 14–18 pt is flat, and the tone reproduction is good, which is conducive to the shelf display effect. The heavy feeling of the material and the finger press feedback when opening the box will directly form a judgment on the value of the product.
Production process
The selected thickness must match the actual printing method (such as the rigidity limit of digital printing on the paper path). Thick paper needs to be scored at the folding area to avoid cracking of the surface layer; finishing processes such as embossing and foil stamping are more stable on the adhesion of higher-thickness substrates.
Logistics factors
The transportation packaging must adopt a corrugated board structure to withstand the stacking and impact force. The increase in the quality per unit area will directly increase the freight; thick materials occupy a larger volume in the warehouse and need to be included in the cost of storage space.
Sample verification
Before mass production, caliper samples of the target thickness should be obtained from paper mills, processors or printers, and the actual printing press, die-cutting or finishing equipment should be used for testing. You can quickly get hand feedback by bending and touching with your bare hands.
Conclusion
Paper thickness is not a single geometric parameter, and its value will directly affect printability, logistics cost and end consumers' perception of packaging quality. In the selection of packaging materials, it is first necessary to distinguish between the indicators of two different dimensions, GSM (mass per unit area) and caliper (thickness).
When selecting materials, we should not only pay attention to "how much GSM is needed", but also clarify the specific requirements of specific products for thickness (caliper), bending stiffness and surface treatment.
The degree of matching between material characteristics and application scenarios directly determines the comprehensive performance of packaging in protection, display and cost control.
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