Content owner: Packaging Factory Direct. Published and reviewed: . This is a purchasing and specification guide, not a declaration that a particular board is suitable for a product, regulation or distribution system.
Key takeaways for packaging buyers
- Use GSM to describe mass per area and PT or mm to describe thickness.
- Convert points to millimeters mathematically, but do not convert GSM to thickness without board-specific data.
- Do not compare folding carton, rigid-box greyboard and corrugated board using one number alone.
- Identify the grade, manufacturer or approved-equivalent rules, test method, tolerance and converting process.
- Approve the actual printed, creased, folded and assembled sample for the intended product.
What GSM, points and millimeters actually measure
| Term | Property | Unit meaning | Useful purchasing role |
|---|---|---|---|
| GSM | Grammage | Grams per square meter | Compares mass per area under a defined method; often used in paper and paperboard specifications |
| PT or point | Caliper / thickness | 0.001 inch per point | Communicates a nominal thickness in an inch-based convention |
| mm | Caliper / thickness | Millimeter | Communicates thickness in SI units, common for rigid board and many international RFQs |
| Board construction | Layer and structural system | Grade, ply, flute, liner and medium description | Explains what the sheet is, which GSM or thickness alone cannot do |
| Performance requirement | Application-dependent behavior | Specified test and acceptance value | Connects the material and converted box to the product and distribution need |
ISO 536 specifies a method for determining paper and board grammage. ISO 534 addresses thickness, density and specific volume. TAPPI/ANSI T 410 and T 411 similarly treat grammage and caliper as separate measurements. That separation is the reason a supplier should not silently replace one property with the other.
Exact point-to-millimeter conversion
TAPPI's writing guidance defines one point as 0.001 inch. Because one inch is exactly 25.4 millimeters, one point is exactly 0.0254 mm. The following values are mathematical unit conversions, not claims about the measured tolerance or performance of a real sheet.
| Nominal points | Exact millimeters | What still needs confirmation |
|---|---|---|
| 10 pt | 0.2540 mm | Actual measured caliper and tolerance |
| 12 pt | 0.3048 mm | Actual measured caliper and tolerance |
| 16 pt | 0.4064 mm | Actual measured caliper and tolerance |
| 18 pt | 0.4572 mm | Actual measured caliper and tolerance |
| 24 pt | 0.6096 mm | Actual measured caliper and tolerance |
| 36 pt | 0.9144 mm | Actual measured caliper and tolerance |
Why GSM cannot be converted to thickness universally
Grammage tells a buyer how much a defined area weighs. Caliper tells a buyer how thick the sheet is under a defined measurement method. The relationship between them depends on apparent density or specific volume. A denser sheet can place similar mass into less thickness; a bulkier sheet can occupy more thickness at a similar mass.
Fiber mix, number of plies, fillers, coatings, calendaring or compression, moisture and production variation can affect this relationship. A generic chart that maps every 350 GSM board to one point value therefore removes information that the buyer needs. Use a current technical data sheet or measured sample for the exact grade, and state the method and tolerance when thickness is critical.
Do not specify every packaging structure the same way
Folding cartons
A folding-carton RFQ should identify the paperboard grade or an approval process for equivalents, grammage where relevant, target caliper and tolerance, coated side, grain direction, surface requirements and converting operations. Foil, embossing, lamination, heavy ink coverage, scoring and folding can change how the converted carton behaves. The product load, panel geometry and closure matter alongside the board.
Wrapped rigid boxes
A wrapped rigid box normally combines a structural board with a separate wrap paper, adhesive and sometimes an insert. Specify the structural greyboard thickness in mm or points, its tolerance and construction; then specify the wrap paper and decoration separately. Adding the wrap-paper GSM to the greyboard thickness is not a meaningful shortcut. Verify corners, hinge movement, lid fit, wrapped edges and insert height on an assembled sample.
Corrugated mailers and shipping boxes
Corrugated board is a system of liners, fluted medium and adhesive. State the intended board construction, flute or combination, relevant component specifications, box style, dimensions, joint and the required application-specific evidence. A total GSM figure alone cannot communicate flute geometry, liner arrangement, converting quality or finished-box performance. Review the packed product and distribution route rather than selecting corrugated material by a single paper number.
| Box type | Material fields to control | Converted-sample checks |
|---|---|---|
| Folding carton | Grade/equivalent rule, GSM, caliper tolerance, grain, coating and finish | Crease, fold, print, glue, closure and product fit |
| Rigid box | Structural board thickness, tolerance, wrap paper, adhesive and insert | Panel flatness, corners, lid/base fit, hinge or magnet operation and appearance |
| Corrugated mailer | Flute and board construction, liners/medium, box style and performance requirement | Fold quality, closure, product restraint, packed dimensions and route-relevant validation |
Build a comparable paperboard RFQ
| RFQ field | What to state | Why it prevents disputes |
|---|---|---|
| Material identity | Grade, supplier code where required, or documented approved-equivalent process | Stops unlike materials from being compared only by color or GSM |
| Grammage | Target and tolerance under the agreed test method, when relevant | Controls mass per area rather than implying thickness |
| Caliper | Target in mm or points, tolerance, test method and conditioning where required | Controls the physical property used in fit and converting |
| Surface and grain | Coated side, shade, grain direction and printing surface | Connects the sheet to print, crease and fold behavior |
| Conversion | Printing, coating, lamination, foil, embossing, scoring, folding and gluing | Defines the finished carton rather than an unconverted sheet |
| Application | Product dimensions and mass, box geometry, packing method and distribution conditions | Provides the context needed to evaluate a material choice |
| Approval evidence | Technical data, measured sample, production-intent sample and acceptance record | Creates a reference for production and incoming inspection |
Sample approval and incoming quality checks
- Confirm the data source: request the current technical data for the exact proposed grade, not a generic family name.
- Agree on methods and tolerances: record whether grammage, caliper or both are acceptance properties and how they will be measured.
- Review the blank: check grain, surface, print side and planned crease or fold direction.
- Review the decorated sheet: confirm that coatings, lamination, foil and embossing are represented.
- Approve the converted box: evaluate dimensions, fit, panels, closures, appearance and packing process with the intended product.
- Retain a controlled reference: identify the revision and store records or samples under conditions appropriate to the project.
- Inspect production consistently: use the agreed sample plan, tools, conditioning and decision rules; do not compare one uncontrolled reading with a nominal marketing number.
Common GSM and thickness specification errors
- Using GSM as a synonym for thickness: the two properties answer different questions.
- Using an online GSM-to-PT chart as a material standard: it cannot know the actual board's density, coating or manufacturing data.
- Calling a point value a grammage: points express thickness, not grams per square meter.
- Specifying a rigid box by wrap-paper GSM only: the structural board, adhesive and assembled construction remain undefined.
- Specifying corrugated board by total GSM only: flute, liner arrangement, box design and application evidence remain undefined.
- Assuming higher GSM always means stronger packaging: material composition, geometry, conversion and use conditions also affect performance.
- Approving an unprinted blank as the final reference: decoration and conversion can change the finished box.
Use this guide with material selection and sampling
Start with the site's custom paper box material guide when comparing broad material families. For premium setups, review the rigid box material, thickness and finish guide; for folding formats, use the cardstock product boxes RFQ guide. Then confirm the selected board through the custom packaging sample process rather than relying on a unit label alone.
Primary sources and limitations
- ISO 536:2019: determination of grammage
- ISO 534:2011: determination of thickness, density and specific volume
- TAPPI/ANSI T 410: grammage of paper and paperboard
- TAPPI/ANSI T 411: thickness (caliper) of paper and paperboard
- TAPPI Writing Guide: point terminology
Standards may require purchase for their complete procedures, and specifications can be revised. Confirm the applicable edition, conditioning, apparatus and tolerances with the buyer, material supplier and testing party. Board labels alone do not prove fitness, regulatory compliance or distribution performance.
FAQ about GSM, points and millimeters
Is GSM the same as paperboard thickness?
No. GSM is mass per unit area, while points and millimeters describe thickness or caliper. Two boards with similar grammage can have different thickness because their furnish, coating, compression, moisture and bulk can differ.
Can GSM be converted directly to points or millimeters?
Not from GSM alone. A valid relationship requires the specific board's measured caliper or apparent density under defined test conditions. A generic online conversion can misstate the actual material.
How many millimeters are in one paper point?
One point is 0.001 inch, which is exactly 0.0254 millimeter as a unit conversion. This converts a stated thickness unit; it does not predict the thickness of a board from its GSM.
What should a folding-carton paperboard RFQ specify?
Identify the board grade or approved equivalent, grammage where relevant, target caliper and tolerance, coated side, grain direction, print and finish processes, converting requirements, product load and the approved sample or measurable acceptance criteria.
Does higher GSM always make a stronger box?
No. Grammage alone does not determine box performance. Fiber composition, board structure, caliper, moisture, grain direction, creasing, geometry, adhesive, print and finish processes, product load and distribution conditions also matter. Validate the finished pack for its intended use.
Specify paperboard without unit confusion
Send the box style, dimensions, product load, proposed grade, grammage, caliper, printing, finishes, quantity and destination. We can review the fields needed for a project-specific sample and quotation.
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