Calculate the missing value from GSM, density, and thickness with g/m², kg/m³, lb/ft³, µm, mm, and inches, using any two of the inputs.
| GSM to Thickness | Thickness to GSM |
|---|---|
| 70 gsm = 87.5 µm | 50 µm = 40 gsm |
| 80 gsm = 100.0 µm | 75 µm = 60 gsm |
| 90 gsm = 112.5 µm | 100 µm = 80 gsm |
| 100 gsm = 125.0 µm | 125 µm = 100 gsm |
| 120 gsm = 150.0 µm | 150 µm = 120 gsm |
| 150 gsm = 187.5 µm | 175 µm = 140 gsm |
| 170 gsm = 212.5 µm | 200 µm = 160 gsm |
| 200 gsm = 250.0 µm | 250 µm = 200 gsm |
| 250 gsm = 312.5 µm | 300 µm = 240 gsm |
| 300 gsm = 375.0 µm | 400 µm = 320 gsm |
| Formulas: t(µm) = G(g/m²) / density(g/cm³); G(g/m²) = density(g/cm³) × t(µm). | |
| GSM to Thickness | Thickness to GSM |
|---|---|
| 20 gsm = 21.7 µm | 10 µm = 9.2 gsm |
| 25 gsm = 27.2 µm | 12 µm = 11.0 gsm |
| 30 gsm = 32.6 µm | 15 µm = 13.8 gsm |
| 40 gsm = 43.5 µm | 20 µm = 18.4 gsm |
| 50 gsm = 54.3 µm | 25 µm = 23.0 gsm |
| 60 gsm = 65.2 µm | 30 µm = 27.6 gsm |
| 80 gsm = 87.0 µm | 40 µm = 36.8 gsm |
| 100 gsm = 108.7 µm | 50 µm = 46.0 gsm |
| 120 gsm = 130.4 µm | 75 µm = 69.0 gsm |
| 150 gsm = 163.0 µm | 100 µm = 92.0 gsm |
| Formulas: t(µm) = G(g/m²) / density(g/cm³); G(g/m²) = density(g/cm³) × t(µm). | |
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GSM to Thickness Formula
The core relationship between GSM, density, and thickness is derived from the definition of areal density. GSM (grams per square meter) represents the mass of material distributed across a flat area, while density represents how tightly that mass is packed into a given volume. Thickness is the third variable that closes the equation.
Where grammage is in g/m², bulk density is in g/cm³ and thickness is in µm. No additional factor of 1000 is used for these units. In SI units, t(m) = (G/1000)/ρ, with G in g/m² and ρ in kg/m³. Choose the value to solve for, enter the two required quantities, and select Calculate.
This formula assumes an even sheet. Porous products require the bulk density of the finished sheet, including its internal air space, rather than the solid density of the constituent material. The calculator’s material examples illustrate the arithmetic and are not specifications for a particular product.
What is GSM?
GSM stands for grams per square meter and is the international standard unit for expressing the areal density (also called basis weight or grammage) of sheet materials. It is governed by ISO 536 for paper and board and is the primary specification buyers and manufacturers reference when sourcing paper, plastic film, nonwoven fabric, geotextiles, and packaging substrates. A higher GSM value indicates more mass per unit area, which generally correlates with a thicker and more rigid material, although the actual thickness depends heavily on the material's density.
In North America, paper weight is often expressed in pounds per ream (basis weight), where the ream size varies by paper grade. For bond/writing paper, a ream is 17 x 22 inches (500 sheets), so 20 lb bond is roughly 75 gsm. For cover stock, a ream is 20 x 26 inches, so 80 lb cover is approximately 216 gsm. The GSM system eliminates this confusion by using a single universal measurement area of one square meter regardless of paper grade.
Material Density Reference for GSM Conversions
The accuracy of a GSM-to-thickness conversion depends on the finished product’s bulk density. The ranges below are approximate examples, not guaranteed design or purchasing values. Obtain a supplier specification or derive bulk density from measured grammage and caliper for the actual sheet.
Paper and Paperboard
Newsprint typically falls between 0.57 and 0.62 g/cm³ due to its high-porosity groundwood pulp. Uncoated woodfree office paper (the standard copy paper found in most printers) ranges from 0.70 to 0.80 g/cm³. Coated papers, where a mineral pigment layer such as calcium carbonate or kaolin clay is applied to the surface, are denser: single-coated papers sit around 0.90 to 1.05 g/cm³, and double-coated glossy papers can reach 1.05 to 1.15 g/cm³. Kraft linerboard used in corrugated packaging ranges from 0.60 to 0.75 g/cm³, while the fluting medium inside corrugated board is even less dense at roughly 0.55 to 0.65 g/cm³. Tissue paper is among the lowest-density paper products at 0.10 to 0.20 g/cm³ because of its highly bulked fiber structure.
Plastic Films
Plastic films are generally non-porous, so their bulk density closely matches the polymer's solid density. LDPE (low-density polyethylene), the most common grocery bag and stretch-wrap material, has a density of 0.91 to 0.93 g/cm³. HDPE (high-density polyethylene) is slightly denser at 0.94 to 0.97 g/cm³. Polypropylene (PP), widely used in food packaging and BOPP labels, falls between 0.89 and 0.91 g/cm³, making it the lightest of the commodity plastics. PET (polyethylene terephthalate), used in bottles and thermoformed trays, has a substantially higher density of 1.33 to 1.40 g/cm³. PVC (polyvinyl chloride) ranges from 1.30 to 1.45 g/cm³ for rigid formulations and can be lower when plasticized. Nylon (PA6) films sit around 1.12 to 1.15 g/cm³.
Nonwoven Fabrics and Textiles
Nonwoven bulk density varies widely with fiber, loft, bonding and compression. Spunbond, meltblown, needle-punched and thermal-bonded products cannot be assigned a reliable density from their construction name alone. Use the finished product’s measured or specified density.
Why Two Materials with the Same GSM Have Different Thicknesses
A 100 gsm sheet of standard copy paper (density approximately 0.75 g/cm³) will measure about 133 µm (0.133 mm) thick. A 100 gsm LDPE plastic film (density approximately 0.92 g/cm³) will only be about 109 µm thick. And a 100 gsm spunbond polypropylene nonwoven (bulk density approximately 0.15 g/cm³) will be roughly 667 µm (0.67 mm) thick. All three weigh the same per square meter, but the nonwoven is nearly five times thicker than the copy paper and six times thicker than the plastic film. This illustrates why GSM alone is never sufficient to specify thickness; you must account for how the material's internal structure distributes that mass through its cross-section.
GSM Ranges by Application
Different end-use applications require specific GSM ranges because of the mechanical, optical, and tactile properties needed. Thermal receipt paper is typically 48 to 65 gsm. Standard office copy paper is 75 to 90 gsm. Letterhead and premium stationery range from 90 to 120 gsm. Brochure and flyer paper is 130 to 170 gsm. Magazine covers and promotional cards use 200 to 250 gsm. Business cards and postcards require 270 to 400 gsm for sufficient rigidity. Corrugated box liner uses 125 to 440 gsm depending on flute type. For plastic packaging, grocery bags are 15 to 30 gsm HDPE film, food-grade cling wrap is 8 to 15 gsm LDPE, and heavy-duty shipping bags are 70 to 120 gsm LDPE. In nonwovens, disposable surgical gowns use 35 to 45 gsm SMS fabric, agricultural row covers range from 17 to 50 gsm spunbond, and geotextile drainage fabric can range from 100 to 600 gsm needle-punched polyester.
Measuring Thickness: Standards and Methods
Physical thickness of sheet materials is measured using a dead-weight micrometer that applies a controlled static load to a sample between two parallel platens. The two dominant international standards for this measurement are TAPPI T 411 (used primarily in North America) and ISO 534 (used internationally). These standards differ in the applied pressure, platen diameter, and measurement dwell time, which means a single sheet of paper can yield different thickness readings depending on which standard was used. TAPPI T 411 uses a 50.0 kPa load with a 16.0 mm diameter platen, while ISO 534 uses a 100 kPa load with a 16.0 mm platen for single-sheet measurement. The higher pressure in ISO 534 compresses the sample slightly more, so ISO readings for bulky materials tend to be slightly lower than TAPPI readings for the same sheet.
For plastic films, ASTM D6988 provides guidance for determining test-specimen thickness; ISO 4593 also addresses film and sheet thickness by mechanical scanning. Use the material specification and applicable test method for the required instrument, pressure, resolution and sampling. Nonwoven methods such as ISO 9073-2 also depend on specimen structure; a universal pressure or resolution does not apply to every sheet.
Bulk Density vs. Solid Density
When using the GSM to thickness formula, the most common source of error is confusing solid-phase density with bulk density. Solid-phase density is the density of the raw material itself without any air pockets or internal porosity. Bulk density is the effective density of the finished sheet, including all internal voids. For non-porous materials like extruded plastic films, these two values are essentially the same. For porous materials such as paper, nonwovens, and foamed sheets, they can differ by an order of magnitude.
Cellulose fibers have a solid density of approximately 1.50 g/cm³, but a sheet of 80 gsm copy paper has a bulk density of about 0.75 g/cm³ because roughly 50% of the sheet volume is air trapped between fibers. Using the solid fiber density (1.50) in the calculator would predict a thickness of only 53 µm, while the actual measured caliper of 80 gsm copy paper is about 100 to 110 µm. Similarly, polypropylene spunbond nonwovens use a polymer with a solid density of 0.90 g/cm³, but the fabric's bulk density can be as low as 0.12 g/cm³ because the fabric is mostly air. Always use the bulk density of the finished product, not the density of the raw material, when performing these conversions.
Factors That Affect the GSM-to-Thickness Relationship
Calendering compresses a sheet and changes its bulk density, so sheets of the same grammage may have different thicknesses. Moisture and coating also affect mass, dimensions and pore structure. Their effects are product dependent; a single percentage rule cannot reliably predict the caliper change.
For plastic films, the orientation process matters. Biaxially oriented polypropylene (BOPP) is stretched in both the machine and transverse directions, which increases the polymer chain alignment and very slightly increases density compared to cast PP film. The blown film process used for LDPE bags produces a slightly different crystallinity (and therefore density) than the cast film process. Temperature during extrusion and the speed of the chill roll also influence the final film density by controlling the degree of crystallization. These variations are small for plastics (typically less than 2%), but they can be significant in precision applications like capacitor films or optical films where thickness tolerances are specified in fractions of a micrometer.
Unit Conversion Reference
One micrometer (µm) is 0.001 mm or 0.0001 cm. One inch is exactly 25,400 µm, and one mil is 25.4 µm. A paper caliper point often means one mil; this differs from a typographic point (1/72 inch). For density, 1 g/cm³ = 1000 kg/m³ ≈ 62.428 lb/ft³.
