GSM is one of the first numbers a merchandiser or developer writes on a fabric specification, and one of the first checks a laboratory is asked to make. It is also one of the easiest to misunderstand. The letters stand for grams per square metre: the mass of the cloth divided by its area. Two fabrics can share the same GSM and still differ in thickness, drape, stretch and cover, because GSM is only mass per unit area. It is not a grade of quality, and it is not the same as stitch density on a knitted fabric.
This article explains what the unit means, how to calculate it from a cut specimen, which official test methods apply to woven and knitted cloth, and why the state of the fabric (relaxed or stretched, grey or finished, conditioned or oven-dry) must be stated with the result. Worked numbers are arithmetic illustrations or conversions from a named official factor, not invented product ranges. For how laboratories approach fabric tests in general, see Fabric Testing: Methods, Equipment and What Results Mean. If you are relating fabric weight to stitch density or yarn count on a knitting machine, see Knitting Machine Gauge Explained: Needles, Yarn and Fabric.
What GSM means
Mass per unit area is an SI quantity. When the mass is expressed in grams and the area in square metres, the result is in grams per square metre (g/m²). In merchandising and mill language this is almost always called GSM. The same quantity can be reported in ounces per square yard, which is still common for denim and some exported woven cloths; that conversion is covered later.
The idea is simple. Weigh a specimen whose area you know, then divide. In symbols:
GSM = mass / area
with mass in grams and area in square metres. If the area is more convenient to measure in square centimetres, the same relationship is:
GSM = mass × 10,000 / area
because one square metre is 10,000 square centimetres. That factor is arithmetic from the unit, not a special textile constant.
A specimen of 100 cm² has an area of 0.01 m², so dividing by 0.01 is the same as multiplying by 100. If such a specimen has a mass of 1.80 g, the GSM is 180 g/m². That 1.80 g figure is only an illustration of the arithmetic. Always use the area you actually cut. If you use a circular cutter, use the area marked on the cutter, not an assumed 100 cm².
How to measure it in the laboratory
A shop-floor weighing of an unconditioned offcut can be a useful check, but it is not a substitute for a named test method. Three current standards are widely cited.
ISO 3801:1977, confirmed in 2023 and adopted in the UK as BS 2471:2005, is written for woven fabrics. Its official abstract notes that, for some fabrics, mass per unit length and mass per unit area are related simply by the width, but that variations in structure can make the two quantities distinct. The methods apply to woven fabrics, including stretch types, made up full width or folded down the middle, and to complete pieces as well as sample lengths.
For small specimens, including knitted fabrics, the usual European method is EN 12127:1997, published in the UK as BS EN 12127:1998. The official scope states that it describes methods for the determination of the mass per unit area using small samples in the standard atmosphere for testing and/or oven-dry, and that it is applicable to woven and knitted fabrics and may also be applicable to fabrics produced by other techniques.
In the United States, ASTM D3776/D3776M covers the measurement of fabric mass per unit area (weight) and is applicable to most fabrics. The NBS record notes that the standard was reapproved in 2025. Use the edition named in the purchase order or laboratory quality system.
The atmosphere itself is defined, for textile testing, by ISO 139:2005, including Amendment 1 (2011), also published as BS EN ISO 139:2005+A1:2011. The official scope is that the standard defines the characteristics and use of a standard atmosphere for conditioning, for determining the physical and mechanical properties of textiles, and a standard alternative atmosphere that may be used if agreed between the parties. Quote the current ISO 139 values from the standard itself rather than from memory or from an old textbook; they are part of the method, not a detail to invent.
How those methods are applied in a laboratory can be stated more concretely without copying a paid standard in full. FILK, an accredited German laboratory, describes its DIN EN 12127 procedure as follows. The fabric is conditioned for at least 24 hours in a relaxed, flat state under the specified standard atmosphere. At least five representative specimens, each of at least 100 cm², are prepared. They are cut with a cutting device or with scissors; if scissors are used, the actual length and width are measured to obtain the area. Creases, distortions and unrepresentative areas are avoided. Each conditioned specimen is weighed on a suitable balance. Alternatively, or in addition, an oven-dry mass may be determined after drying at 105 plus or minus 3 degrees Celsius until constant mass is reached. Mass per unit area is calculated from mass and area, expressed in g/m², and reported as a mean; variability and minimum and maximum values may also be given where required.
That description is FILK’s account of an accredited method, not a substitute for the text of EN 12127. Follow the method named in the specification you are working to. Specimen numbers, conditioning times and whether an oven-dry result is required can differ.
Converting ounces per square yard
In some trades, especially denim, mass per unit area is still quoted in ounces per square yard (oz/yd²). The official conversion comes from the NIST Guide to the SI, Appendix B.8: to convert ounce (avoirdupois) per square yard into kilogram per square metre, multiply by 3.390575 × 10⁻². Multiplying that factor by 1,000 to express the result in grams per square metre gives:
g/m² = oz/yd² × 33.90575
The labels on the chart below are calculated from that factor and rounded to the nearest gram per square metre. They are not measured fabrics. A figure often used in jeans specifications, 14 oz/yd², corresponds to 475 g/m² on that rounding. How denim weight is used in the trade is discussed in Jeans Fabric: Denim Twill, Indigo Warp and Weight.
Do not mix the two unit systems in one calculation, and do not treat a rounded conversion as more precise than the weighing.
What changes the number
Because GSM is mass divided by area, anything that changes either term will change the result.
Moisture is the most common reason two weighings of the same cloth disagree. A fabric taken from a humid store or straight off a stenter is not in the same state as a specimen that has been conditioned in the standard atmosphere of ISO 139. An oven-dry result, where the method asks for one, will be lower than the conditioned result because the moisture has been driven off. Always say which state was used.
Area is just as important as mass. A knitted fabric that is still under take-down tension, or a woven fabric pulled wide on a stenter, has a larger area than the same cloth after relaxation, so it will read lighter. EN 12127 is written for small samples of woven and knitted fabrics; knitted cloth may need a prior relaxation, agreed between the parties, before the conditioned weighing. Stitch density, yarn count and loop length all influence knitted GSM, but they are not themselves a GSM measurement. Those relationships are discussed in the machine-gauge article.
The finishing state must be reported. Grey, dyed, coated, raised, compacted or washed fabrics of the same greige construction will not necessarily have the same mass per unit area. Residual size, moisture, coating and cropped fibre all add or remove mass. If a buyer specifies a finished GSM, a grey GSM from the knitting or weaving shed is the wrong number to ship against.
Sampling matters. ISO 3801 distinguishes mass per unit length of a piece from mass per unit area, because structure can vary across the width. Selvedges, crease lines and unrepresentative areas should be avoided on small specimens, as FILK notes for EN 12127. A result from one small swatch describes that swatch; it does not automatically describe the whole roll.
What GSM is not
GSM is not thickness. A dense, compact fabric and a lofty, raised fabric can share a mass per unit area and feel entirely different in the hand. It is not opacity, stiffness or bursting strength, although those properties often change when the construction that produces the GSM changes. It is not, on its own, a measure of quality.
It is also not a substitute for a construction formula. Mills sometimes estimate GSM from thread count, yarn linear density and crimp, or, for single jersey, from stitch length and stitch density. Those estimates are planning tools. They omit finish add-on, moisture and any change of width in dyeing and finishing. When an estimate and a measured GSM disagree, the measured, conditioned value against a named method is the one to report. Garment merchandisers who put GSM on a fabric order should also name the method, the fabric state and the tolerance, not only a single number.
Paper and board are also sold by grammage in grams per square metre. ISO 536:2019 specifies a method for determining the grammage of paper and board. The unit is the same; the test method is not. Do not use a paper method for a textile, or a textile method for a sheet of paper, and then compare the numbers as if they were obtained in the same way.
FAQ
What does GSM stand for? Grams per square metre. It is the mass of the fabric divided by its area, reported in g/m².
How do I calculate GSM from a 10 cm by 10 cm swatch? A 10 cm by 10 cm square has an area of 100 cm², or 0.01 m². GSM is the mass of that swatch in grams, multiplied by 100. The same shortcut applies only when the area really is 100 cm². If you cut another size, use mass × 10,000 / area in cm².
Is a GSM cutter required? No. EN 12127, as described by FILK, allows a cutting device or scissors, provided the area is known. A circular cutter is convenient because its area is fixed and marked, but the marked area must be the one used in the calculation. Conditioning and a suitable balance matter at least as much as the cutter.
Which standard should I name on a test report? Name the method required by the specification. For small samples of woven or knitted fabric in Europe this is often EN 12127. For woven pieces, ISO 3801 or BS 2471 may be specified. In many US programmes, ASTM D3776 is specified. If the document is silent, agree the method with the customer before you test.
Why is my result different from the mill’s ticket? Common reasons are a different fabric state (grey against finished, or conditioned against oven-dry), incomplete relaxation, a specimen that includes a selvedge or a crease, a cutter area that was assumed rather than read, or a ticket that used ounces per square yard without conversion. Check those points before you assume the cloth is out of specification.
Can I read GSM from yarn count alone? No. Yarn count is linear density of the yarn, not mass per unit area of the fabric. You also need the construction, and after wet processing you need a measurement. Use a named method on a conditioned specimen.
Sources
- ISO 3801:1977 (EVS catalogue; official abstract; confirmed 2023) and BS 2471:2005 (BSI Knowledge).
- EVS-EN 12127:2000 / EN 12127:1997, and BS EN 12127:1998 (official scope: small samples, standard atmosphere and/or oven-dry, woven and knitted fabrics).
- FILK, DIN EN 12127 accredited procedure.
- ASTM D3776/D3776M, NBS publication record (applicable to most fabrics; reapproved 2025).
- EVS-EN ISO 139:2005 and EVS-EN ISO 139:2005/A1:2011; BS EN ISO 139:2005+A1:2011.
- NIST Guide to the SI, Appendix B.8 (ounce avoirdupois per square yard to kilogram per square metre: 3.390575 × 10⁻²).
- ISO 536:2019 (grammage of paper and board).
No comments:
Post a Comment