How to Calculate Fabric GSM
Fabric GSM is the weight of one square metre of fabric in grams, written as g/m². To calculate it, cut a piece of known area, weigh it on a precise scale and divide the weight by the area. Denim weight is often quoted in ounces per square yard. One oz/yd² is about 33.9 g/m².
The GSM formula
GSM (g/m²) = Sample weight (g) ÷ Sample area (m²)
When the area is in square centimetres, convert it first: 100 cm² = 0.01 m². With a 100 cm² sample, multiply the weight in grams by 100 to get g/m².
How do you measure fabric GSM step by step?
You measure fabric GSM by cutting a sample of known area and weighing it on a scale that reads to 0.01 g. You need a cutting tool (a circular GSM cutter, or a ruler and scissors), the scale and a flat table.
A quick check outside a lab is useful for screening. When there is a dispute on an order, a lab report to a named standard is what counts.
- Lay the fabric flat, without tension, and leave it in the room for at least a day. When moisture changes, weight changes.
- Take the sample away from the selvedge, close to the middle of the roll.
- Cut a piece of known area. Circular cutters usually cut 100 cm². A 10 × 10 cm square cut with scissors does the job too.
- Weigh the piece and note the grams.
- Divide the weight by the area. Cut at least three samples from different spots and use the average.
Worked example
A 100 cm² circular sample weighs 3.40 g.
- 3.40 g ÷ 0.01 m² = 340 g/m²
- 340 ÷ 33.906 = about 10 oz/yd²
If a 20 × 25 cm piece cut with scissors weighs 17.2 g, the area is 500 cm², or 0.05 m². 17.2 ÷ 0.05 = 344 g/m².
Why scale precision changes the result
On a 100 cm² sample, every 0.1 g on the scale equals 10 g/m² in the result. A scale with one decimal place can read the same fabric as 340 or as 350 g/m². On denim, that is a gap of about 0.3 oz. Small samples need a scale that reads to two decimal places (0.01 g).
Which standards define fabric weight testing?
ISO 3801, EN 12127 and ASTM D3776 define how fabric weight is tested. ISO 139 sets the conditions the fabric sits in before the test. For woven fabric, labs work to ISO 3801; for small samples, to EN 12127. Buyers in the United States usually ask for ASTM D3776.
| Standard | Used for |
|---|---|
| ISO 3801 | Woven fabric weight per metre and per m², 5 methods |
| EN 12127 | Small samples, 5 pieces of 100 cm² |
| ASTM D3776 | 4 options: roll, full width, small swatch, narrow fabric |
| ISO 139 | Conditions for the fabric before testing |
ISO 139 sets the standard atmosphere at 20 ± 2 °C and 65 ± 4 % relative humidity. If both parties agree, 23 °C and 50 % humidity can be used instead, with the same tolerances. The fabric stays in this atmosphere until weighings 2 hours apart change by no more than 0.25 %.
Under the small-swatch option of ASTM D3776, the result applies only to that swatch. It is not assumed to represent the whole lot. The weight of a swatch card is not the weight of the roll.
How do you convert oz/yd² to g/m²?
To convert oz/yd² to g/m², multiply by 33.906, because 1 oz/yd² equals 33.906 g/m². This conversion factor comes from the definitions of the ounce (28.3495 g) and the square yard (0.8361 m²).
- oz/yd² to g/m²: multiply by 33.906.
- g/m² to oz/yd²: divide by 33.906.
Common denim weights in g/m²
| oz/yd² | g/m² | g/m (150 cm) |
|---|---|---|
| 6 | 203 | 305 |
| 8 | 271 | 407 |
| 10 | 339 | 509 |
| 12 | 407 | 610 |
| 14 | 475 | 712 |
| 16 | 542 | 814 |
| 18 | 610 | 915 |
| 21 | 712 | 1,068 |
The last column is the weight of one running metre of fabric 150 cm wide. Values are rounded to whole numbers.
Round g/m² values in oz/yd²
| g/m² | oz/yd² | g/m (150 cm) |
|---|---|---|
| 200 | 5.9 | 300 |
| 250 | 7.4 | 375 |
| 300 | 8.8 | 450 |
| 350 | 10.3 | 525 |
| 400 | 11.8 | 600 |
| 450 | 13.3 | 675 |
| 500 | 14.7 | 750 |
What is the difference between weight per metre and GSM?
Weight per metre is the weight of one running metre of the roll, so it depends on fabric width. GSM does not. Some fabric suppliers quote weight per running metre (g/m) instead of per square metre. To compare the two, use the width:
g/m² = Weight per metre (g/m) ÷ Fabric width (m)
A fabric 150 cm wide that weighs 510 g per metre is 510 ÷ 1.50 = 340 g/m². The same weight per metre on a 140 cm width is 364 g/m². A weight per metre with no width is not enough to compare offers. Ask whether the width includes the selvedge or is the cuttable width.
Does washing change fabric weight?
Yes, washing changes fabric weight. Denim shrinks in the wash, so the same fibre sits in a smaller area and the weight per square metre goes up. Stone and abrasion processes remove fibre and can bring the weight down.
Which effect wins depends on the fabric and the wash recipe. No single percentage applies.
A swatch card does not always say whether its weight is before or after washing, so ask the supplier. If the finished garment has a target weight, the order documents should state two values:
- fabric weight before washing,
- expected weight after washing, with the accepted tolerance.
Why does the swatch weight differ from the roll?
When a swatch and the delivered roll show different weights, look at the measuring conditions first. Check these in order before you call the supplier:
- Where was the sample cut? A piece taken near the selvedge weighs differently from one taken in the middle of the roll.
- Was the fabric conditioned? Fabric from a damp warehouse weighs more than fabric that has rested in a dry room.
- Which width was used? When GSM is calculated from weight per metre, full width and cuttable width give different results.
- Is the figure before or after washing? Two values can only be compared in the same state.
- Is it the same lot? Rolls from different dye or weaving batches can differ.
- How many decimal places does the scale show? On a 100 cm² sample, 0.1 g equals 10 g/m².
If all six answers match and the gap remains, send the fabric for a lab test to a named standard.
How should fabric weight appear on an order?
On an order, write fabric weight with its unit, wash state, tolerance and test method. Weight affects fabric price, drape, and how needles and machines are set for sewing. Heavier fabric also means a heavier finished garment, and freight charged by weight rises with it.
- Unit: g/m² or oz/yd². If both appear, state which one governs.
- State: before or after washing.
- Tolerance: the accepted lower and upper limit.
- Method: the test standard, for example ISO 3801 or ASTM D3776.
Fabric weight standards describe how to test. They do not set an acceptance tolerance. The buyer and the supplier agree the tolerance and write it into the order.
Our denim production page shows how we cut and sew light and heavy denim. Woven fabrics such as chino and gabardine are on our woven and non-denim production page.
You can see styles in different weights in the women's denim trousers and men's denim trousers collections. Explore our production line.
Frequently asked questions
What does GSM mean?
GSM stands for grams per square metre. A 300 GSM fabric weighs 300 grams per square metre.
Can you find fabric GSM without a scale?
Not precisely. With the supplier's weight per metre and the fabric width, you can calculate it with the formula above. Estimating by hand feel or thickness is not good enough for an order.
How many grams is 12 oz denim?
12 oz/yd² denim is about 407 g/m² (12 × 33.906).
Sources
- ISO 3801:1977, Textiles, Woven fabrics, Determination of mass per unit length and mass per unit area
- ISO 139:2005, Textiles, Standard atmospheres for conditioning and testing
- EN 12127:1997, Textiles, Fabrics, Determination of mass per unit area using small samples
- ASTM D3776/D3776M-20 (2025), Standard Test Methods for Mass Per Unit Area (Weight) of Fabric
- NIST, definitions of the ounce and the yard (SI conversions)
