There are two accepted methods, and they answer different questions. Measure a swatch when you want to know what you produced. Calculate from loop geometry when you want to decide what to produce.
This is the standard laboratory method and the one most quality departments use daily.
A 100 cm² swatch weighing 1.6 g is 160 GSM. The arithmetic is trivial; the discipline is not. Take five cuttings from different points across the width and along the length, then average — a single reading cannot tell a real weight problem apart from ordinary local variation.
This derives weight from the loop structure itself, before any fabric exists. It is how you set stitch length to hit a target GSM.
The constant 1550 is 39.37 squared — square inches in a square metre. Tex is grams per 1000 metres of yarn, which is what turns a length into a weight.
| From | To tex | Example |
|---|---|---|
| Ne (cotton count) | tex = 590.5 ÷ Ne | 30s Ne = 19.7 tex |
| Nm (metric count) | tex = 1000 ÷ Nm | 50 Nm = 20.0 tex |
| Denier | tex = denier ÷ 9 | 180 den = 20.0 tex |
| Structure | Typical GSM | Common use |
|---|---|---|
| Single jersey | 120–200 | T-shirts, innerwear |
| Pique | 180–240 | Polo shirts |
| 1×1 Rib | 180–260 | Cuffs, collars, bodywear |
| Interlock | 200–280 | Premium tees, babywear |
| Fleece / terry | 240–380 | Sweatshirts, hoodies |
There are two accepted methods. The direct method: cut a known area with a GSM cutter (usually 100 cm²), weigh it in grams, and multiply by 100 to get grams per square metre. The calculated method: derive weight from loop geometry using wales per inch, courses per inch, stitch length and yarn count. Use the direct method to verify finished fabric and the calculated method to set the machine before knitting.
For the direct method, GSM = swatch weight in grams ÷ swatch area in square metres. With a standard 100 cm² cutter this simplifies to GSM = weight in grams × 100. For the calculated method, GSM = (WPI × CPI × 1550 × stitch length in mm × tex) ÷ 1,000,000.
A GSM cutter cuts a circular swatch of exactly 100 square centimetres. Weigh that swatch on a precision balance in grams and multiply by 100. A swatch weighing 1.6 g gives 160 GSM. Take five cuttings from different parts of the roll and average them.
Multiply WPI by CPI to get loops per square inch, scale to a square metre by multiplying by 1550, multiply by stitch length in millimetres to get total yarn length, then convert that length to weight using the yarn's tex value. The full expression is GSM = (WPI × CPI × 1550 × SL × tex) ÷ 1,000,000.
Single jersey cotton usually falls between 120 and 200 GSM, with 140–180 the most common apparel range. Interlock and rib run heavier at 180–280 GSM. Fleece and terry structures go higher again.
Tex equals 590.5 divided by Ne, so a 30s Ne cotton yarn is roughly 19.7 tex. The calculator accepts Ne, tex, denier or Nm and converts automatically.
Calculated GSM describes greige fabric straight off the machine. Relaxation, scouring, dyeing, compacting and finishing all change both dimensions and weight, so finished GSM commonly differs by 5–15%. Use the calculated figure to set the machine and the measured figure to confirm what you actually produced.
At least five cuttings from different areas across the width and along the length of the roll, then average. A single reading cannot distinguish a genuine weight problem from ordinary local variation.
The same stitch length and tension settings that determine GSM also determine whether you get holes, drop stitches and lycra breaks. Knit-I watches the fabric as it forms and halts the machine the moment a defect appears — so a setting drift costs you metres, not a whole roll.
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