How GSM is measured
The quick factory method uses a round GSM cutter that punches a disc of 100 cm² from the fabric. The disc is weighed on a balance reading to 0.01 g, and the weight is multiplied by 100 to give grams per square metre. Several discs are cut across the width, avoiding the selvedge, and the results are averaged.
Formal methods include ASTM D3776 (mass per unit area of fabric) and ISO 3801 (woven fabrics). Both expect the fabric to be conditioned first, because cotton, wool and jute take up moisture from the air. In a humid monsoon week a cotton towel can read heavier than the same towel in a dry, air-conditioned lab. For a dispute, an accredited lab tests conditioned samples to the standard named in the order.
Rugs and carpets are often specified by pile weight in ounces per square yard; 1 oz/yd² is about 33.9 g/m². Paper and board use the same idea, called grammage.
Worked example: a towel order that came in light
A buyer orders bath towels at 550 GSM with a tolerance of ±5%, so the accepted range is 522.5 to 577.5 GSM. The inspector cuts discs from five towels taken from different cartons. Average results: 548, 541, 515, 519 and 552 GSM. Two towels are below 522.5. The inspector checks the carton marks and finds the two light towels come from one dyeing lot. The report records each reading with photos of the balance, and the result goes to the buyer as a finding on GSM, with the lot identified, so the buyer can decide on that lot before shipment.
Light towels are usually a cost saving in yarn or pile height, and they also absorb less water. GSM is therefore a quality check, not only a number on the label.
How Clear To Load uses it
On towels, bed linen, greige and finished fabric, the inspector cuts and weighs samples during a pre-shipment inspection and reports every reading against your specification. Put the target GSM, the tolerance and the test method in your purchase order; without a tolerance, a small shortfall is hard to call.