To run a GSM test on fabric, cut a sample of known area, weigh it on a precise scale, and convert the result to grams per square metre. A round cutter of 100 square centimetres makes this simple: the weight in grams multiplied by 100 is the GSM.
Many buyers set a tolerance of plus or minus five per cent unless the contract says otherwise. Whatever you choose, write it in the specification so that nobody argues about it later.
How do you test GSM step by step?
Standard methods exist, such as ISO 3801 for woven fabrics and ISO 12127 for small samples, and the same principle applies in all of them.
- Condition the fabric in a standard atmosphere, since moisture changes the weight. Cotton in particular can gain or lose weight with humidity.
- Take samples from several places across the width and along the length. Avoid the first metre or two of a roll and the area within a few centimetres of the selvedge.
- Cut each sample with a GSM cutter or a template, and check that the cut is clean, with no stretched edges.
- Weigh each sample on a balance accurate to at least 0.01 gram.
- Calculate GSM for each sample and average the results.
What tolerance should you allow?
A tolerance of about five per cent is common in the trade, but it is a convention and not a law. Light fabrics may need a tighter percentage because small weight differences are visible in drape and opacity. Heavy canvas and denim often vary more.
Decide whether the tolerance applies to the whole lot average or to each roll. A lot average of 150 GSM with a five per cent tolerance can hide individual rolls that are well outside it, so say how many samples you will test.
Which mistakes distort the result?
Most disputes come from method, not from the fabric.
- Testing finished fabric against a greige specification, or the reverse; finishing changes weight.
- Weighing fabric that has not been conditioned, or that has been left in a damp warehouse.
- Using a hand-cut square with uneven edges, instead of a cutter.
- Reporting GSM when the contract specifies ounces per square yard, without converting properly.
What does GSM tell you, and what does it not?
GSM is a weight, not a quality grade. Two fabrics of 180 GSM can feel and perform very differently, depending on yarn count, construction and finish. Read what GSM means and how to choose the right fabric weight for how to specify it with other properties, and see reading a laboratory certificate for what to check on a test report.
How do you convert between GSM and ounces?
Some markets, especially for denim and canvas, specify weight in ounces per square yard. One ounce per square yard is about 33.9 GSM, so a 12 oz denim is about 407 GSM.
Always convert with the same factor on both sides of a contract, and write both numbers in the specification. Denim weights in oz available from Indian mills gives more examples.
Next step
We have lots tested at independent laboratories and check that the method and the tolerance match your specification. To set this up for your fabric, book a free sourcing review.