Does a "same fabric" quote mean the same GSM after washing?
A GSM figure on a quote is a starting point. Without a test method and a laundering condition, two quotes for the "same" fabric are not comparable.
We received two quotes for a 180 GSM cotton jersey T-shirt. The prices are close, and both factories say it is the same fabric. Our last order felt thinner after a few washes. How do we know the weight is really the same, before and after washing?
Main answer
Not necessarily. A GSM figure tells you the mass per square metre of a fabric under one set of conditions. It does not tell you how the fabric was measured, whether it was measured in greige or finished state, or what happens to it after laundering. Two quotes can carry the same number and still describe fabric that behaves differently on the shelf and in the customer's wash.
Ask how the weight was measured
Fabric weight is normally measured as mass per unit area, for example under ASTM D3776/D3776M. The result depends on how the specimen was taken and conditioned. Ask each supplier which method they used, whether the figure is for finished fabric, and what tolerance they quote. A quote that says "180 GSM" with no method or tolerance gives you nothing to compare against at inspection.
It also helps to request the fabric's construction details: fibre content, yarn count and knit structure. Two jerseys can reach the same weight with different yarns, and that changes hand-feel and durability even when the GSM matches.
Separate weight from wash behaviour
Feeling thinner after washing is usually a question about dimensional change and surface change, not about the original GSM figure. Shrinkage can make a fabric denser, and surface wear can make it look thinner. Those are measured by different tests. Laundering procedures such as ISO 6330, or AATCC's home-laundering procedures used in methods like TM150 for garments, give a repeatable wash so you can measure before and after rather than relying on feel.
If your issue is "thinner after five washes", write that into the test request: measure the fabric weight and dimensions before washing and again after an agreed number of cycles under a named procedure. Agree the number of cycles with your lab and the factory. No standard sets one for you.
Write it into the order
The practical fix is a specification, not a better quote. State the fibre content, construction, finished weight with method and tolerance, the laundering procedure and the number of cycles. Then approve a sealed fabric sample. When bulk fabric arrives, it is checked against that sample and those numbers, not against the word "same".
If the two factories cannot tell you the method behind their GSM figure, that tells you something too. You are then comparing prices, not fabrics.
Sources and further reading
- ASTM D3776/D3776M — Mass per Unit Area (Weight) of Fabric
- ISO 6330:2021 — Domestic washing and drying procedures for textile testing
- AATCC — Changes in the 2026 Manual (TM150, TM179, TM207 revisions)
Standards and rules are summarised from their published scope or official guidance. Use the current edition and confirm requirements for your market.
Three perspectives on this question
Purchasing perspective
I ask both suppliers to restate the quote with the test method and tolerance next to the GSM figure. If one price is lower, I want to know whether it is the same construction or a lighter yarn that reaches the same weight. That question is cheaper to ask before the deposit than after the shipment.
Merchandising perspective
For the line plan, I keep the approved fabric sample and its test sheet together under one reference. The customer's complaint was about how the tee felt after washing. So I add a washed sample to the approval set, not only a new one. Then the buying team can see what they actually signed off.
QC perspective
At incoming inspection we cut specimens from the bulk roll and weigh them using the same method named on the order, then compare with the sealed sample. If wash behaviour is part of the spec, we send specimens for the agreed laundering procedure. A tolerance with no method behind it cannot be checked, so we ask for the method first.
Working on a similar program?Send your tech pack, fabric references and test requirements. We reply with a quote and the questions we need answered.
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