What GSM actually measures
GSM is mass per unit area. Take a known area of fabric, weigh it, divide the weight by the area in square metres, and express the result in grams. That is the entire measurement.
The simplicity is the problem. Everything that makes the measurement useful or useless sits in the choices made around it:
- What area was cut, and from where on the roll. Fabric mass varies across the width of a roll and along its length, and it varies more in some constructions than others.
- What condition the fabric was in. Greige (unfinished) fabric, fabric after dyeing, and fabric after washing and compacting all have different masses. A supplier quoting greige weight and a buyer measuring finished weight will disagree by a predictable margin — and each will believe the other is wrong.
- Which method was used. ASTM D3776 and ISO 3801 are not interchangeable procedures. Most contracts specify one, and most disputes happen where the contract specified none.
Why buyers care, and where the number misleads
GSM is used as a shorthand for a set of properties that buyers actually want: opacity, drape, durability, warmth, and the perceived “substance” of a garment.
It correlates with those things loosely. It does not determine them.
A few worked comparisons make the point:
| Fabric | GSM | What the number tells you | What it does not |
|---|---|---|---|
| Single jersey, combed cotton | 160 | Lightweight, suitable for summer T-shirts | Whether the yarn is combed or carded |
| Single jersey, carded cotton | 160 | Identical mass | Noticeably rougher surface, lower durability, cheaper |
| Cotton–polyester, 65/35 | 160 | Identical mass | Different shrinkage, pilling and moisture behaviour |
| Interlock, combed cotton | 220 | Heavier, more opaque, less stretch | That it is warmer — that depends on the construction, not the mass |
The left column is what a specification controls. The right column is what a specification without fibre content and construction leaves to chance.
The three states of a fabric weight
For any fabric that is dyed and finished, there are at least three different weights, and the difference between them is routinely 5–12%:
- Greige weight — measured off the loom or knitting machine, before any wet processing.
- Finished weight — measured after dyeing, washing, drying and any compacting or heat-setting.
- After-wash weight — measured after the garment is laundered under the conditions the end consumer will use.
Knitted cotton fabrics commonly lose weight through processing; some finished and compacted fabrics regain mass through the finish applied. Either way, the number a supplier quotes first is usually the flattering one, and the number a buyer measures is usually the one that matters commercially.
Practical rule: specify the weight at a named stage, using a named method, with a stated tolerance. “180 GSM ± 5% finished weight, ISO 3801” is a specification. “180 GSM” is an opening position in a negotiation.
Tolerances and what to put in a contract
Fabric weight is a variable, not a constant, so a specification needs a tolerance. Common contract practice for knitted fabric sits around ±5%, and for woven fabric around ±3–5%, but the appropriate figure depends on the construction and on what the garment is for.
Two things are worth writing into the purchase order beyond the number itself:
- The measurement method and specimen size. This is what makes an independent test result comparable to a supplier’s claim.
- The consequence of a failed test. Whether a roll outside tolerance is rejected, renegotiated or accepted with an adjustment determines whether the tolerance was real.
GSM in the garment context
For reference, these are the ranges generally encountered across common garment types. They are given as orientation, not as a standard — actual choices vary widely and should come from the fabric supplier’s own test data.
| Garment | Typical fabric | Typical GSM range |
|---|---|---|
| Light summer T-shirt | Single jersey, cotton or blend | 120–160 |
| Standard T-shirt | Single jersey, cotton | 160–200 |
| Heavyweight T-shirt | Single jersey or interlock | 200–260 |
| Polo shirt | Piqué | 180–240 |
| Hooded sweatshirt | French terry or fleece | 280–400 |
| Denim jeans | Twill, cotton or stretch | 300–450 |
| Shirting | Poplin or broadcloth | 100–150 |
| Shirting, heavier | Oxford or twill | 140–200 |
| Suiting | Worsted woven | 200–320 |
| Bedsheeting | Plain weave percale | 110–140 |
Ranges are presented as typical commercial practice. They are not measurements taken from a specific source and should be treated as an orientation figure until a supplier’s own test report replaces them.