GSM, Explained Without the Marketing

GSM is grams per square metre. It's the most useful number a brand can print, and the one most often left off.

What it measures

Weigh a square metre of cloth and that's the gsm. Fixed area keeps the comparison honest. A 480gsm loopback has more than twice the material of a 220gsm jersey.

What it does not measure

Fibre quality. Short and long staple cotton weigh the same and wear nothing alike. Short fibres work loose and pill.
Construction. Loopback terry and brushed fleece can hit the same number and feel nothing alike.
Finish. Sizing adds weight and washes out, so the garment lightens after a cycle.
Cut and stitch. Heavy cloth in a badly built garment fails at the seams.

GSM rules things out. It doesn't pick a winner.

Useful ranges, by garment

T shirts. 140gsm is a summer tee you can see through, 180 to 200gsm everyday, 220gsm and up boxy and structured.
Hoodies and sweats. 280gsm is light, 350gsm standard, 420gsm and up properly heavy.
Denim. Measured in ounces, out of habit. 12oz is light, 14oz standard, 16oz and above heavy.
Knitwear. Sold by gauge, but a lambswool crew at 400 to 500gsm is a winter garment.

Compare within a row, never across.

Converting ounces

One ounce per square yard is just under 34gsm. Convert first: 12oz looks smaller than 350 and is heavier.

8oz. About 270gsm. A light sweat.
10oz. About 340gsm. Near a standard hoodie.
12oz. About 405gsm. Light denim, or a heavy sweat.
14oz. About 475gsm.
16oz. About 540gsm.

The other way, divide by 34. A 480gsm hoodie is about 14oz.

The words around the number

Jersey. A plain knit with a smooth face.
Loopback. Loops on the inside. Brushed fleece raises them into a pile.
Rib. Raised lines that stretch and pull back. Cuffs, hems, collars.
Ring spun and open end. Ring spun yarn is finer and smoother. Open end is coarser and faster to make.
Loom state. Cloth as it left the machine, before finishing.

Same number, different fibre

Cotton takes on water, softens and settles into the shape it's given. Polyester in the blend springs back instead, so the garment gives less over a year. A few percent elastane does the same and barely moves the gsm.

Weight is not thickness

GSM counts grams, not air. Warmth comes from trapped air, and loft insulates better at the same weight but crushes flat under a shell. Wear comes from density. Packed mass abrades slower, so a dense 350 outlasts a lofty 400 at the elbows. More on that in does a heavier hoodie last longer.

Weight is not weather protection

A knit is yarn with gaps in it, at 280gsm and 480gsm alike. Heavy cloth slows wind without stopping it, so the same hoodie is warm indoors and thin on a platform. Cotton absorbs rain, and heavier cloth holds more. A heavy hoodie insulates. Something else faces the weather.

Weight is not colour, or pill resistance

Two weights dyed in one lot won't match. Flat jersey reflects light off one plane, a brushed face scatters it, so the same black reads softer. Pills come from fibre, not grams. Short staple works loose at 480gsm as readily as at 220gsm, and density can't hold a fibre too short to stay put.

Heavier is not automatically better

Drape. Very heavy jersey stops falling and starts standing.
Drying. A 480gsm hoodie holds water and takes a day flat on a rack. See air drying a heavy hoodie.

Pick the weight for the job, then ask whether the garment carries it.

What weight does to the fit

Light jersey follows the body. Heavy jersey keeps its own shape, so two garments cut alike sit like different sizes. At 220gsm and up a tee stands off the waist, and sizing down only shortens the body. A 420gsm hoodie needs armhole room for what goes under it. Measure something you already own instead of trusting the letter.

The first month

Heavy cloth arrives stiff and softens where it moves, elbows, cuffs and shoulders first. A brushed pile lies flatter after a few washes and reads smoother than new.

A Canadian year by weight

June to August. 180 to 200gsm jersey. A 220gsm tee holds heat nobody wants in a Toronto August.
Spring and autumn. 280 to 350gsm sweats as the outer layer. What goes over and under matters more than the last 40 grams.
Deep winter. 420gsm and up, as a mid layer under a shell, not as a coat.

Where the weight sits

The figure covers the body cloth. Unquoted parts fail first.

Rib. Cuffs, hem and often the collar are a separate fabric, almost never quoted. Heavy panels pull thin rib open in a season.
Hood. A lined hood is two layers. At 400gsm that's 800 on the back of the neck.

A quoted gsm is a target

Mills knit to a nominal weight and cloth lands around it. Yarn and tension move it either side within one order. Washing, brushing and setting move it again. Read it as a band, not a measurement.

Shrinkage moves it again

Shrinking cloth loses area, not grams, so the gsm climbs as the body shortens and the chest tightens. The first wash pulls both ways: sizing rinses out and drops the weight, while the knit relaxes in and raises the density. Pre shrunk means less movement left, not none. A hot dryer finds the rest.

When a brand quotes the whole garment

Some pages give the finished piece in grams instead of the cloth.

It moves with size. A large uses more cloth than a small, so the figure tracks the size on the scale, not the quality of the knit.
It counts everything. Rib, a lined hood, a zip and the drawcord all land in the total.
It doesn't convert. Without the cloth area there's no gsm in it.

Ask for both, and which size was weighed.

What the extra grams cost

More cloth. A 480gsm hoodie takes more cotton than a 280gsm one cut from the same pattern.
Heavier parcels. Courier rates climb with weight.
Slower to build. Thick cloth wants bigger needles, stronger thread and flat seams where four layers meet.

A heavy garment priced like a light one saved the money somewhere.

What the label has to say

In Canada the label owes two things by law: who the dealer is, and what the fibre is. Any fibre at 5% or more of the mass gets its generic name and percentage. Composition is mandatory. GSM is a choice.

Reading a product page

Heavyweight, no figure. An adjective where a number should be. Treat the weight as unknown.
A figure, no composition. Cotton and a polyester blend at 350gsm wear differently. Half a spec.
A figure, no garment. 220gsm is heavy for a tee and light for a hoodie. Check which row it belongs in.

Comparing two hoodies side by side

Same garment row.
Same composition.
Same state. Loom state flatters a number that washed cloth won't.
The unquoted parts. Rib at cuffs and hem, and a lined or single hood.
The price. If the heavier one costs less, find where the saving went.

Checking it yourself

Without a scale, hold the cloth to a window. Thin jersey shows the gaps between yarns. Hang it off one hand: light cloth collapses over the fingers, heavy cloth keeps a fold.

With one, cut a 10cm square from a swatch or offcut and weigh it dry. That's a hundredth of a square metre, so multiply by 100. Weigh it again after the first wash to see the sizing go.

1.8 to 2.0g. Everyday tee jersey.
2.2g and up. A structured tee.
2.8g. A light sweat.
3.5g. A standard hoodie.
4.2g and up. Properly heavy.

Use a scale that reads to 0.1g, or the rows blur.

What heavy cloth asks for

Fold it, because a hanger pulls all week on two points at the shoulder. Wash it inside out and less often. Dry it flat. The symbols on the tag are decoded in what the care symbols mean.

Questions after the number

Is that finished, washed cloth, or loom state?
What staple length is the cotton, and is the yarn ring spun or open end?
Which mill, and in which country?
Has it been pre shrunk, and how much movement is left?
On a hoodie, what weight is the rib at the cuffs and hem?

A brand that can answer has walked its supply chain. One that answers in adjectives hasn't.

Why brands hide it

It's checkable. Premium heavyweight can't be tested. 480gsm can, with a kitchen scale.

The short version

Ask for the number. Compare within a category. Treat gsm as a floor test, not a quality score, then look at the fibre, the rib and the seams.

Sources

Competition Bureau Canada, Labelling textile requirements in a nutshell.