A guide to fabric weight: what does GSM actually mean?

Fabric weight is the first number you should ask about, and the last one most new importers think about. It is measured in grams per square metre (GSM), and it decides how a t-shirt feels, what it costs, how long it lasts, whether it suits your market's climate — and even what it costs to ship.

What the number actually measures

GSM measures the density of the knit, not thickness alone. Two fabrics at 180 GSM can feel completely different depending on the yarn and how it was spun. So never specify weight on its own: state the yarn type and spinning method alongside it.

Cotton types and spinning

Combed cotton: the fibres are combed to strip out the short ones, so the yarn comes out softer, stronger and far less prone to pilling after washing. The right choice for any brand that wants a repeat customer.
Carded cotton: only roughly carded. Slightly coarser and around 15–20% cheaper. Perfectly suited to promotional tees that will be worn a handful of times.
Ring-spun: the yarn is twisted more finely, producing a softer, stronger thread. Around 10–15% dearer than open-end, and the difference is obvious in the hand.
Slub jersey: deliberately uneven yarn giving an irregular texture and a hand-made look. Popular in oversized and streetwear.

Blends

100% cotton: the most breathable and the best fit for hot climates, but it creases and shrinks more.
65/35 or 50/50 poly-cotton: creases and shrinks less, costs less, dries faster — but breathes less. The practical choice for uniforms and promotional tees.
Tri-blend: cotton, polyester and rayon. Very soft with a fluid drape, at a higher price. For markets that pay for feel.

Weights and what they suit

140–160 GSM: very light. For promotional tees, very hot markets, and base layers. It shows what is underneath when it is a light colour — watch this on white.
170–190 GSM: the world's best-selling weight. A balance of price, hand feel and durability. If you are starting out and want one weight you cannot get wrong, it is 180.
200–220 GSM: heavy and substantial. For a tee sold at a higher price, for temperate markets, and for uniforms.
230–260 GSM: very heavy. For oversized and streetwear, where the fabric has to hold its own shape rather than drape onto the body.
280–350 GSM: French terry and fleece, for hoodies and sweatshirts.

Shrinkage — the problem that shows up too late

Cotton shrinks. The question is not whether but how much. Untreated fabric can shrink 7–8% after the first wash, which turns an L into an M. The treatment is called compacting, or pre-shrinking.

Write it into the purchase order: shrinkage no more than 5% in length and 3% in width after three washes, and ask for the test report. Without that clause in writing, you have no basis for a claim.

Colour fastness

Colour fastness is graded from 1 to 5. Ask for grade 4 or above for washing, rubbing and perspiration. Dark colours — deep red and black especially — are the most likely to bleed onto lighter garments in the machine. If your order carries dark colours, ask specifically for a wet rub test.

Weight changes your freight cost

This is the point most people miss: a heavier tee takes more volume in the carton, and sea freight is usually charged by volume (CBM) rather than weight. The difference between 180 and 240 GSM can mean 25% more cartons for the same piece count. Work this out before you settle on a weight, not after.

What to ask the factory for before confirming

A fabric swatch in exactly the weight and colour you want — not an approximate colour.
A shrinkage and colour-fastness report from a laboratory.
Yarn type and spinning method written into the purchase order.
A pre-production sample built to the final specification in full.

Three recurring mistakes

Choosing weight on price alone. The gap between 160 and 180 GSM might be ten cents, but it is the gap between a tee that gets reordered and one that gets complained about.
Copying a competitor's weight without testing your market. What works in a temperate market can be stifling in a hot one.
Not putting shrinkage in writing. The most common complaint after a container lands traces back to that missing clause.