What '100% cotton' actually tells you — and what it doesn't

The composition label is the most-read and least-understood line on a garment. "100% cotton" feels like it settles the question. It does not. It tells you what the fibre is, and stops there — and almost everything you care about is decided after that.

A cotton t-shirt, a cotton oxford shirt, a pair of cotton jeans and a cotton towel are all 100% cotton. The label cannot tell them apart.

What the label is actually for

Fibre content is on the label because it is legally required, and the reason it is required is care, not quality. Cotton and wool and polyester behave differently in hot water and under an iron, and the label exists so you do not destroy the garment.

It is a safety notice that people read as a quality rating. That is the whole misunderstanding in one sentence.

The four things that decide how fabric behaves

Weight

Usually given in GSM (grams per square metre) for knits, or ounces for denim. This is the most useful number a brand can publish about a fabric, and the one most often missing.

For t-shirts, roughly:

• 120–150 GSM — light. Drapes, often slightly sheer, cheap to make.
• 160–190 GSM — standard. What most good t-shirts are.
• 200 GSM+ — heavy. Holds its shape, feels substantial, takes longer to dry.

A 140 GSM and a 220 GSM t-shirt are both "100% cotton" and are not remotely the same garment. If a brand publishes GSM, that is usually a sign they are proud of it.

Construction

How the fibre is turned into cloth, and it matters more than the fibre itself.

Knitted fabric is looped, so it stretches in every direction and recovers. T-shirts, jersey, sweatshirts.

Woven fabric is threads crossed at right angles. It has almost no give unless elastane is added. Shirts, jeans, chinos, most trousers.

This single distinction predicts more about how something fits than the fibre does. A woven cotton shirt that is slightly too small stays too small. A knitted one gives.

Yarn quality

Within cotton there is enormous variation, and this is where price usually goes. Staple length — how long the individual fibres are — determines how smooth and strong the yarn is and how much it pills. Long-staple cotton (you will see it sold as Supima, Pima or Egyptian) makes a fabric that stays smooth; short-staple cotton goes fuzzy and bobbly at the friction points after a dozen washes.

Two 100% cotton t-shirts at £8 and £45 are usually telling you about staple length and weight, not about the word "cotton".

Finishing

What is done to the fabric after it is made. Mercerising adds sheen and strength to cotton. Enzyme washing softens it. Sanforising pre-shrinks it — which is why some jeans shrink two sizes and others do not move.

Finishing is almost never disclosed, and it is often the difference between a shirt that looks the same after a year and one that does not.

What a blend is telling you

A blend is a compromise, and it is usually worth asking which way.

• Cotton with 2–5% elastane — added for stretch and recovery. Almost always an improvement in a fitted garment. In trousers it is the difference between sitting down comfortably and not.
• Cotton with polyester — cheaper, dries faster, wrinkles less, pills more, and holds smell more stubbornly. In a sweatshirt this is a reasonable trade. In a summer shirt it is usually a cost decision.
• Wool with nylon — added at the heel and toe of socks, or through a knit, for abrasion resistance. Genuinely useful; pure wool socks wear through.
• Anything with viscose, rayon, modal or Tencel — these are all processed cellulose. They drape beautifully and feel cool, and they are weaker when wet, so they need gentler washing than the fibre name suggests.

A high synthetic percentage is not automatically bad. A fleece is better as polyester. A waterproof shell has to be synthetic. The question is whether the synthetic is there to do a job or to save money, and you can usually tell by whether the garment needs that property.

Reading between the lines

Some phrases are worth knowing.

"Cotton-rich" means less than 100% and more than half. It is a phrase chosen to avoid giving a number.

"Premium cotton" means nothing. It is not a defined term. "Supima" and "Pima" do mean something — they are certified and licensed.

"Performance fabric" means synthetic, almost always polyester or nylon, and is a fair description if the garment is for exercise.

No weight, no yarn detail, no construction on a product page usually means there is nothing flattering to say. The brands with good fabric tend to talk about it at length.

The short version

• The composition label is a care instruction, not a quality rating.
• Weight, construction, yarn and finishing decide how a garment behaves. The fibre name decides almost nothing on its own.
• Knitted stretches and forgives; woven does not.
• A brand that publishes GSM and staple length is usually telling you something real. Silence on both is its own answer.