Fabric weight: the number that tells you how a garment will behave

Fabric weight is expressed either in grams per square metre (GSM), used for most fabrics internationally, or in ounces per square yard, still common for denim and for wool suiting in Britain and America.

It is the most useful single specification a garment can carry, because it predicts how the cloth will hang, how opaque it will be, how warm it is, and roughly how long it will last.

Rough guide by garment

T-shirts: 120–150 GSM is lightweight and will be somewhat transparent, particularly in white. 150–180 is the standard mid-weight most brands use. 180–220 is heavy — it holds its shape, hangs away from the body rather than clinging, and lasts considerably longer. Above 220 is the boxy, structured territory of Japanese and American workwear brands.

Shirting: 100–120 GSM is fine and summery. 120–140 is a normal year-round shirt. Above 150 is a heavier winter shirt or a flannel.

Denim: under 12 oz is lightweight, comfortable immediately, and wears out faster. 12–14 oz is the standard range. 14–16 oz is heavy and takes weeks to break in. Above 18 oz is a specialist proposition that will feel like cardboard for a month.

Wool suiting: 7–9 oz is summer weight and will crease readily. 9–11 oz is a genuine year-round weight and the sensible default. 11–13 oz is winter cloth, drapes beautifully, and resists creasing well. Above 14 oz is heavy overcoating territory.

Sweatshirts: 250–300 GSM is standard. 350–450 is heavyweight and will hold its shape for years.

Weight is not the whole story

Three other variables interact with it, and ignoring them produces bad predictions.

Yarn count. In shirting, a higher number — 80s, 100s, 120s — indicates a finer yarn. Finer yarn produces a smoother, softer, more lustrous cloth, and a weaker one. A 140s shirt feels wonderful and wears out considerably faster than a 100s. Yarn count is a measure of fineness, not of quality, and it is regularly sold as the latter.

Weave. A plain weave (poplin) is crisp and comparatively opaque. Twill is more drapey and resists creasing. Oxford is a basket weave, more textured and durable. Two fabrics at identical weight in different weaves behave completely differently.

Fibre. Linen at 150 GSM behaves nothing like cotton at 150 GSM — it is stiffer, more open in structure, more absorbent, and creases immediately.

Ply, and the thing labels do not say

Two-ply means the yarn is made from two finer yarns twisted together. It is stronger, smoother and more durable than a single-ply yarn of the same thickness, and it resists pilling better.

Two-ply shirting is a genuine quality marker, and it is one of the few specifications that reliably indicates a better garment. It is frequently listed as "2-ply" or "two-fold".

Using this when buying

If the weight is listed, read it. More brands publish it than used to, particularly those selling online.

If it is not listed, hold it up to light. Transparency is a direct proxy. A white t-shirt you can read through will be transparent when worn.

Feel the hang, not the surface. Pick the garment up by the shoulders and let it fall. Good drape falls in long soft folds. Poor drape either sticks out stiffly or collapses into limp clinging.

Scrunch and release. How fast it recovers tells you how it will look after an hour in a chair.

The one place light beats heavy

Genuine heat. In hot weather, an open, light, loosely woven natural fibre — linen, seersucker, open-weave cotton, tropical-weight wool — moves air and dries fast. Weight is a liability there, and a heavy t-shirt in humidity is miserable.

Everywhere else, and particularly for anything you want to keep for years, the heavier version of the same garment is almost always the better buy.