The GSM Test for Winter Coats

A wool overcoat is sold on cut and photographed on a hanger, which is the one situation in which every coat performs well. The thing that decides whether it still holds a shoulder line in January of year three is fabric weight, written as grams per square metre and usually buried on the inner care label.

What the weight tells you

Below roughly 400 gsm a wool coat behaves like a heavy cardigan: it drapes, it creases at the elbow, and within two seasons the front edge goes soft. Between 500 and 750 gsm the cloth has enough body to carry structure without interlining doing all the work. Above 800 gsm you are buying a blanket with sleeves, which is excellent on a platform at dawn and miserable indoors.

400–480 gsm — spring weight. Fine for a mild maritime winter, wrong for anywhere with real frost.
500–650 gsm — the useful band. Holds a lapel, survives a rucksack strap, still packs into a bin.
700–850 gsm — heavy melton and double-face. Warmest, least forgiving to a tailor, worst on a train.

Cloth weight sorted by hand: three coats of the same cut, three very different recoveries after being crushed.

The four-second shop test

Hold a sleeve against a window. Dense long-staple cloth blocks the light almost entirely and springs back when you crush it in a fist; a loose short-staple weave glows at the edges and keeps the crease you just made. Then do the part nobody does: put the coat on and reach forward as though for a door handle. If the back pulls across the shoulder blades in the shop, it will split a lining seam by March.

Everything hangs well on a hanger. The test is what the shoulder seam does after an hour of carrying a bag.

Cost per wear, upkeep included

The cost-per-wear argument is normally used to justify the expensive option. Run honestly it includes upkeep, and it often argues the other way. A £430 coat worn 512 times over four winters, with two cleans a year at £18, one replaced button and a £45 cuff relining, comes to £1.12 a wear and still falling. A £120 coat that lasted 78 wears with no repair possible cost £1.54 a wear, and was cold at the bus stop.

Coat	Weight	Wears logged	Total upkeep	Per wear
Harlow melton, £430	620 gsm	512	£189	£1.12
High-street wool blend, £120	380 gsm	78	£0	£1.54
Vintage double-face, £95	760 gsm	240	£130	£0.94

None of that is a general law. It is what the arithmetic said in three specific cases, and the point is that the arithmetic is cheap to run before you buy rather than after.

What we would actually do

Pick the 500–650 gsm band, insist on a replaceable viscose or bemberg lining, and check that the side seams have enough allowance for a tailor to let out. A bonded or fused lining cannot be replaced at all, which quietly sets an expiry date on an otherwise good coat.