Wool Fabric Weights: Per Metre, GSM and Worsted Counts

Wool cloth comes with its own vocabulary of numbers. Suiting merchants quote "280 g", "9 oz" or "13 oz" cloth; coatings are described as heavyweight; worsted yarns carry counts like 2/48 or 2/60; and fine suitings are labelled Super 120s or Super 150s. Some of those numbers use a different basis from the GSM figures on most other fabrics, which makes comparison confusing. This guide explains each one, converts them with the site's Fabric Weight from Dimensions tool, Fabric Weight Converter and yarn count converter, and covers the shrinkage behaviour that makes wool different from every other fibre.

Weight per running metre, not per square metre

Most fabrics are described by GSM, grams per square metre. Many suiting and tailoring cloths are instead described by weight per running metre, or per running yard, of the cloth at its full width, which for suitings is often around 150 cm. A "280 g" suiting, in that convention, is one where a metre cut from the roll weighs 280 g.

To compare it with a GSM figure, divide by the width in metres. At 150 cm wide, 280 g per running metre is 186.67 GSM. You can confirm that in the dimensions tool: 186.67 GSM over 1 m of 150 cm cloth gives 280 g per running metre. In the converter, 186.67 GSM is 5.51 oz/yd².

A heavier cloth at 400 g per running metre, the kind of weight you might see in tweeds and winter suitings, works out at 266.67 GSM, or 7.87 oz/yd².

Ounce figures in tailoring are often per running yard on the same basis, not per square yard, so a "9 oz" suiting is not the same as a 9 oz/yd² denim. When a merchant quotes weight, ask whether it is per running metre or yard, or per square metre, and at what width.

Common wool cloth weights

• Lightweight suitings, for warm climates and summer, sit at the low end of the per-metre range, and are often tropical or open weaves.
• Mid-weight suitings are the all-season middle, the kind of figure in the example above.
• Heavier suitings, tweeds and flannels are for winter and outdoor wear.
• Coatings are heavy: a wool coating at 500 GSM is 14.75 oz/yd², and at 650 GSM, 19.17 oz/yd², in the converter's Very heavy band.

A long coat in a 650 GSM coating, cut from 2.5 m of 150 cm cloth, uses 3.75 m² of fabric weighing 2.44 kg according to the dimensions tool. That is before lining, interlining, and the canvas inside a tailored front, which explains why a good winter coat feels as heavy as it does.

Woollen and worsted

Wool yarns are spun in two broad ways:

• Worsted: long fibres are combed so they lie parallel, then spun into a smooth, strong, fine yarn. Worsted cloths are smooth, crisp and hard-wearing: most suitings, gabardines and fine trouserings.
• Woollen: shorter fibres are carded but not combed, giving a fuzzier, loftier, warmer yarn. Woollen cloths are soft and bulky: tweeds, flannels, blankets and many coatings.

Reading worsted counts

Worsted yarns are often described in the metric count Nm, kilometres per kilogram, with the number of plies first. A 2/48 yarn is two singles of Nm 48 twisted together. Two singles weigh twice as much per metre as one, so the plied yarn's resultant is roughly Nm 24.

The converter takes tex as input, so turn Nm into tex first: tex = 1000 ÷ Nm. Nm 24 is 41.67 tex, which the converter shows as 375 denier and 416.67 dtex. A finer 2/60 yarn has a resultant of about Nm 30, or 33.33 tex: 300 denier. Twist contraction makes plied yarns slightly heavier than the simple halving suggests, but the difference is small for comparison purposes.

Finer counts, all else equal, make lighter, smoother cloth. Fine suitings use high counts and often two-ply yarns in both warp and weft for strength.

What Super numbers mean

Labels such as Super 100s, Super 120s and Super 150s describe the fineness of the wool fibre, not the yarn. Under the labelling code of the International Wool Textile Organisation, each Super number corresponds to a maximum average fibre diameter in microns, with higher numbers meaning finer fibre: Super 100s, for example, corresponds to 18.75 microns or finer. Finer fibre makes softer, lighter, more lustrous cloth, but also more delicate cloth that creases more and wears faster. A Super 100s or 110s cloth is often a better choice for a suit worn hard every week than a very high number.

Wool and shrinkage

Wool can shrink in two ways:

• Relaxation shrinkage, as with other fibres, when tension from weaving and finishing relaxes in moisture and heat.
• Felting, which is unique to animal fibres. The surface of a wool fibre has microscopic scales; with heat, moisture and agitation, fibres migrate and lock together. Felting shrinks and thickens cloth dramatically and cannot be reversed.

Machine-washable wools are treated to resist felting; most suiting and coating wools are not. Tailors pre-shrink wool cloth with steam before cutting, traditionally by a process called London shrinking, in which the cloth is dampened, rested and dried slowly. You can do a simpler version with a steam iron held above the cloth, without pressing.

To see how much a cloth moves, mark a 50 cm length on an offcut, steam it as you will when pressing the garment, let it dry flat, and measure. If it now measures 49.5 cm, the Fabric Shrinkage Calculator reports 1%. Never test wool cloth by washing it warm unless the label says it is washable; you will be testing felting, and the sample will be ruined.

How much cloth for a garment

Pattern envelopes give yardage for tailored garments, and you should follow them for layout reasons: wool cloth often has a nap or a directional weave, and checks and stripes need matching. As a rough area-based estimate, if a two-piece suit's pieces add up to about 3.4 m², the Fabric Yardage Calculator gives 2.58 m on 145 cm usable width with 10% waste. Pattern matching on checks can add a good deal to that.

Care

Brush wool garments after wearing, let them rest between wears so creases drop out, and air them rather than cleaning often. Store them clean, because clothes moths are attracted to soiled wool, and use sealed storage or moth deterrents for long-term storage. Dry-clean or hand-wash according to the label.

What the tools cover

The dimensions tool links GSM, width and weight per running metre, which is exactly the conversion tailoring weights need. The converter gives GSM and ounces per square yard, and the yarn count converter handles worsted counts once they are in tex. None of them can tell you fibre fineness, cloth quality, or how a particular cloth will respond to steam; for that, handle a sample and test an offcut.