How to Read a Fabric Spec: GSM, Yarn Count & Construction

A fabric spec can look like code: 40x40 / 133x72 / 58", followed by a GSM figure. Once you can read it, it tells you almost everything about how a fabric will feel, drape, and wear. This guide breaks down GSM, yarn count, and fabric construction in plain terms, so any buyer or product developer can judge a fabric from its numbers alone.

What is GSM in fabric? GSM stands for grams per square metre, the weight of one square metre of a fabric. It is the industry’s default shorthand for how heavy or substantial a fabric is, and unlike thread count it transfers cleanly across cotton, blends, and knits. A higher GSM generally means a thicker, warmer, more durable cloth; a lower GSM means a lighter, more breathable one.

Key takeaways

• GSM = grams per square metre = fabric weight. It is the most reliable single number for comparing fabrics.
• Yarn count (Ne) measures yarn fineness. Higher count = finer yarn. 40s is finer than 20s.
• Construction (ends x picks, or EPI x PPI) is how many warp and weft yarns sit in one inch.
• Warp runs lengthwise along the roll; weft runs across the width.
• Thread count vs GSM: thread count counts yarns per inch; GSM weighs the cloth. GSM is harder to fake.
• Match GSM to end use. Light for shirting and voile, heavy for drill and canvas. Higher is not automatically better.

What is yarn count (Ne)?

Yarn count describes how fine or coarse a yarn is. In the English cotton system, count is written as Ne (number English) and defined as the number of 840-yard hanks that weigh one pound. Because it is an inverse measure, a higher number means a finer, lighter yarn:

• 10s to 20s Ne: coarse, heavier yarns used in denim, canvas, and drill.
• 30s to 40s Ne: medium to fine, the workhorse range for shirting, poplin, and bedsheets.
• 50s to 80s+ Ne: very fine yarns for voile, lawn, and premium shirting.

Count drives the hand feel: finer yarns produce a softer, smoother, more refined surface. Two fabrics with an identical GSM can feel completely different if one uses a finer yarn spun to a higher count. See our full range of made-to-construction fabrics for how count is specified per quality.

What are warp and weft?

Every woven fabric is built from two thread systems that interlace at right angles.

• Warp yarns run lengthwise, parallel to the selvedge, along the length of the roll. Individual warp yarns are called ends.
• Weft yarns run across the fabric, from selvedge to selvedge. Individual weft yarns are called picks.

The density of each is measured per inch: EPI (ends per inch) for the warp, and PPI (picks per inch) for the weft. EPI and PPI together are the “construction” of the cloth.

How to read a fabric construction like 40x40 / 133x72

A standard woven fabric spec is written in a fixed order. Take this example:

40 x 40 / 133 x 72 / 58“

• 40 x 40 is the yarn count: 40s Ne in the warp, 40s Ne in the weft.
• 133 x 72 is the construction: 133 ends per inch (EPI) by 72 picks per inch (PPI).
• 58“ is the finished width in inches.

Read together, this is a fine, medium-density, plain-weave cotton, the classic profile of a shirting or poplin. If the two count numbers differ, say 30 x 16, the fabric uses a finer warp and a coarser weft, common in twills and drills where the weft carries the weight. A worked read of a spec always follows the same logic: count first (yarn fineness), then density (yarns per inch), then width, then weave and GSM.

Construction-notation cheat sheet

Field in spec	Example	What it means
Warp count x weft count	40 x 40	Yarn fineness in Ne (higher = finer)
EPI x PPI (ends x picks)	133 x 72	Warp yarns and weft yarns per inch
Width	58“	Finished fabric width, edge to edge
Weave	Plain / twill / satin	Interlacing pattern of warp and weft
GSM	118 g/m2	Weight per square metre

GSM vs thread count

The two are often confused. Thread count is the total of warp plus weft yarns in a square inch (for 133 x 72, that is 205). GSM is the actual measured weight of a square metre of that cloth.

They diverge because thread count ignores yarn thickness. Two fabrics can share a thread count yet weigh very differently if one uses heavier, lower-count yarn. Thread count also became a marketing number on bedding, where inflated figures are common, so it can mislead. GSM measures mass directly and works across fibre types, which is why mills, dyeing houses, and technical buyers lean on it as the honest baseline. Use thread count to judge density and cover; use GSM to judge substance.

You can estimate GSM from a spec with a common approximation: GSM = (EPI ÷ warp count + PPI ÷ weft count) x 25.54. Treat it as a planning figure, not a substitute for weighing a finished sample.

Typical GSM ranges by fabric use

GSM should suit the application. The ranges below are practical guides for cotton and cotton-blend wovens.

Fabric / use	Typical GSM	Notes
Voile, lawn	70 to 100	Sheer, very light, fine high-count yarns
Summer shirting	100 to 130	Breathable, fine plain weaves
Poplin	110 to 150	Smooth, crisp, fine weft rib
Sheeting / bedding	120 to 180	Percale and sateen constructions
Twill / lightweight denim	180 to 260	Diagonal weave, more body
Drill, workwear	240 to 320	Dense, durable, uniform and utility
Canvas, upholstery	300 and above	Heavy, rigid, industrial end uses

At KSJ Spins and Weavers, made-to-construction weaving covers roughly 80 to 330 GSM in widths up to 63 inches, supplied greige or RFD. That span, from voile-weight shirting to workwear drill, is set on the loom by choosing count, EPI, PPI, and weave to hit a target GSM. See our weaving capabilities for the loom mix and quality controls behind each construction.

Putting a full spec together

A complete, unambiguous fabric spec names six things: fibre or blend, yarn count (warp x weft), construction (EPI x PPI), weave, width, and GSM. For example: 100% cotton, 40 x 40, 133 x 72, plain weave, 58“, ~118 GSM, RFD. From those six fields alone, a mill can quote, a buyer can compare, and a garment maker can predict drape and durability.

If you are working with blends, yarn count and GSM interact with fibre choice too. Our guides on greige vs RFD fabric and poly-viscose, modal, and poly-cotton blends cover the next layer of detail once you can read the base spec.

Frequently asked questions

What is GSM in fabric? GSM stands for grams per square metre, the weight of one square metre of fabric. It is the single most reliable way to compare how heavy or substantial a fabric is, because it works the same across cotton, blends, and knits. A higher GSM usually means a thicker, heavier, more durable fabric; a lower GSM means a lighter, more breathable one.

What is yarn count (Ne)? Yarn count in the English cotton system (Ne) is the number of 840-yard hanks that weigh one pound. A higher number means a finer, thinner yarn: 40s cotton is finer than 20s cotton. Count controls how soft, fine, and smooth a fabric feels, and combined with thread density it largely determines the finished GSM.

What is the difference between thread count and GSM? Thread count is the total number of warp and weft yarns per square inch, while GSM is the fabric’s actual weight per square metre. Two fabrics can share a thread count yet weigh differently if their yarn thickness differs. GSM measures mass directly, so it is the more honest, cross-fibre way to judge how substantial a fabric really is.

What do the numbers in 40x40 / 133x72 mean? The first pair is yarn count: 40s warp by 40s weft. The second pair is construction: 133 ends per inch (warp yarns) by 72 picks per inch (weft yarns). Together they describe a fine, medium-density plain-weave cotton typical of shirting or poplin. Add the width, usually in inches, to complete the spec.

Is a higher GSM always better? No. GSM should match the end use, not simply be maximised. Voile and summer shirting are intentionally light at 80 to 120 GSM for breathability, while workwear drill and canvas run 240 GSM and above for durability. The right GSM is the one that suits the garment, drape, and performance the buyer needs.

Need a fabric built to an exact GSM and construction? Contact KSJ Spins and Weavers with your spec, or share a target end use and we will recommend a construction to match.