A fabric spec sheet is meant to make two parties agree on what they’re buying. In practice, most of the numbers on it get misread — GSM gets treated as a quality score, thread count gets treated as a luxury rating, and both get compared across fabrics where the comparison is meaningless.
Once you understand what each figure actually measures, you can predict how a fabric will behave before you order it, and spot when a spec is being used to flatter a mediocre product.
This guide covers GSM, thread count, weave and knit structures, finishes and testing standards. It’s part of our wider knitting and fabric section.
GSM: Weight, Not Quality
GSM means grams per square metre. A 200 GSM fabric weighs exactly 200 grams per square metre, whatever the fibre or weave.
It’s the single most useful spec because it predicts drape, opacity, warmth and durability. What it is not is a quality score.
| GSM | Typical fabrics | Uses |
|---|---|---|
| 80–120 | Voile, lawn | Summer shirts, linings, scarves |
| 120–180 | Poplin, percale | Dress shirts, bedsheets, light dresses |
| 180–250 | Twill, flannel | Workwear, uniforms, heavier shirting |
| 250+ | Denim, canvas, corduroy | Jackets, upholstery, bags |
For T-shirt jersey specifically: 110–140 GSM is fast-fashion territory chosen for cost and shipping weight; 160–200 GSM is where premium tees sit for better opacity, drape and durability; 220+ is heavyweight.
GSM Doesn’t Measure Thickness
This trips people up. A tightly woven high-thread-count fabric at 180 GSM can feel thinner than a loosely woven fabric at the same 180 GSM, because the first packs fine yarns densely while the second uses bulkier yarns loosely.
Two fabrics at identical GSM can also drape completely differently — a 180 GSM cotton poplin and a 180 GSM polyester satin behave nothing alike, because weave structure and fibre properties matter as much as weight.
Thread Count: Useful, and Widely Abused
Thread count is warp threads plus weft threads per square inch. It applies to woven fabric only — it’s meaningless for knits.
Higher generally means a finer, denser weave. But the relationship between thread count and quality breaks down fast, and here’s why.
The inflation trick: manufacturers can count individual plies within a multi-ply yarn rather than counting yarns. A fabric made from two-ply yarn at 400 actual threads gets labelled 800. Some push further with three- and four-ply.
The result is that very high thread counts — 800, 1000, 1500 — often indicate lower-quality multi-ply yarn rather than superior fabric. Genuinely fine single-ply cotton rarely exceeds about 600, and most excellent bedding sits between 200 and 400.
Weave affects the number too. Sateen weaves float threads on the surface rather than distributing them through the thickness, which raises visible thread count without a proportional GSM increase. A 180 GSM sateen may carry a 400 thread count label while a 200 GSM percale shows 300 — the percale has more actual substance.
Practical guidance for bedding:
- Percale at 200–400 for a crisp, cool, breathable feel
- Sateen at 300–600 for a smoother, silkier feel
- Above 600, be sceptical
Fibre quality matters more than the count. Long-staple cotton at 300 outperforms short-staple cotton at 800.
Weave Structures
Plain Weave
The simplest interlacing — each weft yarn passes over one warp, under the next. Maximum interlacing points make it strong, stable and breathable. It creases readily and has no inherent stretch.
Poplin, percale, muslin, chambray, canvas and voile are all plain weaves at different weights and yarn counts.
Twill
Twill offsets the interlacing points, producing the visible diagonal line. Fewer interlacings let yarns pack more densely, which makes twill stronger and more drapey than plain weave at the same weight, and considerably better at hiding creases and soil.
Denim, gabardine, chino and herringbone are twills.
Satin and Sateen
These minimise interlacing further, letting long yarn floats sit on the surface. Smooth, lustrous and fluid — and fragile, because those floats snag and abrade easily.
Technically satin uses filament yarn (silk, polyester) and sateen uses spun yarn (usually cotton), but the structural principle is identical.
Knit Structures
Knits are loops rather than crossed yarns, which is where their stretch comes from — the geometry, not the fibre. Cotton jersey stretches; cotton poplin doesn’t, despite being the same fibre.
Single jersey — one needle bed, smooth face and looped back, the standard T-shirt fabric. Light, drapes well, and curls at cut edges. 130–220 GSM typical.
Interlock (double jersey) — two needle beds, identical on both sides, doesn’t curl. Denser and more stable, which is why it’s used for polos and uniforms. 180–320 GSM.
Rib — alternating knit and purl columns, strong crosswise stretch and excellent recovery. Used for cuffs, collars and waistbands.
Pique — textured surface with a fine geometric pattern. The polo shirt fabric; more breathable than flat jersey.
French terry — loops on the back, soft and absorbent, year-round weight.
Fleece — those loops brushed into a nap, trapping air for warmth.
Yarn Specifications
Yarn count (Ne) — in the English cotton system, higher numbers mean finer yarn. 30/1 is finer than 20/1. Finer yarn generally produces smoother, softer fabric.
The /1 indicates single-ply; /2 is two-ply, which is stronger and smoother but heavier.
Spinning method matters more than most specs suggest:
- Open-end (rotor) — fast and cheap, produces coarser, weaker, fuzzier yarn
- Ring-spun — twists fibres into a fine, tightly aligned, smooth, strong yarn
- Combed ring-spun — short fibres removed first, then ring-spun. The premium standard
Two fabrics at identical GSM and thread count can feel entirely different because of this, and it’s why “combed ring-spun” is the most reliable quality phrase on a cotton spec.
Staple length — longer fibres make stronger, smoother, less pill-prone fabric. Pima and Egyptian cotton are extra-long staple.
Micron count for wool — lower is finer and softer. Merino under 19 microns is next-to-skin soft.
Finishes
Finishes change how a fabric behaves after it’s woven or knitted.
Mercerisation (cotton) — caustic soda treatment under tension. Increases lustre, strength and dye uptake.
Compacting / pre-shrinking — mechanically shrinks fabric before cutting so garments shrink less later. Look for “compacted” or “pre-shrunk” on knit specs.
Sanforising — a controlled pre-shrinking process for wovens.
Brushing / napping — raises surface fibres for softness and warmth.
Calendering — pressing between rollers for smoothness and sheen.
Enzyme wash — removes surface fuzz, improving softness and reducing pilling.
DWR (durable water repellent) — surface treatment making water bead. Wears off and needs reapplying.
Anti-pilling, antimicrobial, wicking — functional finishes, most of which diminish with washing.
Testing Standards
Spec sheets reference test methods. The method matters as much as the number — a result without its method is uninterpretable.
Martindale / Wyzenbeek — abrasion resistance. Higher rub counts mean more durable. Used heavily for upholstery.
Pilling test — resistance to surface pilling, usually graded 1–5 where 5 is best.
Colourfastness — tested separately to washing, light, rubbing (crocking) and perspiration. Graded 1–5.
Hydrostatic head — waterproofness, in mm. Relevant for outerwear.
MVTR / breathability — moisture vapour transmission, g/m²/24h. Higher is more breathable.
Air permeability — how much air passes through. Relevant to windproofing and comfort.
Dimensional stability — shrinkage after washing, as a percentage.
Tensile and tear strength — resistance to pulling and tearing.
When comparing suppliers, insist on both method and target. “Good colourfastness” means nothing; “ISO 105-C06, grade 4 minimum” is a specification.
Reading a Spec Sheet
A typical fabric spec contains:
- Composition — fibre percentages
- Construction — weave or knit type
- GSM — weight
- Width — usable width, which affects marker efficiency and cost
- Yarn count — fineness and ply
- Thread count or courses/wales — density
- Finish — treatments applied
- Shrinkage — with the test method
- Colourfastness — with methods and grades
Check that width is usable width rather than including selvedge, and that GSM is specified as finished rather than greige (untreated), since finishing changes weight.
Choosing Fabric for a Project
- Knit or woven? Set by the garment. Patterns are drafted assuming a specific stretch.
- GSM? Match to season, drape and opacity.
- Fibre? Comfort, care, cost.
- Stretch and recovery? Check both — fabric that stretches but doesn’t recover bags at knees and elbows.
- Shrinkage? Pre-wash natural fibres before cutting.
- Order swatches for anything significant. No spec captures hand-feel.
Common Misunderstandings
“Higher GSM means better quality.” It means heavier. A 160 GSM combed ring-spun fabric outperforms a 240 GSM carded open-end one on softness, durability and pilling.
“1000 thread count is luxury.” Usually the opposite — reached by counting plies rather than yarns. Be sceptical above 600.
“Thread count applies to my T-shirt.” It doesn’t. Thread count is for wovens. Knits use courses and wales.
“Same GSM means same thickness.” No — yarn fineness and weave density change thickness independently of weight.
“Natural fibres are always better.” Depends on use. Polyester genuinely outperforms cotton for activewear moisture management.
Where to Go From Here
- Knitting & Fabric — the full guide to knit and woven structures
- Yarn & Thread — fibre types, counts and plies
- Knitting Basics & Techniques — hand knitting from beginner onward
- Quilting — quilting cotton specifications and batting
Related areas:
- Fabric Dyeing — fibre content determines which dyes work
- Fabric Printing — fabric structure limits which printing methods are viable
- Garment Manufacturing & QC — fabric inspection and the 4-point system
- T-Shirts — how these specs show up in a finished garment
Frequently Asked Questions
What does GSM mean, and what’s a good number? Grams per square metre — fabric weight. There’s no universally good figure; it depends on use. Around 180–220 GSM for everyday T-shirts, 120–180 for shirting, 250+ for denim and canvas. Higher means heavier, not better.
Is a higher thread count always better? No. Above roughly 600, high counts usually come from counting plies within yarns rather than genuinely finer yarn, which produces heavier, stiffer, less breathable fabric. Most excellent bedding sits between 200 and 400. Fibre quality and weave matter more than the number.
What’s the difference between thread count and GSM? Thread count measures yarn density — threads per square inch in woven fabric. GSM measures mass per unit area. They’re related but measure different things, and thread count doesn’t apply to knits at all.
Why do two fabrics with the same GSM feel so different? Because GSM measures weight, not structure. Yarn fineness, spinning method, weave type and finish all change hand-feel independently. A 180 GSM poplin and a 180 GSM satin behave nothing alike.
What does “combed ring-spun” mean? Two separate processes. Combing removes short fibres, leaving only long ones. Ring spinning twists fibres into a fine, tightly aligned yarn. Together they give the smoothest, strongest, least pill-prone cotton — and it’s the most reliable quality indicator on a cotton spec sheet.
What’s the difference between percale and sateen? Both are weave types for bedding. Percale is a plain weave — crisp, cool, breathable, matte. Sateen floats threads on the surface — smoother, silkier, slight sheen, warmer. Preference rather than quality; percale generally lasts longer since sateen’s surface floats abrade.
Which testing standards should I ask a supplier for? At minimum, colourfastness to washing and rubbing, dimensional stability (shrinkage), and pilling resistance. For outerwear add hydrostatic head and MVTR; for upholstery add Martindale abrasion. Always ask for both the test method and the target grade — a number without its method is meaningless.
Should I order swatches before buying fabric? Yes, for anything significant. No specification captures hand-feel, drape, or true colour. Screen colour is unreliable and GSM tells you nothing about how a fabric moves.
