That Yarn · Journal
Wool and Cashmere: Micron, S-Count, and What to Choose
A finer fiber is not always a softer one. What micron actually measures, why S-counts are older and stranger than they look, and the number that predicts prickle better than either.
Two numbers follow wool everywhere. One is a micron count — 19.5µm, 21µm — and it sounds like physics. The other is an S-count — 64s, 70s, 80s — and it sounds like a grade. Buyers treat them as interchangeable, converting one to the other with a table and moving on.
They are not interchangeable. One is a measurement; the other is a historical estimate of spinning yield. They correlate, but the correlation is loose enough that relying on it will occasionally mislead you badly. And neither number, on its own, predicts whether a garment will itch.
This guide explains what each system measures, gives the one authoritative conversion table that exists, and then introduces the figure that actually governs next-to-skin comfort — a figure that appears on almost no consumer label.
Micron: a measurement
A micron (µm) is one thousandth of a millimetre. Fiber diameter is measured in microns by instruments — a Sirolan Laserscan or an Optical Fibre Diameter Analyser — that scan thousands of individual fibers from a sample and compute the distribution.
What gets printed on a label is the mean fiber diameter, the average of that distribution. A wool described as 19.5µm has an average fiber diameter of 19.5 microns. It does not mean every fiber measures 19.5µm. Some are finer. Some are considerably coarser. Hold that thought — it becomes the whole argument later.
Finer fibers bend more readily when pressed against skin. A fiber end that buckles under light pressure does not trigger the skin's pain receptors; one that stays rigid does. This is the entire physical basis of softness, and it is why micron is the single most useful number on a fiber label.
S-count: an estimate, and an old one
The S-count — properly the Bradford Count, also called spinning count or the English Worsted Yarn Count System — predates instruments entirely.
In nineteenth-century Bradford, then the centre of the world wool trade, graders judged fleece by eye and hand. Their unit was spinning yield: how many 560-yard hanks of single yarn could a skilled spinner produce from one pound of combed top? Finer wool packed more fibers into that pound, so more hanks could be spun from it.
64s = approximately 64 hanks of 560 yards, spun from one pound of top
The number was never a diameter. It was a prediction about spinning, made by a person, and different graders reached different conclusions about the same fleece. The system also ignores crimp entirely — and crimp affects how a fiber drafts and spins as much as diameter does.
Bradford values run from roughly 28s for coarse carpet wool to 70s for very fine wool, with 80s at the extreme upper end of what nineteenth-century spinners could achieve.
Why it survives
Micron measurement replaced Bradford for all commercial and technical purposes decades ago. S-counts persist in three places: among shepherds and breed associations, in the vocabulary of hand spinners, and — most consequentially for buyers — in marketing copy, where a high number reads as a promise of luxury.
Treat an S-count as a rough dialect, not a specification. When a mill gives you both, the micron figure is the one to trust. When a mill gives you only an S-count, that is itself a piece of information about the mill.
The conversion table — and its limits
In 1968 the United States Department of Agriculture assigned each Bradford count a range of average fiber diameter, plus a maximum permitted standard deviation. This is the only conversion with formal standing, and it applies to United States wool grading.
| Bradford count | Average fiber diameter | Max. standard deviation | Character |
|---|---|---|---|
| 80s | 17.70–19.14 µm | 4.09 µm | Superfine merino |
| 70s | 19.15–20.59 µm | 4.59 µm | Fine merino |
| 64s | 20.60–22.04 µm | 5.19 µm | Merino / fine crossbred |
| 62s | 22.05–23.49 µm | 5.89 µm | Fine crossbred |
| 60s | 23.50–24.94 µm | 6.49 µm | Crossbred |
| 58s | 24.95–26.39 µm | 7.09 µm | Medium crossbred |
| 56s | 26.40–27.84 µm | 7.59 µm | Medium |
Read the third column. The standard was never a simple diameter equivalence — it also bounded how variable the fibers could be. A grade was a statement about the whole distribution, not just its centre.
Two cautions worth stating plainly
First, the Bradford scale ends at 80s. There is no official 90s or 100s wool grade. Those numbers belong to the Super-S system, which grades finished worsted fabric, not fiber, and which mills define for themselves. A Super 150s cloth from one weaver may be finer or coarser than a Super 150s from another. Seeing "100S wool fiber" on a label means the seller has borrowed a fabric term for a fiber, and the number carries no standardised meaning.
Second, modern breeding routinely produces wool at 14–16µm, finer than anything the 1968 scale contemplated. The scale cannot describe such fiber, which is another way of saying it has been outgrown.
The number that predicts prickle
Here is where mean diameter stops being enough.
Textile physicists established in the 1990s that the sensation of prickle does not come from average fineness. It comes from the proportion of fibers coarser than about 30µm — the small tail of the distribution, whose stiff ends refuse to buckle and instead press into the skin.
This proportion is called the prickle factor. Its complement is the comfort factor.
Comfort factor = % of fibers finer than 30 µm · Prickle factor = 100% − comfort factor
The research finding is consistent: once roughly five percent or more of the fibers exceed 30µm, most wearers register a fabric as prickly. Below that threshold, they do not.
The consequence is counter-intuitive, and it was demonstrated experimentally. In a 1992 study, knitted fabric made from 23.2µm wool was found to be less prickly than fabric made from finer 21.5µm wool. The finer wool had a wider distribution, and therefore more coarse fibers in its tail. Mean diameter said one thing; comfort said another.
Two wools of identical micron count can feel entirely different. The one with a tighter distribution — a lower standard deviation, a lower coefficient of variation — will be the kinder one against skin.
This also explains why the USDA grades bounded standard deviation, and why quality-conscious mills report coefficient of variation alongside mean diameter. It is not decoration. It is the part of the specification that tells you about the tail.
Why cashmere escapes the problem
Cashmere is not merely fine. Commercial definitions constrain the tail directly: fiber must average at or below roughly 19µm, with a strict cap on the proportion of coarse fibers permitted. Dehairing — the mechanical separation of soft undercoat from stiff guard hair — exists to remove exactly the fibers that would otherwise cause prickle.
Fineness alone does not make cashmere soft. Fineness plus a ruthlessly narrow distribution does.
Cashmere: what the grades mean
Cashmere comes from the soft undercoat of the cashmere goat, grown as insulation against continental winters and combed or shorn in spring. The coarse outer guard hair is a separate fiber, removed during dehairing.
Most commercial cashmere falls between 15 and 19µm. Within that band, the trade recognises tiers — but these are market grades, not scientific boundaries, and the thresholds shift between suppliers and regions.
| Market tier | Typical diameter | Character |
|---|---|---|
| Ultrafine | ≈ 14–15.5 µm | Exceptional softness; scarce, expensive |
| Fine | ≈ 15.5–17 µm | The heart of good commercial cashmere |
| Standard | ≈ 17–19 µm | Soft, more substantial, more durable |
Two qualifications matter more than the tiers themselves.
Fiber length is as commercially significant as diameter, and it is rarely disclosed. Short cashmere fibers spin into yarn that pills readily, however fine they are. A long-stapled 17µm cashmere will outlast a short-stapled 15µm one, and wear better throughout its life. Where a mill reports both figures, the pair tells you far more than either alone.
And dehairing quality determines everything. Incompletely dehaired cashmere retains guard hair — coarse fiber, well above 30µm, precisely the tail that causes prickle. This is why some cashmere disappoints despite an impressive micron count on the label.
What micron does not tell you
Diameter is the most useful single number. It is not the only one that governs how a fiber behaves.
| Property | What it governs | Usually disclosed? |
|---|---|---|
| Mean diameter | Baseline softness, spinning limit | Yes |
| Distribution (CV, SD) | Prickle, evenness, yarn strength | Sometimes |
| Staple length | Strength, pilling resistance | Rarely |
| Crimp | Elasticity, loft, memory | Rarely |
| Coarse fiber content | Prickle, harshness | Rarely |
| Processing and finishing | Handle, lustre, drape | No |
Processing deserves particular mention. Superwash treatment alters the fiber's scale structure and can make a 26µm wool feel softer than an untreated 23µm one. Overzealous scouring can leave a fine wool dry and lifeless. Very often the harshness a knitter blames on the sheep was introduced in the mill.
A number describes the fiber that arrived at the mill. It does not describe the yarn that left it.
Choosing, in practice
For next-to-skin wear
Look for a mean diameter below roughly 19.5µm, and if the specification offers it, a comfort factor above 95%. In Bradford terms this is 70s and finer. In cashmere it is anything properly dehaired. Where two candidates share a micron count, prefer the one with the lower coefficient of variation.
For everyday knitwear
The 19.5–22µm band — 64s to 70s — offers the useful compromise. Soft enough over a layer, resilient enough to hold a shoulder line, and considerably more durable than superfine merino, which abrades faster precisely because its fibers are thin.
For structure, outerwear, and texture
Wool above 24µm has body, resilience, and stitch definition that fine merino cannot match. It felts well, it holds a cable, it survives. Medium wool is not a lesser fiber. It is a different one, chosen for the projects that punish softness.
For hand spinning
Consider staple length and crimp before micron. A fine, short-stapled top is difficult to draft evenly and produces a weaker yarn. A slightly coarser top with good length and crimp will spin more forgivingly and wear better. Our guide to choosing wool top for hand spinning works through this in detail.
Reading a specification with intent
When a spec sheet arrives, read it in this order.
- Mean fiber diameter. The baseline. Everything else adjusts it.
- Standard deviation or CV. If present, this is the tail. Lower is kinder.
- Staple length. If present, this predicts pilling and strength.
- Comfort factor, if the mill reports it. Above 95% is comfortable next to skin.
- S-count, last. Use it to sanity-check the micron figure, never to replace it.
Most mill-end cone yarn arrives with less information than this. Where a figure is absent, it is absent — worth noting as a gap rather than filling with an estimate. A cone described honestly as "wool, undisclosed micron" is more trustworthy than one carrying a confident 100S that means nothing.
Frequently asked questions
What is the difference between micron and S-count?
Micron is an instrument measurement of average fiber diameter. S-count, or Bradford count, is a nineteenth-century estimate of how many 560-yard hanks could be spun from a pound of combed wool. They correlate, but S-count was a judgement about spinning yield, not a measurement of thickness.
Is a lower micron count always softer?
No. Softness depends more on the proportion of fibers coarser than 30µm than on the average. A study found knitted fabric from 23.2µm wool could feel less prickly than fabric from finer 21.5µm wool, because the finer wool had a wider distribution and more coarse fibers in its tail.
Does 90S or 100S wool exist?
Not as a fiber grade. The Bradford scale ends at 80s. Numbers like 100s and 150s come from the Super-S system, which grades finished worsted fabric and is defined by each mill for itself. On a fiber label, such numbers carry no standardised meaning.
What is comfort factor?
The percentage of fibers in a sample finer than 30µm. Its complement is the prickle factor. Research indicates that once about five percent of fibers exceed 30µm, most wearers perceive the fabric as prickly. A comfort factor above 95% is generally comfortable against skin.
How fine is cashmere compared to wool?
Most commercial cashmere measures between 15 and 19µm, with ultrafine grades around 14–15.5µm. Sheep wool ranges from about 14µm for the finest bred merino to well over 30µm for carpet wool. Cashmere's softness comes from fineness combined with a very narrow distribution and thorough dehairing.
Why does my fine merino sweater still itch?
Likely because its fiber distribution is wide, leaving enough coarse fiber ends above 30µm to trigger prickle, or because processing left the yarn harsh. Mean diameter alone cannot predict comfort; coefficient of variation and finishing both matter.
Which number should I trust when buying yarn?
Mean fiber diameter in microns, first. Standard deviation or coefficient of variation, second, if disclosed. Treat S-count as a rough cross-check. Where a figure is not disclosed, treat it as unknown rather than estimating from the numbers that are.
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