The Micron Count Guide: Understanding Merino Wool Grades and Softness
If you've spent any time reading merino wool product pages, you've seen numbers like 17.5, 18.5, or 21 micron tossed around without much explanation. They show up next to price tags and fabric specs as though everyone already knows what they mean.
Most people don't. And the brands using those numbers are counting on that.
Micron count is the single most important specification in merino wool. It determines how the fabric feels against your skin, how it performs during movement, and — more than most shoppers realize — how long it holds up. This guide breaks down what those numbers actually mean, where the grades fall, and how to read them when you're comparing merino activewear.
What Is a Micron Count?
A micron is a unit of measurement equal to one-millionth of a meter. In merino wool, the micron count refers to the average diameter of the individual wool fibers in the fabric.
Thinner fibers bend more easily when they press against skin. That bendability is what makes fine merino feel soft rather than scratchy. Thicker fibers are more rigid — they push against nerve endings instead of flexing around them, which is why coarser wool can feel like it's prickling you.
So when a brand lists its micron count, it's telling you how fine the individual fibers are. Lower number, softer feel. It's that direct.
Up close and personal with different fiber types
Merino Wool Grades: What the Numbers Mean
Not all merino is the same. The wool industry grades merino into broad categories based on fiber diameter, and each grade has a distinct feel and set of trade-offs.
Ultrafine merino (below 16.5 micron) is the rarest and softest grade. It feels almost silky — closer to cashmere than what most people picture when they think of wool. It's used primarily in luxury knitwear and next-to-skin base layers where softness is the priority above all else. The trade-off is durability: ultrafine fibers are more delicate and more prone to pilling under abrasion.
Superfine merino (16.5–18.5 micron) is the sweet spot for performance apparel. Fibers in this range are soft enough to wear directly against skin all day — no prickling, no itch — while being robust enough to handle regular use, washing, and movement. This is the grade most premium activewear and base layer brands are working with when they take their fabric seriously.
Fine merino (19–20 micron) still feels comfortable for most people, though some with sensitive skin may notice a slight texture difference compared to superfine grades. It's a common choice for mid-layer garments and everyday knitwear where the fabric sits over a base layer rather than directly against skin.
Medium merino (21–22 micron) is where you start to notice the difference. Most people can wear 20-micron merino without complaint, but above 22 micron, the fibers become rigid enough that the prickle factor increases noticeably. This grade works well for outer layers, socks, and accessories where direct skin contact is limited.
Broad merino and traditional wool (above 23 micron) is the territory of outerwear, blankets, and upholstery. It's durable and warm, but it's not what you want against your skin during a workout.
Why 18.5 Micron Matters for Activewear
There's a reason the superfine range (and 18.5 micron specifically) shows up so often in premium merino activewear. It sits at the intersection of two competing demands.
On one side, softness. Activewear touches skin directly, often during movement and sweat. Anything that irritates at rest will irritate more during a run, a flow, or a set of deadlifts. And that's the last thing you want to be focusing on when you're in the zone. At 18.5 micron, the fibers are fine enough to flex against skin without triggering the itch response. You put it on and you stop thinking about it.
On the other side, durability. What you gain in softness you lose in garment longevity. Ultrafine merino below 16 micron is extraordinarily soft, but those thinner fibers are more vulnerable to abrasion — the friction from a barbell, a backpack strap, or just repeated washing. At 18.5 micron, the fiber has enough structural integrity to hold up under regular athletic use without sacrificing the next-to-skin comfort that makes merino worth choosing in the first place.
Aiua's entire product line — tees, tanks, and quarter-zips — is built on 18.5-micron superfine merino. It's the finest grade we can use while still standing behind the durability of the garment under real training conditions.
How GSM Weight Interacts with Micron Count
Micron count tells you how fine the fibers are. GSM — grams per square meter — tells you how much fabric is there. They work together.
A high micron count at a low GSM gives you a coarse, thin fabric: scratchy and flimsy. A low micron count at a high GSM gives you a soft, heavy fabric: comfortable but potentially too warm and too dense for athletic use.
For activewear, the pairing matters. Aiua uses 150gsm fabric — a lightweight construction that breathes well during exertion, layers easily under a jacket or sweater, and doesn't cling when wet. And with the 18.5-micron fiber, the result is a fabric that feels substantial without feeling heavy. Cool when you're warm. Warm when the air is cool. Present without being noticeable.
For reference, most merino base layers fall between 120gsm and 200gsm. Below 120, the fabric can feel too sheer for standalone wear. Above 200, it starts to trap heat in ways that limit its range. The 150gsm midpoint is versatile across seasons and activity levels — gym sessions in June, layered runs in October.
What to Look for When Comparing Merino Products
When you're reading product pages and comparing merino garments, micron count and GSM are the two specifications that tell you the most about how the fabric will actually feel and perform. Here's what to watch for.
If a brand doesn't list its micron count, that's usually a signal. Premium merino brands are specific because they have something specific to be proud of. A vague label like "fine merino" without a number could mean anything from 17 to 22 micron — a range that spans the difference between silk-soft and noticeably textured.
If a brand lists micron count but not GSM, you're getting half the picture. An 18.5-micron fabric at 120gsm will feel and perform very differently from 18.5 micron at 200gsm. Both are soft; they're not the same garment.
If a brand uses merino blends, check the blend ratio. A shirt labeled "merino blend" might be 20% merino and 80% polyester. The micron count of the merino fibers matters less when most of the fabric is synthetic. The merino properties — temperature regulation, odor resistance, moisture management — diminish proportionally with the blend ratio.
The Specs Behind the Feel
Micron count isn't something most people think about when they're reaching for a shirt before the gym. And it shouldn't be — the whole point of getting the specs right is that you don't have to think about the fabric once it's on.
But if you're the kind of person who reads product pages carefully, who compares before you buy, who wants to understand what you're paying for — the micron count is where to start. It's the most revealing number on the label.
Explore Aiua's extra-fine merino collection — 18.5 micron, 150gsm, no synthetic fabric — and feel the difference the intentional specs make. [Shop the Collection →]
All Aiua garments are certified OEKO-TEX Standard 100 (tested for harmful substances), Responsible Wool Standard (ethical sourcing), and BSCI verified (fair labor practices). Fabric specifications: 18.5-micron superfine merino wool, 150gsm.