What Is Merino Wool?
Merino wool comes from merino sheep, a breed whose fleece is far finer than ordinary wool.
Fineness is the entire difference. Ordinary wool runs 25 to 40 micron and is too coarse to wear against skin. Merino generally runs 15 to 24 micron, which is fine enough to be worn as a first layer rather than over one.
The two numbers that describe any wool garment
Micron is the average diameter of the fiber. Lower is finer and softer. Below roughly 22 micron, most people stop feeling prickle.
GSM is grams per square metre, meaning how heavy the fabric is. Roughly 150 gsm is what the industry calls lightweight, 200 is midweight, 250 and up is heavyweight. Lighter fabric layers invisibly and works in warm weather. Heavier fabric holds more warmth and is harder to wear under things.
Between them, these two numbers tell you more than any adjective on a product page.
What merino actually does
- Regulates temperature in both directions. Wool fibers trap air when it is cold and move moisture vapor away when it is warm, which is why the same shirt works across a wider range than cotton does.
- Resists odor. Wool manages moisture in a way that gives odor-causing bacteria far less to work with. In practice this means several days of wear between washes.
- Handles moisture without feeling wet. Wool can absorb a substantial share of its weight in moisture vapor before it feels damp against skin.
- Biodegrades. Wool is a protein fiber and breaks down at the end of its life. Synthetic fabrics do not, and they shed microplastic in the wash.
The blend question
Many garments sold as merino are not entirely merino. To reach a light fabric weight, brands commonly blend in polyester or nylon, which is cheaper and easier to knit. This is disclosed on the label but rarely in the marketing.
A blend is not automatically worse. Synthetic content adds abrasion resistance and dries faster. It does reduce the odor resistance and the biodegradability, and it means the garment sheds plastic when washed. The point is simply to know which one you are buying.
Worsted and woolen spun
Worsted spinning combs the fibers parallel before twisting them. The resulting yarn is smoother, stronger and pills far less. Woolen spinning leaves the fibers jumbled, which makes a loftier and warmer but fuzzier yarn that pills more readily. Worsted is slower and more expensive. Next-to-skin garments are better for it.
How to judge a brand
Ask three questions. Does it publish the micron count? Does it publish the fabric weight in gsm? Does it tell you where the fabric was knit, rather than only where the company is headquartered? A brand that answers all three is telling you something, and a brand that answers none is also telling you something.
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