Most merino can be machine washed cold on a gentle or wool cycle with a mild detergent, then dried flat. The garments that get ruined are almost never ruined by washing — they are ruined by heat and agitation, usually in a tumble dryer. The routine below keeps base layer tops and knitwear in shape for years.
Check the label first
Merino reaches you in two forms and they tolerate different treatment. Untreated wool felts when heat and agitation combine, because the fibre surface is covered in microscopic scales that interlock and cannot be separated again. Machine-washable merino has been treated — usually a chlorine-Hercosett process that smooths or coats those scales — so it survives ordinary washing.
The label tells you which you have. A wool cycle symbol or "machine washable" means treated. A hand-wash symbol means untreated, and it means what it says. When there is no label at all, treat the garment as untreated and hand wash it.
The routine
- Turn the garment inside out. This puts the abrasion on the inner surface, which reduces pilling on the side people see.
- Close zips and fasten anything with a hook. Hard fastenings snag fine knits — including other garments in the same load.
- Wash cold on a wool or delicate cycle. Cold water and a low spin speed. Heat plus agitation is the combination that felts wool, and removing the heat removes most of the risk.
- Use a small amount of mild or wool-specific detergent. Ordinary biological detergents contain protease enzymes that digest protein — and wool is protein. Repeated use degrades the fibre.
- Skip the fabric softener. It coats fibres, reduces absorbency and undermines the moisture behaviour you paid for. Merino does not need it.
- Reshape while damp. Lay the garment out and gently push it back to its intended dimensions before drying. Wool sets as it dries.
- Dry flat, away from direct heat. Hanging a wet wool garment lets its own weight stretch it out of shape. Flat on a rack or towel is correct.
Detergent matters more than people expect
Wool is keratin, the same protein family as hair. Most mainstream laundry detergents are enzymatic, and the enzymes that make them effective on food and blood stains work by breaking down protein. Used on wool repeatedly, they thin the fibre and shorten the garment’s life.
- Use: a wool-specific detergent, or a mild non-enzymatic liquid.
- Avoid: anything labelled biological or enzyme-based, and anything containing bleach.
- Use less than you think. Excess detergent does not rinse out fully and leaves residue that attracts dirt and dulls the fabric.
Drying is where garments die
A tumble dryer combines the two things wool cannot tolerate — heat and mechanical agitation — which is why it is responsible for most ruined merino. Untreated wool will felt irreversibly. Even treated, machine-washable merino will shrink and lose its hand over repeated dryer cycles.
Tumble drying is also a documented source of microfibre release into the air, so skipping it is marginally better environmentally as well as for the garment. Flat drying takes longer and costs nothing.
Where it goes wrong
- Hot water. The most common cause of shrinkage. Cold water washes merino perfectly well — wool releases odour readily and does not need heat.
- Tumble drying. The most common cause of permanent damage. See above.
- Biological detergent. Slow damage rather than sudden, which makes it easy to miss until a garment thins out.
- Overloading the drum. More garments means more agitation and more friction, which drives pilling.
- Washing with denim or Velcro. Abrasive fabrics and hooks destroy fine knits. Wash merino with soft items or on its own.
- Washing too often. Every cycle wears a garment. Merino resists odour well enough that airing is frequently sufficient — our guide on how often to wash merino covers when it is genuinely needed.
- Hanging wet knitwear. Gravity plus a saturated garment equals a stretched, misshapen one.
If a garment has already shrunk
If the wool was treated and the shrinkage is mild, there is sometimes a partial recovery. Soak the garment in cool water with a capful of hair conditioner for around 20 minutes, which relaxes the fibres, then gently stretch it back towards shape on a flat surface and let it dry pinned out. Expect to recover some of the loss, not all of it.
If the garment has felted — the surface looks matted and dense, and the knit structure is no longer visible — it is not recoverable. Felted wool still has uses as it is, but it will not return to a wearable base layer.
Washing socks and heavier knits
Merino socks are usually blended with nylon and tolerate washing better than pure wool tops. They still benefit from a cold wash and flat drying, and they collect more dirt and body oil than a shirt does, so they need washing every wear. Turn merino socks inside out to get the debris out of the pile.
Heavier knitwear and merino jackets should be washed rarely — spot clean where possible, air after wearing, and run a full wash only when genuinely needed. When you do, the routine above applies, but support the garment’s weight when moving it wet: a saturated heavy knit will stretch under its own mass if you lift it by the shoulders.
Once clean and fully dry, store wool properly — damp storage invites mildew and dirty wool attracts clothes moths. Our guide to storing merino covers off-season storage.
Related merino guides
Sources
- Woolmark: how to wash wool
- Woolmark: how to care for wool
- Icebreaker: how to care for merino wool
- Environmental Science and Pollution Research: microplastic fibres from machine washing and tumble drying
Image: Merino Mart catalogue. Browse base layer tops.