GSM (grams per square metre) tells you how much fabric there is, not how itchy it is. Every Merino.tech weight uses the same superfine 17.5-micron Australian merino wool; heavier fabric simply traps more warm air. Choose your weight by temperature and by how hard you will be working.
Merino weights compared
| Weight | Warmth | Best for | Typical temperature* |
|---|---|---|---|
| 165gsm lightweight | Light | Summer, running, cycling, travel and layering under warmer tops | 15°C and above, or colder when working hard |
| 190–200gsm light-midweight | Moderate | Everyday wear, spring and autumn walks, layering under jeans | 5–18°C |
| 250gsm midweight | Warm | Skiing, snowboarding, winter hillwalking, cold commutes | -7°C to 7°C |
| 320gsm heavyweight | Very warm | Deep cold, fishing, stalking, winter camping, spectating | Below -4°C, or whenever you are static |
*Worn as a base layer under suitable outer layers. Your activity level and how warm you run make a big difference.
How to choose your merino weight
If you run hot or work hard
Go one weight lighter than you think. Sweat is what makes you cold when you stop, and lighter merino breathes and dries faster. Runners, cyclists and fast walkers usually prefer 165gsm, even in winter.
If you feel the cold or stand still
Go heavier. 320gsm holds the most warmth for fishing, shooting, watching football from the stands or waiting for the kids at the ski school.
The classic UK layering system
A 165gsm merino top next to the skin, a 250gsm merino or fleece mid layer and a waterproof shell handles almost anything the British weather throws at you. Add or remove the mid layer as conditions change.