Our sizes: EU 36-38, 39-42, 43-46 (and 47-48 on some models). Our socks run slightly small, so if you're between two sizes, always go up.
The most reliable method is to measure your foot: stand with your heels against a wall and measure to the tip of your longest toe. Do it at the end of the day, when your feet are at their largest.
Up to 24.5 cm, take 36-38. Between 24.5 and 27 cm, take 39-42. Between 27 and 29.5 cm, take 43-46. Above that, take 47-48.
Go by your foot measurement, not by the size of your hiking boots.
If you're unsure, send us your measurement and Lisa will advise you within a few hours.