UK and European sock and shoe sizes
Sock sizes are based on length and volume. If you are between sock sizes and if you have a narrow foot (width D or less) then you should choose the smaller sock. For unisex socks, women should choose the smaller sock if they are on the border between sizes because women's feet, for a given size, often have a smaller volume than men's feet.
Generally, technical socks should have a tighter fit than normal socks. The below table contains details for X-Socks, Thorlo, Bridgedale and Sealskinz socks.
Click this link for a detailed shoe size conversion chart.
shoe size chart
sock size chart