Depends on the manufacturer... and their definitions of 8, 10, 12, etc.
But, commonly, XXS=6, XS=8, S=10, M=12, L=14, XL=16, XXL=18
Take care of the US numeric sizes, which are 2 smaller than European, US4=UK6.
Sadly, there's little consistency in clothing sizes, even within numeric sizing, let alone conversions between numeric and descriptive sizes (S, M, etc). Not to mention the issue of 'vanity sizing'...
Wiki page on clothing sizes
Many outdoor manufacturers stick to numeric sizes for women, or quote both numeric and descriptive sizes. e.g. Sprayway.