American Size to British Size [+8REP]

Ok, basically im ordering a TWLOHA tshirt and the sizes are American sizes which are different to the British sizes, could anybody tell me what a Male British size Medium would be in American size.

+Rep will be given for people who help, supply links to websites that could help with this.

Thanks!

Don't worry!
I have found a site now, and I have found out the sizes.

Don't post

Oh no, i was just about to tell you, no rep for me then XD

It's 4 sizes down from British to USA.