You can order a ring of any size in any sizing system.
If you are not sure about your size, this page might be helpful: findmyringsize.com
If you want to buy the ring as a gift and you don't know the precise size, you may also choose a standard size: Small (UK- M, USA - 6) Medium (UK - P, USA - 7,5) Large (UK - S, USA - 9).
