How do I figure out the correct ring size?

You can go to a professional jeweller and get your ring size measured. (In that case, kindly provide us with the "inside diameter mm" measurement). 

Alternatively, print out this Ring Size Chart and check it with an existing similar-style ring. Please remember that the same finger on opposite hands may be a different ring size, and so always measure the correct finger. If in doubt, the UK average is a size L1/2, the US average is a size 6.5 and the average ring size in the Netherlands is 53.1mm.

Lastly, you can request a sizer 

What if I choose the wrong size? 

You can have most rings resized and our first resizing, within your 45 days standard exchange/refund period, is complimentary (after this period, we may charge a small fee). Usually resizing 2 sizes up or down is no problem. Do bear in mind that it may not be possible to resize ring bands that have small gems 'pavé' set in the bands.