If you are looking for a ring but aren't sure of what size your finger is to fit that ring, you can find a conversion chart online to help figure this out.
Do an Internet search engine search for "online ring conversion chart" to find a chart and a site to assist you. Most likely the site will be able to convert your measurements into different several different foreign conversions if need be. You can also do a U.S finger size on most of these sites.
The site will most likely ask you to wrap a string or piece of paper around the finger you wish to measure. You will be asked to mark the point where the two pieces of paper or string meet and then be asked to measure the distance between the two marks onto a ruler. You should then divide that number by 3.14 to get the diameter of your finger. You can then put this information into the site to get the size of ring you need for your finger.