Before guessing a ring size the most accurate way to get one would be to go to your nearest jeweller and ask them to measure your intended finger. However if this is not possible the next best option would be to get a ring which already fits the finger you want the ring for, measure the inside diameter of the ring. From this measurement (mm) use the conversion chart below to see which ring size suits you best. You can also measure the circumference of your finger by wrapping a narrow strip of paper around the finger you intend to use. Again use the below chart to find out your ring size. If the size fits between two numbers rather use the bigger one as it is more likely to be accurate. Remember that fingers swell when hot and shrink when cold.
Your ring should be easy to put on and a little tricky to take off to ensure it won't fall off.