I make a lot of rings. A lot.
And quite often I get a request to resize a ring that a customer has bought. There are loads of reasons for this, because ring sizing is tricky.
Its very hard to estimate a ring size; half a mm out and the ring is wobbly on your finger, and it is so disappointing to unpack a ring and find that the size is not right, so here's a few tips to get it right first time around.
By far the best method is to go to a jeweller and have your finger sized. They always have a set of sizer rings that you can try and really get a feel for how it sits on your finger.
Allow enough room to accommodate your knuckle.
Bear in mind, different fingers on the same and the opposite hand may have different sizes. Therefore, measure the specific finger on which you are planning to wear the ring.
Measure your finger at the end of the day when your finger is likely to be largest.
Measure with a ring the same width as the one you are planning to buy as a wider ring will need to be bigger to be comfortable.
A good rule is "Slide on, wiggle off". It should be easy to get the ring on, but if it slides straight off again it is too large. If it stays on until you wiggle it off you're less likely to lose it accidentally.
If you can't get to a jeweller, second best would be to buy a ring sizer.