Your ring should fit snuggly on your finger without risk of falling off. Not too tight. Not too loose. You do not want to be hurting yourself or putting too much pressure on the ring or your finger getting it on and off. It should just be able to fit over the largest part of your finger which is often the knuckle for most people.
Keeping this in mind; it is really important to check your ring size more than once which applies to both a simple home check or even a proper sizing in-store with a jeweler. Your fingers will swell in warmer weather and vice-versa in cold weather. This is particularly important for wedding and engagement rings as they will be worn everyday for a long period of time. Check your size on a cold day and again on a warmer day. Checking two or three times is not going too far because getting it right the first time could save you time, frustration and money on potentially expensive resizes.
Think About The Type Of Ring
Did you know that wide rings require a bigger size? Bands approximately 6mm wide or more generally require a slightly larger size for most people. Narrower rings allow your skin to move to either side of the band and often leave that tell-tale 'ring mark' on your finger when you take it off. Wide bands don't allow this so much which means there is 'more finger' under the band.
And don't forget that some rings may not allow sizing to your size. This is most true of rings with larger and/or relatively 'open' settings. For example, if you have very small fingers and the ring needs to be sized down a lot then the setting could be at risk. As part of the band (usually the bottom opposite the setting) is cut out to size down, it can pull the setting open at the top. Your jeweler will be able to discuss this with you but for those of you buying on-line, you should keep this in mind.
Make Regular Checks
Don't forget to give your rings regular checks. Check them for wear and look for any cracks in the band or any obvious thinning of the metal which is causing serious bending. It is always much better for your ring and your bank account to get onto band wear earlier rather than later. And if you are getting to the point where you can only get your ring off occasionally because it is too tight or it is regularly slipping off; it's probably time for a resize. No one wants to have their ring cut off or worse, have it slip off and lose it!