This is a great question. I get emails and telephone calls all the time about this. Especially from folks that want to order a ring from us online, either here or at our Titanium-Jewelry.com website.
Seriously, this is not how you do it. (read below)
The internet proposes many ways to size your finger. Including tying string around your finger and measuring it. (Seriously, this is LAME.) However rings are perfectly round - our fingers are not. Many of these procedures leave you with an approximate size at best. You then wind up with the wrong size ring. And this ends up being a hassle for you.
What's the best way?
The most accurate way is to be sized with a hard finger gauge. One that features separate rings for each individual size. These can be found in hard plastic or metal. They come in 7mm and 3mm flat fit and 6mm comfort fit. The 7mm wide sizer gauges are good to size your finger for a flat fit ring. The 3mm finger sizer is good for comfort fit band sizing if a 6mm comfort fit sizing gauge is not available.
Variables to consider:
Comfort fit: To be most accurate you need to know what kind of ring you are going to wear. Is the inside of the band a rounded comfort fit or is it a flat fit ring? Comfort fit rings slide over the knuckle easier than flat inside rings and therefore feel looser than the flat ones.
Width: Wide rings take up more space on your finger and therefore will feel tighter than a thin width ring.
How should it feel?
If you plan on rarely taking your ring off then the perfect size ring for you should slide on your finger relatively easy and take a bit of effort to get back off. Someone who needs to take their ring off every day for work might want it more loose than that.
Where to get help?
You can stop by your friendly neighborhood jewelry store like Yates & Co. Jewelers. Or if this isn't convenient, shoot me an email and I'll pop a plastic sizer in the mail to you - for free. (provided you are in the U.S.) Sales@YatesJewelers.com
GIA Graduate Gemologist