If you've chosen to shop online for a ring either because you want something hand-crafted from a unique material, or you just dislike jewellery stores, you'll still need to know the right ring size before you buy. Grab a roll of scotch tape and this handy ring sizing chart from Blue Nile to get a good idea of what size you should buy.
Reader Wayne shared this tip at Google+ that helped him size his next ring, even though he didn't have one to use with the sizing chart.
Wrap a piece of scotch tape around your ring finger in reverse (so you can slide it off when it's sealed.) Move it up to your thickest knuckle if you have particularly large ones, and close off the loop. Then print out the sizing chart, making sure to not stretch or alter the dimensions of the printed image. The chart asks you to put a ring you already own on the chart so you can see the size, but you can use the scotch tape loop instead.
Scotch tape is flexible, and it's certainly no replacement for a proper fitting, but it can give you the freedom to size your finger without subjecting yourself to a jewelry store and the scorn of the salespeople if that's all you really need. If you -- like many people -- are turning to the web for better pricing on jewellery, or handmade rings from artisans who sell them online, this is a great way to size, buy, and then get it adjusted locally if you need to.