Take a string or strip of paper and wrap it around the base of the desired finger.

Mark the point where the end meets the string or paper. 

Measure the string or paper on a ruler inmillimetersto the point where you marked it.  

Find the closest corresponding ring size on the chart below:

Your fingers change size over the course of the day. Keep in mind if you're very warm, your fingers will be bigger and if you're cold they'll be smaller. Measuring when you're a bit warm will ensure that the ring will fit all the time. 

Keep in mind, your dominant hand will almost always be slightly bigger. Make sure to measure each finger individually.

If your knuckle is bigger than the base of your finger, measure the widest point as well as the base of your finger and pick a size that is in between both. 

When using a string or paper to measure your finger, don't let it get loose but don't pull it tight. Doing either will skew the size of your ring. 

For best results, measure 3 or 4 times and compare results. You want to make sure the sizing is accurate.

If you have a current ring that fits you can measure the inside diameter and use that to find your ring size.