There is an app called Ring Sizer by Jason Withers. It is very easy to use and precise. I would suggest using one of your current rings with the app, to find out your ring size.
Below is a ring sizing chart which is not to scale. To figure out your ring size use a precise measuring tool to measure the inside of a ring and compare it to this chart.
Alternatively if you are in the Waterloo Region, I have ring sizers that we can use to get your accurate size.