Measure size
Knowing your ring size is not something that can be taken for granted.
So how do you measure your finger to get a fair measurement?
My tip is to take a ring that fits your finger, place it on a tape measure/ruler and measure from inside to inside. This will give you a measurement that should then be read in millimeters.
Let's say you get 17 mm, then your ring size is 17.
It doesn't get any more difficult than that.