Ring sizing guide
Take a piece of string or dental floss (avoid something that stretches)
Measure your finger by wrapping the string around mark the string at the point of overlapping.
Measure the string with a ruler or measuring tape.
Now go to the ring sizer chart and find your ring size
If you have a bigger knuckle, measure the knuckle as well and choose a size nearer to the knuckle size.
If you have a ring that fits your finger, measure the inside of the ring and compare to the chart.