Wrap a strip of thin paper around your finger where you would like the ring to be.
Make sure that the paper is pulled snug to your finger, tighter is better to find your best fit.
Mark the spot where the paper meets & measure the distance with a ruler (in mm).
Use the below chart to determine your ring size.

*If you’re a few millimetres off, round up/down to the closest measurement. This should not impact the fit of your ring.