We use standard US sizing chart. Please visit

http://www.ringsizes.co/

1. Cut up a thin piece of paper (or string) and wrap it around your finger firmly. Make sure you fit it around your finger firmly, how you would want the ring to fit.

2. Mark the thin piece of paper to get the circumference. 

3. Lay the piece of paper and measure the circumference.

4. Please visit http://www.ringsizes.co/ to measure your circumference (in mm/cm) against the US sizing column (We use standard US sizing)

5. Please choose the ring size that is closest to your circumference if you are in-between sizes.

If you have an existing ring
Alternatively, please measure the internal diameter if your current ring and use the chart to measure diameter against US sizing 
http://www.ringsizes.co/