For the most accurate result, it is always recommended that you get your size professionally measured at your local jeweler. Make sure to ask for the inner diameter or inner circumference of the ring in millimeters as sizing systems can vary between countries and conversion charts are not always accurate.
Note that your size might fluctuate during the day, usually depending on the temperature, so keep that in mind when getting your size measured.
If you can't visit a local jeweler, you can find your size using a ring you already own, preferably a simple, slim wedding band style ring. Measure the inner diameter of your ring (try to be as precise as possible) and match it to the chart below.
*Typically, wide rings need to be larger in size than slim/narrow rings, in order to fit properly.
When ordering a ring, choose the size you normally wear in a narrow band ring. The width of the ring you ordered will be taken into account during the production process and the size will be adjusted accordingly.