A first method to find out your ring size is to use a tape measure. A second option is to measure with an existing ring and compare its size with a measurement chart.
To obtain the best result, you should leave enough space for your knuckle. Attention: In the course of the day your finger may vary in size, depending on different factors like temperature and humidity. It is therefore advisable to measure your finger at the end of the day, when it is normally at its largest. There may also be a difference between the fingers on your left and right hand. Make sure you measure the finger you will wear the ring on.
Download the measurement card here.