Keep in mind that your ring size can change throughout the day and can be affected by weather conditions, medication and changes in your weight. Your fingers tend to be larger in the evenings and when it is hot.
So we recommend measuring your finger once in the morning and again in the evening to make sure you get the right fit.
In general a ring is the perfect fit when it sits well enough to not fall off, but loose enough to fit over your knuckle without too much difficulty.
If you have a ring and you want to know its size you can also print out this PDF (make sure to print the actual size and do not choose "adjust to page“). To be sure that the sizes are displayed correctly, just take your ruler and measure the diameter of a circle. It should match with the length that you read on your ruler. Now place your ring on the circle that you see most fitting. Make sure that the inner wall of your ring will fully cover the circle line.