Best option: get sized by us or at your nearest jewellery store, otherwise use the following methods.
Method 1: Use a ring that already fits you well and measure the inside diameter at the widest point as shown below.
Method 2: Cut a strip of paper long enough to wrap around your entire finger at the base (paper width no more than 1/4”) and make your mark. Measure against a ruler in millimeters. That’s your circumference.
As a last option you can also use a soft tape measure, however, this may not give as accurate a measurement as the previous methods.