Green Reach

You can measure your ring size using either string or, perhaps a more common item in your home, floss. Take one of these measurement tools and wrap it around the base of your finger, marking where the string or floss first overlaps with a pen. Then, line that up with a ruler and take down its length in millimeters

With standard ring sizes, every half-size matches up to a 0.4 millimeter increment, starting with size 3 equaling 14 mm, size 3.5 equal to 14.4 mm, size 4 equal to 14.8 mm, and so on. Common ring sizes for women are 6 (16.5 mm), 6.5 (16.9 mm), and 7 (17.3 mm). For men, the most common sizes are 10 (19.8 mm), 10.5 (20.2 mm), and 11 (20.6 mm)

Step 1 - Get a non-stretchy string or paper about 6"(152MM) long and 1/4"(6.5MM) wide.
Step 2 - Wrap around the base of your finger. It has to fit snug, but not too tight.
Step 3 - Mark the point on the string/paper where it overlaps forming a circle.
Step 4 - Measure the length of the string/paper from the starting point to the mark.