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).
You can also measure your ring size using a piece of jewelry that you already have and know fits you. Print out a true-to-size ring size chart and place your ring on the circles until you find the one that matches up with the inside circumference of your ring.