Here are two ways to do it – measure the circumference of your finger or internal diameter of the existing ring.
MEASURE YOU FINGER
Step 1: Take either a length of string or floss and wrap around your finger. At the place where it overlaps, mark the point with a pen.
Step 2: Unwrap it and lie it flat beside a ruler or measuring tape. Measure circumference of your finger in inches or mm.
Step 3: Use the below chart to determine your ring size.
Tip: make sure you don't pull the thread or string too tightly, as it can be stretchy and then it may seem that you need a bigger ring size than you actually require.
MEASURE EXISTING RING
Step 1: Select an existing ring that fits the desired finger.
Step 2: Measure the internal diameter of the ring (in inches or mm).
Step 2: Use the below chart to determine your ring size.
Tip: make sure you compare the internal diameter, not the outer part of the ring.