Method 1: Measure with paper & ruler

• Step 1: Prepare a ruler, a pair of scissors, a pen & a piece of paper
• Step 2: Cut a piece of paper about 10 cm long and 1 cm wide.
• Step 3: Use the cut piece of paper to wrap tightly around the finger you want to measure.
• Step 4: Mark the intersection point.
• Step 5: Remove and use a ruler to measure the length of the paper from the starting point to the marked part. 

Method 2: Manual Measurement

• Step 1: Use a string (or measuring tape/thread/small paper) to wrap around the ring finger, marking the junction.
• Step 2: Measure the length of the wire you just measured.
• Step 3: Take that length minus the ring thickness multiplied by 2 (depending on the ring model, the thickness will be different).
• Step 4: Then divide by 3.14 to get the ring diameter.