1. Determine which finger the ring will be worn on: Different fingers have different sizes. 
  2. Measure the finger at the end of the day: Fingers tend to be slightly larger at the end of the day than in the morning. This is because your fingers expand throughout the day as you use them.
  3. Use a ring sizer: Ring sizers are small, plastic or metal tools that come in a variety of shapes and sizes. They usually have markings on them to indicate ring sizes. To use a ring sizer, slip it onto the finger you want to measure and adjust it until it feels comfortable. Take note of the size indicated on the sizer.
  4. Use a piece of string or paper: If you don't have a ring sizer, you can use a piece of string or paper to measure the finger. Wrap the string or paper around the base of the finger and mark where it overlaps. Measure the length of the string or paper in millimeters and use a ring size chart to determine the size.