While a jeweler can easily figure out what size ring you need, that option isn’t always available to you. When you are buying jewelry online or for someone else, you might need to figure out how to size a finger for a ring.
Use a ring size chart printed from the internet. (See Resources) A good chart will also include the measurements, for example a size 5 is approximately 15.6 mm in diameter. Wrap a small piece of thread or string around the finger you will wear the ring on so that it is snug but not too tight. Mark the measurement and measure against a ruler.
Take a good-fitting ring and place the ring on the chart until the circumference of the ring matches that on the chart, instead of using the string measurement.
Use a printed ring sizing strip. (See resources). Follow the printing directions carefully. Cut out the strip and wrap around desired finger. The strip will indicate what ring size your finger wears.
Go to a jewelry store and ask to have your finger sized. You don't have to make a purchase to get your finger sized. The jeweler will use something that looks like a key ring with lots of rings on it and will try on the rings until he finds the proper fit.