Step 1
Take a piece of non-stretchable string, ribbon, thread, or a thin strip of paper. Wrap it comfortably around your ring finger, close to the knuckle.

Step 2
With a pen, carefully mark the spot where the string overlaps. 

Step 3
Unwrap your marked length. With a ruler, measure it in either in millimeters or inches.

Step 4
Using the ring size chart below, find the closest ring circumference measurment to your string length measurement and view the corresponding US ring size. Always size up if between sizes. 

Step 1
Select an existing ring that fits the desired finger.

Step 2
Measure the internal diameter of the ring (in mm).

Step 3
Using the ring size chart below, find the closest ring diameter measurment to your internal diameter measurement and view the corresponding US ring size. Always size up if between sizes. 

Make sure to measure the finger where you plan to wear the ring
since the finger size on your left and right hands may differ.

Measure your finger in the evening
Measure your ring size at the end of the day when fingers are at their largest

Measure a few times
Be sure to measure your finger size a few times to make sure your calculations are accurate.

Measure your finger close to your knuckle
This is where the ring will usually sit, so you want it to be comfortable.

Control for variables
Avoid Measuring when it is too hot or cold, after an intense workout or if you are sick. Your fingers may have shrunk or swollen up to half a size. Alcohol and salt can make your fingers swell so refrain from measuring your ring size after cocktails and appetizers.

Consider the width of the band
A particularly thick band will feel tighter than a thin band so you may want to buy a full size larger that your normal ring size.