Before you measure your ring size, there are a few pointers to keep in mind:
Consider your knuckle: If you know your knuckle is on the larger side, order half a size bigger than what your finger alone indicates, otherwise you won't be able slide a ring into the right place.
Ensure accuracy with multiple tries: Catbird's site recommends measuring your finger three to four different times because the size fluctuates depending on temperature: It can be smaller the colder you are and more swollen if you're overheated.
How to Measure Your Ring Size using a string or floss:
Take a length of floss or string and wrap it around the base of your finger.
Mark where the floss or string first overlaps with a pen.
Stretch the length of string out along a ruler or measuring tape, and take down the length in millimeters.
Use the ring size chart below to compare your measurement to the standard ring sizes, which go up by 0.4 millimeters every half-size. For instance, a size 3 ring is 14 mm, a size 3.5 ring is 14.4 mm, and a size 4 is 14.8mm. The most common ring sizes for women are 6 (16.5 mm), 6.5 (16.9mm), and 7 (17.3 mm), while the most common men's sizes are 10 (19.8mm), 10.5 (20.2 mm), and 11 (20.6 mm).
Tip:String and floss can stretch, so don't pull them too taut. If you do, your measured ring finger size may be larger than your finger and any rings you purchase may be too big.