You can use the ring size chart below in two different ways, both for finding your ring size based on finger measurement (easily measured at home, see below), or to determine the size of a already perfect fitting ring (so that you can buy the same ring size again). Smart, huh?
Determine ring size based on finger measurent: Use a flexible measuring tape (mm/cm) to measure the circumference around your ring finger (exactly where the ring is supposed to sit). Do not tighten the measuring tape too much, it is supposed to sit quite loose around your finger. Look up your measurement in the ring size chart below. Find your measurement in the column "C" (Circumference), and then find the corresponding ring size ("Ring size" column).
Find ring size based on ring measurement: Use a ruler or measuring tape (mm/cm) to carefully measure the inside diameter of a ring (edge to edge). Make sure to measure where the diameter is largest (center). Look up your measurement in the ring size chart below, in the "DIA" (Diameter) column. Find the corresponding ring size. Note that the ring size is almost the same as the ring's inside circumference. Good to know!