i believe you mean US sizes
Diam = 20.7 mm
Circum = 64.1 mm
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19.5 ring size is
Divide the circumference by pi (~3.14) to get the diameter.
If the ring is 2 cm in diameter, the US ring size is 10 and a quarter. In terms of British and Australian size, it is said to be U.
The three most common ways to describe the dimension or size of a circle is either the diameter or the radius or the circumference (perimeter).When the circumference is given then dividing that by Pi will give you the diameter (circumference / Pi = diameter).
Circumference = π*d = π*80 = 251.33 units