It's about one-tenth as heavy as a Quarter.
In other words, 645.5 miligrams is 0.1138 times the weight of a Quarter, and the weight of a Quarter is 8.787 times that amount.(United States quarter-dollar coin) (a.k.a. Quarter dollar, a.k.a. America the Beautiful quarter, a.k.a. two bits)
The America the Beautiful series quarters, which began to enter circulation in 2017, weigh 5,670 miligrams each. In answer to the common riddle, there are 119 ridges around the edge of the coin in its current design.
It's about one-tenth as heavy as a sheet of Paper.
It's about one-tenth as heavy as an Eye (Human).
In other words, the weight of an Eye (Human) is 11 times 645.5 miligrams.(adult)
It's about one-twentieth as heavy as a Battery (AAA).
In other words, the weight of a Battery (AAA) is 19 times 645.5 miligrams.(Alkaline)
An "AAA" size battery weighs 12,000 miligrams. Batteries are named by the American National Standards Institute (ANSI) Committee C18 and AAA batteries were given their name because they were smaller than the "AA" and "A" size batteries which were invented first.
It's about twenty times as heavy as a Grain of Rice.
In other words, the weight of a Grain of Rice is 0.045 times 645.5 miligrams.(long grain; 14% moisture content)
It's about one-twenty-fifth as heavy as a Compact disc.
In other words, the weight of a Compact disc is 23 times 645.5 miligrams.(a.k.a. CD)
It's about one-thirtieth as heavy as a Mouse.
In other words, the weight of a Mouse is 32 times 645.5 miligrams.(a.k.a. "House mouse", Mus musculus) (adult)