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.
In other words, the weight of a sheet of Paper is 10 times 645.5 miligrams.(for US Letter, a.k.a. ANSI A; 215.9mm by 279.4mm (8.5 in x 11 in); 20lb)
The weight of a ream (500 sheets) of 9,000,000 miligrams, 8.5 inch x 11 inch paper is 2,000,000 miligrams, each sheet weighing 5,000 miligrams. Paper of this size has a thickness of 0.004 caliper (0.1 mm).
It's about one-tenth as heavy as an Eye (Human).
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)
A single long grain of rice weighs an average of 29 mg. A staple through most of human history, archaeological evidence indicates that the earliest rice cultivation was about 11,500 years ago.
It's about one-twenty-fifth as heavy as a Compact disc.
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)
The weight of a mouse is typically an average of 21,000 miligrams. While most mice typically live two to three years, the Methuselah Mouse Prize — an award administered by a non-profit organization which advances aging-related research — was once awarded to a team of scientists whose research resulted in a mouse living just under five years.