
It's about fourfifths as heavy as a Nail.
In other words, 645.5 miligrams is 0.82 times the weight of a Nail, and the weight of a Nail is 1.2 times that amount.
(for 15gauge, 2inch, finishing nails)
It's about threefifths as heavy as a Jelly Bean.
In other words, 645.5 miligrams is 0.59 times the weight of a Jelly Bean, and the weight of a Jelly Bean is 1.7 times that amount.
(for Jelly Belly beans; approximate)
It's about half as heavy as a Paper Clip.
In other words, 645.5 miligrams is 0.46 times the weight of a Paper Clip, and the weight of a Paper Clip is 2.2 times that amount.
(average)
It's about onethird as heavy as a Playing Card.
In other words, the weight of a Playing Card is 2.8 times 645.5 miligrams.
(ISO 216 B8 size, 2.5" x 3.5")
It's about three times as heavy as a Raindrop.
In other words, the weight of a Raindrop is 0.3 times 645.5 miligrams.
(a.k.a. drop, a.k.a. droplet) (average)
It's about onefourth as heavy as a Penny.
In other words, 645.5 miligrams is 0.26 times the weight of a Penny, and the weight of a Penny is 3.8 times that amount.
(United States) (formally onecent coin) (Union shield design, 2010present)The weight of a United States penny is 2,500 miligrams. According to a 2010 report by the United States Mint, the cost of manufacturing and distributing a penny is $0.0179 — more than its facevalue.
It's about onefifth as heavy as a Die (Dice).
In other words, 645.5 miligrams is 0.16 times the weight of a Die (Dice), and the weight of a Die (Dice) is 6.3 times that amount.
(for sixsided, 16 mm, rounded cube) 
