It's about one-thirty-fifth as heavy as a Battery (AA).
In other words, the weight of a Battery (AA) is 37 times 645.5 miligrams.(Alkaline)
An "AA" size battery weighs 24,000 miligrams. AA batteries came into common use during World War II and were given their name by the American National Standards Institute (ANSI) Committee C18 because they were smaller than the "A" size batteries which were invented first.
It's about one-fifty-fifth as heavy as a Light Bulb.
In other words, the weight of a Light Bulb is 53 times 645.5 miligrams.(a.k.a. lamp) (for A19-size, 60w, incandescent bulb by Bulbrite; package weight)
A standard, sixty-watt light bulb weighs about 34,000 miligrams. The tightly-coiled filament inside a bulb of this size would measure about 579mm if uncoiled and stretched out.
It's about one-seventieth as heavy as a Golf Ball.
In other words, 645.5 miligrams is 0.01405 times the weight of a Golf Ball, and the weight of a Golf Ball is 71.17 times that amount.(per R&A USGA Rules of Golf; maximum rule weight)
According to the Rules of Golf (as approved by the United States Golf Association and the Rules Committee of the Royal and Ancient Golf Club of Saint Andrews), golf balls must weigh no more than 45,930 miligrams. Golf balls are not required to have the familiar dimpled pattern, but the design has been popular since its invention in 1905 because it reduces drag while increasing lift.
It's about one-ninetieth as heavy as a Tennis Ball.
In other words, 645.5 miligrams is 0.0112 times the weight of a Tennis Ball, and the weight of a Tennis Ball is 89.3 times that amount.(ITF Standard; for Type 2 ball)
Per International Tennis Federation standard, regulation tennis balls must weigh about 57,700 miligrams. Competition balls are also tested for bounce height, with the standards requiring that a ball dropped from 254 cm must bounce back to a height of 135 cm to 147 cm.
It's about one-one-hundredth as heavy as a Battery (C).
In other words, 645.5 miligrams is 0.0099 times the weight of a Battery (C), and the weight of a Battery (C) is 100 times that amount.(Alkaline)
A "C" size battery weighs 65,000 miligrams. C batteries are typically used in medium-drain devices such as musical instruments.
It's about one-one-hundred-fiftieth as heavy as a Deck of Cards.
In other words, the weight of a Deck of Cards is 140 times 645.5 miligrams.(ISO 216 B8 size, a.k.a. "poker size" 63 x 88 mm, 52-card deck)
A deck of 52 ISO 216 B8-size playing cards weighs 94,000 miligrams. Utilizing nearly 3,700 decks, the record-holding largest card tower was built in early 2010, stood over 3.5 m (11 ft), and weighed over 230,000,000 miligrams.
It's about 150 times as heavy as a Grain of Sand.
In other words, the weight of a Grain of Sand is 0.0067 times 645.5 miligrams.(medium grain)
With tremendous variety in the size of grain and the component material, grains of sand weigh an average of 4.4 mg. In 2007, a team of seven in Myrtle Beach created the world's tallest sandcastle, which measured 15.10 m (49.55 ft) in height.