It's about 0.0000000000008 times as heavy as The Golden Gate Bridge.
In other words, 645.5 miligrams is 0.000000000000802 times the weight of The Golden Gate Bridge, and the weight of The Golden Gate Bridge is 1,250,000,000,000 times that amount.(San Francisco, California and Marin County, California) (Post 1986 weight reduction; total weight of bridge, anchorage, and approaches)
Following the completion of the 1986 project to eliminate excess material, the total weight of the Golden Gate Bridge — including its anchorages and approaches — is 805,000,000,000,000.0 miligrams. The two main cables in the spanning the bridge are spun out of a total of 129,000 km (80,200 mi) of wire.
It's about 0.0000000000001 times as heavy as The Great Pyramid of Giza.
In other words, 645.5 miligrams is 0.00000000000011 times the weight of The Great Pyramid of Giza, and the weight of The Great Pyramid of Giza is 9,100,000,000,000 times that amount.(a.k.a. Pyramid of Khufu, a.k.a. Pyramid of Cheops) (Cairo, Egypt)
The Great Pyramid of Giza has an estimated mass of 5,900,000,000,000,000.0 miligrams. The 2,300,000 stones used in the construction of the pyramid weighed an average 2,300,000,000 miligrams and were quarried from distances of up to 805 km (500 mi) away.
It's about 0.0000000000000000000000000000000003 times as heavy as The Sun.
In other words, 645.5 miligrams is 0.00000000000000000000000000000000032 times the weight of The Sun, and the weight of The Sun is 3,100,000,000,000,000,004,221,441,750,859,776.0 times that amount.The sun has a mass of 1,999,999,999,999,999,879,418,332,743,206,357,172,224.0 miligrams which represents 99.9% of the solar system's total mass. 98.7% of the sun's mass consists of hydrogen and helium — respectively the lightest and second-lightest elements in the universe.
It's about 0.0000000000000000000000000001 times as heavy as The Earth.
In other words, 645.5 miligrams is 0.00000000000000000000000000010806 times the weight of The Earth, and the weight of The Earth is 9,254,099,999,999,999,772,962,324,480.0 times that amount.The mass of the planet Earth is 5,973,599,999,999,999,901,033,752,952,832.0 miligrams. The element iron makes up about 32% of that mass, or 1,917,500,000,000,000,131,719,197,884,416.0 miligrams.