Astronomy Answers
From the Astronomical Dictionary

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large numbers

Here is a list of British English and American English names of large numbers and small fractions, and the prefixes of units that go with them. In the title row, "om" stands for order of magnitude: this indicates how many zeros are involved. The prefix can be used with units: for example, a kilometer or km is equal to 1,000 meters, and a megameter or Mm is equal to 1,000,000 m or 1,000 kilometers.

Table 2: Very Large and Very Small Numbers

British	American	om	prefix	number
		21		1.000.000.000.000.000.000.000
trillion		18	exa = E	1.000.000.000.000.000.000
	quadrillion	15	peta = P	1.000.000.000.000.000
billion	trillion	12	tera = T	1.000.000.000.000
	billion	9	giga = G	1.000.000.000
million	million	6	mega = M	1.000.000
thousand	thousand	3	kilo = k	1.000
hundred	hundred	2	hecto = h	100
ten	ten	1	deca = da	10
tenth	tenth	−1	deci = d	1/10
hundredth	hundredth	−2	centi = c	1/100
thousandth	thousandth	−3	milli = m	1/1.000
millionth	millionth	−6	micro = µ	1/1.000.000
	billionth	−9	nano = n	1/1.000.000.000
billionth	trillionth	−12	pico = p	1/1.000.000.000.000
	quadrillionth	−15	femto = f	1/1.000.000.000.000.000
trillionth		−18	atto = a	1/1.000.000.000.000.000.000
		−21		1/1.000.000.000.000.000.000.000