SI Units and Metric Prefixes

The International System (SI) of weights and measures is the standard in the sciences. It is closely related to the Metric System, and uses the same metric prefixes. Dr. Russ Rowlett, Director of the Center for Mathematics and Science Education at the University of North Carolina at Chapel Hill, has compiled an excellent summary of the SI system, the Metric System, and units in general.

Given the great range of scales encountered in astronomy, one can understand the value of metric prefixes to avoid the use of large exponents. The following table (adapted from Dr. Rowlett's web site) summarizes the common and uncommon metric prefixes that have been approved by the General Conference on Weights and Measures.

yotta- (Y-) 1024 1 septillion 1,000,000,000,000,000,000,000,000
zetta- (Z-) 1021 1 sextillion 1,000,000,000,000,000,000,000
exa- (E-) 1018 1 quintillion 1,000,000,000,000,000,000
peta- (P-) 1015 1 quadrillion 1,000,000,000,000,000
tera- (T-) 1012 1 trillion 1,000,000,000,000
giga- (G-) 109 1 billion 1,000,000,000
mega- (M-)       106 1 million 1,000,000
kilo- (k-) 103 1 thousand 1,000
hecto- (h-) 102 1 hundred 100
deka- (dk-) 10 1 ten 10
deci- (d-) 10-1 1 tenth 0.1
centi- (c-) 10-2 1 hundredth 0.01
milli- (m-) 10-3 1 thousandth 0.001
micro- (µ-) 10-6 1 millionth 0.000001
nano- (n-) 10-9 1 billionth 0.000000001
pico- (p-) 10-12 1 trillionth 0.000000000001
femto- (f-) 10-15 1 quadrillionth 0.000000000000001
atto- (a-) 10-18 1 quintillionth 0.000000000000000001
zepto- (z-) 10-21 1 sextillionth 0.000000000000000000001
yocto- (y-) 10-24       1 septillionth 0.000000000000000000000001

In addition to standard metric units, there are some special units that are commonly used in astronomy.

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