SI Units (Basic science review)

There is an internationally agreed-upon set of units used in the sciences. The International System of Units (in French, Système international d’unités) has 3 parts:

• base units
• prefixes
• derived units

Base units

There are 7 base units from which all other units are derived.

Quantity	Unit	Abbreviation
Length	meter	m
Time	second	s
Mass	kilogram	kg
Temperature	Kelvin	K
Electric current	ampere	A
Luminous intensity	candela	cd
Amount of substance	mole	mol

Prefixes

There are 24 prefixes that can be combined with the base units to create other units that are easier to work with. For instance, a distance of 0.002 meters is more easily thought of as 2 millimeters.

Prefix Name	Prefix Symbol	Base Ten	Decimal
giga	G	109	1000000000
mega	M	106	1000000
kilo	k	103	1000
hecto	h	102	100
deca	da	101	10
—	—	100	1
deci	d	10−1	0.1
centi	c	10−2	0.01
milli	m	10−3	0.001
micro	μ	10−6	0.000001
nano	n	10−9	0.000000001

Derived units

There are an infinite number of possible derived units. There are 22 that have been given special names. Some of the more common derived units are shown below.

Quantity	Units
speed	m/s
acceleration	m/s2
force	kg·m/s2
area	m2
volume	m3
density	kg/m3

Note:

By the rules of exponents, x^-1 = 1/x and x^-2 = 1/x². In the same way, units in the denominator of a derived unit can be written with negative exponents.

For example, acceleration can be written as m/s² or m·s^-2.

Acceptable non-SI units

Some non-SI units are so commonly used that they are recognized as acceptable. Some of the more common ones are shown below.

Name	Symbol	Quantity	Value in SI units
minute	min	time	1 min = 60 s
hour	h		1 h = 60 min = 3600 s
day	d		1 d = 24 h = 1440 min = 86400 s
astronomical unit	au	length	1 au = 149597870700 m
degree	°	plane angle  and phase angle	1° = (π⁄180) rad
arcminute	′		1′ = (1⁄60)°
arcsecond	″		1″ = (1⁄60)′ = (1⁄3600)°
hectare	ha	area	1 ha = 10000 m2
litre	l, L	volume	1 L = 1 dm3 = 1000 cm3 = 0.001 m3
tonne	t	mass	1 t = 1000 kg
dalton	Da		1 Da = 1.66053906892(52)×10−27 kg
electronvolt	eV	energy	1 eV = 1.602176634×10−19 J
bel, decibel	B, dB	logarithmic intensity

References

International_System_of_Units