Units of Measurement and Their Magnitude

Base Units

Measure Unit (Symbol) Notes
Length meter (m) The meter is defined as "the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second."
Mass* gram (g) Presently defined as 1/1000 of a kilogram, it has been historically defined as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a meter, and at the temperature of melting ice."
Time second (s) The second is defined as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom."
Temperature  kelvin (K) Currently, the kelvin is defined as "the fraction 1/273.16 of the thermodynamic temperature of the triple point of water."
Electric Current ampere (A) Historically defined "electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of silver nitrate."  It is presently defined as the "constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2×10-7 newton per meter of length."
Quantity mole (mol) As presently defined, the mole is "the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12."
Luminous Intensity candela (cd)   At present, the candela is "the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian."
* - Officially, the SI unit for mass is the kilogram.  For purposes of identifying the units only, the word gram is given here.


Prefix (Abbreviation)   Magnitude (Power of Ten)   Usage
yotta (Y) 1024 The mass of the Earth is roughly 5973.6 Yg.
zetta (Z) 1021  
exa (E) 1018  
peta (P) 1015 Lead-204 has a half-life of 140 Py, the longest half-life of any known radioactive isotope.  The Earth receives 174 PW of solar radiation.  A light year is about 9.46 Pm.
tera (T) 1012 Visible light has frequencies that vary between 400 and 790 THz.  Pluto varies between 4.4 and 7.4 Tm from the Sun.
giga (G) 109 The age of the universe is roughly 13.7 Gy.
mega (M) 106 One year is equivalent to 31.5576 Ms.  The largest elephant on record was 11 Mg.
kilo (k) 103 The mass of a human can be expressed using tens of kilograms.
hecto (h) 102  
deca (da) 101  
BASE UNIT - no prefix 100 An FM radio wave varies between 2.78 and 3.41 m.  The earthquake that struck Chile on February 27, 2010 moved the entire city of Concepcin 3.04 m to the west.
deci (d) 10-1  
centi (c) 10-2 The earthquake that struck Chile on February 27, 2010 moved the Earth's figure axis by 8 cm.
milli (m) 10-3 Background radiation exposes people to 3.1 mSv of radiation annually.
micro (μ) 10-6 The earthquake that struck Chile on February 27, 2010 shortened the day by 1.26 μs.
nano (n) 10-9 Visible light has wavelengths that vary between 380 and 750 nm.
pico (p) 10-12 Some estimates suggest the radius of a hydrogen atom is 35 pm.
femto (f) 10-15 The proton is approximately 0.84184 fm.
atto (a) 10-18  
zepto (z) 10-21  
yocto (y) 10-24 Hydrogen-5 has a half-life of 80 ys, the shortest half-life of any known radioactive isotope.  The mass of a proton is roughly 1673 yg.  A neutron's mass is slightly larger at 1675 yg.

Derived Units

Measure Unit (Symbol) Derivation
Acceleration meters per second squared
Area square meter (m2) (m)(m)
Density grams per cubic centimeter
Electric Charge coulomb (C)
Energy/Work joule (J)
Force newton (N)
Power watt (W)
Pressure pascal (Pa)
Speed meters per second
Volume liter (L)

Derived Relationships

For water only (since its density is 1 gram per milliliter):

Further Reading

(1) Bloch, Hannah.  A Grander K.  National Geographic, October 2009, p 8 ff.
(2) Castelvecchi, David.  Just How Small Is the Proton? Scientific American, October 2010, p 24.
(3) Karol, Paul J. Weighing the Kilogram. American Scientist. 2014, 102, 426-429.
(4) Petit, Charles.  In Pursuit of the Briefest Beat.  Science News, March 27, 2010, pp 16-20.