|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.|
|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.|
|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 Concepción 3.04 m to the west.|
|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.|
|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.|
|Acceleration||meters per second squared
|Area||square meter (m2)||(m)(m)|
|Density||grams per cubic centimeter|
|Electric Charge||coulomb (C)|
|Speed||meters per second
For water only (since its density is 1 gram per milliliter):
(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.