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. | ||
Magnitude
| 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 Concepción 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.