Tables of SI Units and Prefixes

 

Table I. Basic Units defined by Conférence Générale des Poids et Mesures (CGPM) in the latest SI-brochure of 1998.

Quantity Unit Symbol Definition Length  meter  m  1983, 17th CGPM:  The path travelled by light in vacuum during a time interval of 1/299792458 seconds. This fixes the speed of light to exactly 299792458 m/s. Mass  kilogram  kg  1901, 3rd CGPM:  Mass of the platinum-iridium prototype at BIPM in Sevres. Time  second  s  1968, 13th CGPM:  One second equals 9192631770 periods of the radiation due to the transition between the two hyperfine levels of the ground state of Cesium 133. Electric current  ampere  A  1948, 9th CGPM:  Given two parallel, rectilinear conductors of negligible circular cross-section positioned 1 m apart in vacuum, one ampere is the electric current which, passing through both of them, makes them attract each other by the force of 2.10-7 newtons per every meter of length. This fixes the permeability of vacuum to exactly 2π*10-7 H/m. Temperature  kelvin  K  1968, 13th CGPM:  One degree K equals 1/273.16 of the thermodynamic temperature of the triple point of water. Quantity of substance  mole  mol  1971, 14th CGPM:  The amount of a substance composed of as many specified elementary units (molecules, atoms) as there are atoms in 0.012 kg of Carbon 12. Luminosity  candle  cd  1979, 16th CGPM:  The candle (or 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 W/sr.

Table II. Derived units with assigned names defined by BIPM in the latest SI-brochure (1998) and its supplement (2000).

Quantity Unit Symbol Equals Definition / Note Space and time: Plane angle  radian  rad    The plane angle which,  when centered in a circle, cuts off an arc whose length is equal to the circle radius. Solid angle  steradian  sr    The solid angle which,  when centered in a sphere, cuts off a cap whose surface equals that of a square having the radius as side. Frequency  hertz  Hz  1 s-1 [number of events or cycles]/[time].  Mechanics: Force  newton  N  1 kg.m.s-2 [mass].[acceleration].  Pressure  pascal  Pa  1 N.m-2 [force]/[area]. Also: stress.  Energy  joule  J  1 N.m  [force].[length]. Also: Work, Heat  Power  watt  W  1 J.s-1 [energy]/[time]. Also: Radiant flux  Thermodynamics: Temperature  celsius  oC  1 K  T [oC] = T [K] -273.15 (the offset is exact!).  Electromagnetism: Charge  coulomb  C  1 A.s  [current].[time].  Potential  volt  V  1 W.A-1 [power]/[current]. Only differences are measurable!  Resistance  ohm  Ω  1 V.A-1 [Δpotential]/[current].  Conductance  siemens  S  1 A.V-1 [current]/[Δpotential].  Capacitance  farad  F  1 C.V-1 [charge]/[Δpotential].  Inductance  henry  H  1 V.s.A-1 [Δpotential]/[rate of change of current].  Magnetic flux  weber  Wb  1 J.A-1 [energy]/[current].  Magnetic flux density  tesla  T  1 Wb.m-2 [magnetic flux]/[area]. Also magnetic induction.  Optics: Luminous flux  lumen  lm  1 cd.sr  [luminosity].[solid angle].  Illuminance  lux  lx  1 lm.m-2 [luminous flux]/[area].  Convergence  dioptry  dioptry  1 m-1 Inverse of focal length.  Radioactivity and radiation: Activity  becquerel  Bq  1 s-1 [number of decay events]/[time].  Absorbed dose  gray  Gy  1 J.kg-1 [energy]/[mass].  Dose equivalent  sievert  Sv  1 J.kg-1 [energy]/[mass]. Absorbed dose re-normalized by biological effects.  Chemistry: Katalytic activity  katal  kat  1 mol.s-1 [quantity of substance]/[time].

Table III a. SI Units prefixes with examples of correct usage.

Prefix Symbol Factor Examples of usage Origin Yotta  Y  1024 0.2 YW, 1.23Y [W]  Greek 'octo' (eight, 10008) Zetta  Z  1021 3.33 Zs, 3.33Z [s]  French 'sept' (seven, 10007) Exa  E  1018 1.23 Ekg, 1.23E [kg]  Greek 'six' (10006) Peta  P  1015 7.5 Ps, 7.5P [s]  Greek 'five' (10005) Tera  T  1012 0.5 Tm, 0.5T [m]  Greek 'teras' = monster  Giga  G  109 1.2 GΩ, 1.2G [Ω]  Greek 'gigas' = giant  Mega  M  106 7 MW, 7M [W]  Greek 'megas' = large  Kilo  K, k  103 33 km, 33K [m]  Greek 'kilioi' = thousand  hecto  h  100  Deprecated by SI  Greek 'hekaton' = hundred  deca  da  10  Deprecated by SI  Greek 'deka' = ten  deci  d  0.1  Deprecated by SI  Latin 'decima pars' = one tenth  centi  c  0.01  Deprecated by SI  Latin 'centesima pars' = one hundredth  milli  m, k  10-3 22 mm , 1.2m [m]  Latin 'millesima pars' = one thousandth  micro  µ, u  10-6 2.7 uJ , 2.7µ [J]  Greek 'mikros' = small  nano  n  10-9 2.2 nF, 2.2n [F]  Latin 'nanus' = dwarf  pico  p  10-12 1.5 pA, 1.5p [A]  Spanish 'pico' = minimal measure  femto  f  10-15 4.8 fs, 4.8f [s]  Danish and Norvegian 'femten' = fifteen (10-15) atto  a  10-18 1.2 ag, 1.2a [g]  Danish and Norvegian 'atten' = eighteen (10-18) zepto  z  10-21 0.2 zm, 1.2z [m]  French 'sept' (seven, 1000-7) yocto  y  10-24 1 ys, 1y [s]  Greek 'octo' (eight, 1000-8)

Table III b. Binary prefixes for Bytes which are not a part of SI but which are in common use in informatics [see the note].

Prefix Symbol Factor Value Examples Kilo  KB  210 1024  12345 KB = 12 641 280 bytes  Mega  MB  220 1 048 576  420 MB fits in my PC's dynamic RAM  Giga  GB  230 1 073 741 824  16 GB flash-memory pen drive costs $20  Tera  TB  240 1 099 511 627 776  3.9 TB hard disks are a reality  Peta  PB  250 1 125 899 906 842 624  13.5 PB is the CIA total memory capacity  Exa  EB  260 1 152 921 504 606 846 976  1 EB is still a bit out of reach (AD 2010)  Zetta  ZB  270 1 180 591 620 717 411 303 424  How many ZB to hard-copy a human being ???  Yotta  YB  280 1 208 925 819 614 629 174 706 176  1 YYB is still nothing compared with the Universe

Table IV. Accepted non-SI units compiled according to the US Federal Register (ref.4).

Unit of Symbol Equals Definition / Note Degree of arc  plane angle  o (π/180) rad    Minute of arc  plane angle  '  (1/60) o   Second of arc  plane angle  "  (1/60)'    Minute  time  min  60 s    Hour  time  h  60 min    Day  time  d  24 h  Notice that the duration of a day is not linked to Earth motion!  Liter  volume  L, l  0.001 m3 Often used sub-units are deciliter (dl) and centiliter.(cl).  Gram  mass  g  0.001 kg  A tolerated anomaly: the basic unit of mass (kg) has a prefix.  Ton  mass  t  1000 kg  More precise term: metric ton.  Bit  information  bit  -  The smallest, dimensionless quantum of information  Baud rate  info flux  Baud  1 bit.s-1 [amount of information]/[time]  Neper  ratio  Np  log(A/B)  Measure of a ratio A/B. The logarithms are in base 10.  Bel  ratio  B  0.5 Np  Mostly used as decibel (dB): 1 dB = (1/20) Np.

Table V. Accepted non-SI units with experimental values. For the most recent values of these constantly improving units, see Constants of Physics.

Unit of Symbol Equals Note Electronvolt  energy  eV  1.60217733(49).10-19 J Energy to move an electron across a potential difference of 1 V.  Astronomical unit  length  au, AU, ua  1.49597870(30).10+11 m Mean Earth-to-Sun distance. Also denoted as ua.  Atomic mass unit  mass  u  1.6605402(10).10-27 kg 1/12 of the rest mass of an unbound 12C atom in ground state.

Table VI. Units deprecated by the SI which are still in current use in most countries.

Unit of Symbol Equals Note Nautical mile  length  mile  1852 m    Knot  velocity  knot  1 mile.h-1 A nautical unit.  Are  area  are  100 m2   Hectar  area  ha  100 are  10000 m2  Bar  pressure  bar  100000 Pa  Almost 1 atm = 101325 Pa (an obsolete unit)  Calory  energy  cal  4.1868 J  Note: the conversion factor is fixed by convention.  Ångström  length  Å  10-10 m Used in atomic and molecular physics.  Barn  area  b  10-28 m2 Used in particle physics (collision cross-sections).  Radioactivity and radiation: Curie  Radioactivity  b  3.7*10+10 Bq Note: the conversion factor is fixed by convention.  Röntgen  Radiation dose  R  0.000258 Ci.kg-1 Note: the conversion factor is fixed by convention.  Rad  Radiation dose  rad  0.01 Gy    Rem  Equivalent dose  rem  0.01 Sv