Tables of SI Units and Prefixes
edited by Stanislav Sýkora, Extra Byte, Castano Primo, Italy
Stan's Library, ISSN 2421-1230, Vol.I., First release February 1, 2005. Permalink via DOI: 10.3247/SL1Phys06.002
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Table I. Defining constants

selected by Conférence Générale des Poids et Mesures (CGPM) in the 9-th SI-brochure of 2019, V.4.01 of 2026.
These constants have pre-assigned exact values.

The values were chosen in a way to guarantee compatibility with previous (now historic) units.
Constant Dim Fixed value Definition
ΔνCs Hz 9192631770 Hyperfine transition frequency of Cesium 133
c m.s-1 299792458 Speed of light in vacuum
h J.s 6.62607015 x 10-34 Planck constant
e C 1.602176634 x 10-19 elementary charge
k J.K-1 1.380649 x 10-23 Boltzmann constant
NA mol-1 6.02214076 x 1023 Avogadro constant
Kcd lm.W-1 683 luminous efficacy

Table II. Base Units

These quantities were considered basic in the past but are nowadays classified as derived. They are nevertheless special because they define basic dimensional types T, L, M, I, θ, N, J, (or void), making it possible to classify the type of any quantity as a product of powers of the base types. For example, energy is of type [ML2T-2], while its unit is 1 joule = kg m2 s-2.

Type Quantity Unit Dim New and Old Definitions
T time second s 1 s equals (9192631770 / ΔνCs) , with ΔνCs defined in Table I.
Old: 1 s equals 9192631770 periods of the radiation due to the transition between the two hyperfine levels of the ground state of Cesium 133
L length meter m 1 m equals (c / 299792458) s, with c defined in Table I.
Old: 1 m equals the path light travels in vacuum in 1/299792458 seconds-
M mass kilogram kg 1 kg equals (h / 6.62607015×10-34) s m-2, with h defined in Table I.
Old: 1 kg equals the mass of the platinum-iridium prototype at BIPM in Sevres.
I electric current ampere A 1 A equals (e / 1.602176634×10-19) s-1, with e defined in Table I.
Old: 1 A is the current generating an attractive force of 2.10-7 newtons per every meter of length of two parallel rectilinear conductors of negligible circular cross-section positioned in vacuum 1 m apart (this fixes the permeability of vacuum to 2π*10-7 H/m).
θ temperature kelvin K 1 K equals (1.380 649 x 10-23 / k) kg m2 s-2, with k defined in Table I.
Old: 1 K equals 1/273.16 of the thermodynamic temperature of the triple point of water.
N amount of substance mole mol 1 mol equals (6.02214076 × 1023 / NA), with NA defined in Table I.
Old: 1 mol is the amount of a substance composed of as many pre-defined elementary entities (molecules, atoms) as there are atoms in 0.012 kg of Carbon 12.
J luminosity candle cd 1 cd equals (Kcd/683) kg m2 s-3 sr-1, with Kcd defined in Table I and sr in Table III.
Old: 1 cd is the luminous intensity of a source that emits in a given direction monochromatic radiation of frequency 540 x 1012 Hz and radiant intensity of 1/683 W/sr.

Table III. Other named SI units

defined by BIPM in the 9-th SI-brochure of 2019, V.4.01 of 2026.
Quantity Unit Dim Equals Definition / Note
Space and time
plane angle radian rad 1 m/m 1 rad is the angle subtended at the centre of a circle by an arc that is equal in length to the radius. The full plane angle is numerically 2π.
solid angle steradian sr 1 m2/m2 1 sr is the solid angle subtended at the centre of a sphere by a surface area equal to the squared radius. The full solid angle is numerically 4π.
frequency hertz Hz 1 s-1 [cycles]/[time]. Use only for periodic phenomena, not for transient decays.
Note that in periodic phenomena 1 Hz = 2.π rad/s.
Mechanics
force newton N 1 kg m s-2 [mass].[acceleration]
pressure, stress, strain pascal Pa 1 N m-2 [force]/[area]
energy, work, heat joule J 1 N m [force].[length], but not to be used for torque!
power, radiant flux watt W 1 J s-1 [energy]/[time]
Electric field
charge coulomb C 1 A s [current].[time]
potential, tension, voltage volt V 1 W A-1 [power]/[current]. Only differences are measurable!
capacitance farad F 1 C V-1 [charge]/[Δpotential].
resistance ohm Ω 1 V A-1 [Δpotential]/[current].
conductance siemens S 1 A V-1 [current]/[Δpotential].
Magnetic field
flux weber Wb 1 J A-1 [energy]/[current].
flux density tesla T 1 Wb m-2 [magnetic flux]/[area]. Also magnetic induction.
inductance henry H 1 V s A-1 [Δpotential]/[rate of change of current].
Thermodynamics
Celsius temperature degree C oC 1 K Temperature differences expressed in oC and in K are identical,
but the two scales are shifted; 0 K corresponds to -273.15 oC.
Optics
luminous flux lumen lm 1 cd sr [luminosity].[solid angle].
illuminance lux lx 1 lm m-2 [luminous flux]/[area].
Radioactivity and radiation
(radio) activity becquerel Bq 1 s-1 [number of radionuclid decay events]/[time].
(absorbed) dose, kerma 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.
radiation exposure - C/kg 1 C kg-1 [charge]/[mass]; no SI name; replaces Röntgen which is 2.58x10-4 C/kg
Chemistry
catalytic activity katal kat 1 mol s-1 [amount of substance]/[time].

Table IV. SI prefixes for measurement units
Prefix Symbol Factor Examples of usage Origin
Quetta Q 1030 0.2 QW, 1.23Q [W] Q and Latin 'decem' (ten, 100010)
Ronna R 1027 0.2 RW, 1.23R [W] R and Greek 'ennea' (nine, 10009)
Yotta Y 1024 0.2 YW, 1.23Y [W] Y and 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)
ronto r 10-27 0.2 rm, 1.23r [m] R and Greek 'ennea' (nine, 1000-9)
quecto q 10-30 0.2 qW, 1.23q [W] Q and Latin 'decem' (ten, 1000-10)

Table V. Binary prefixes for Bits and Bytes

which are not a part of SI but which are in common use in informatics [see IEC 80000-13:2025].

Though the binary prefixes are NOT defined by SI, they are widely used in information technology for transmission or storage capacity. They combine an exponent letter (K,M,G,T,P,E,Z,Y,R,Q) with one of the letters "i" (for a bit of information), or "B" (always for "Bytes"), or "b" (usually for "bits"). The distinction between "B" and "b" is unstable; 1 Kb could be interpreted either as 1024 bits, or as 1000 bits, or even 1024 bytes. Here we stick to "i" fors the IEC_80000 convention or "B" for bytes.

Prefix Symbol Factor Value Examples or Notes
kibi, Kilo Ki, KB 210 1024 12345 KB = 12 641 280 bytes
mebi, Maga Mi, MB 220 1 048 576 420 MB fits in my PC's dynamic RAM
gibi, Giga Gi, GB 230 1 073 741 824 16 GB flash-memory pen drive costs $20
tebi, Tera Ti, TB 240 1 099 511 627 776 3.9 TB hard disks are a reality
pebi, Peta Pi, PB 250 1 125 899 906 842 624 13.5 PB is in the reach of a private memory user
exbi, Exa Ei, EB 260 1 152 921 504 606 846 976 1 EB is still a bit rare to see
zebi, Zetta Zi, ZB 270 1 180 591 620 717 411 303 424 1 ZB data centre
yobi, Yotta Yi, YB 280 1 208 925 819 614 629 174 706 176 1 Yi bps exceds human communication capacity
robi, Ronna Ri, RB 290 1 237 940 039 285 380 274 899 124 224 1 RB is nothing compared with the Universe
quebi, Quetta Qi, QB 2100 1 267 650 600 228 229 401 496 703 205 376 1 QB is getting closer

Table VI. Tolerated non-SI units
Unit of Symbol Equals Definition / Note
Degree of arc plane angle o (π/180) rad Angular distance between a rotating planet's pole and its equator is 90o
Minute of arc plane angle ' (1/60) o Typical angular resolution of human eye is 1' (3x10-4 rad)
Second of arc plane angle " (1/60)' Best-case resolution of human eye is 30''
Minute time min 60 s Beware: a day has only 1440 min!
Hour time h 3600 s Also 60 min
Day time d 86400 s Also 24 h. The duration of a day is not linked to Earth motion!
Nautical mile length mile 1852 m Beloved by old navigators
Ångström length Å 10-10 m Atom diameters and lengths of chemical bonds are 1-2 Å
Knot velocity knot (1852/3600) m/s 1 mile/h, a nautical unit
Gal acceleration gal 0.01 m/s2 Standard gravity acceleration on Earth surface is 980.665 gal
Are area are 100 m2 A square of 10 m x 10 m
Hectar area ha 10000 m2 Also 100 are; a square field of 100 m x 100 m
Barn area b 1x10-28 m2 Used in nuclear and particle physics
Liter volume L, l 0.001 m3 Often used sub-units are deciliter (dl) and centiliter.(cl)
Bar pressure bar 100000 Pa Almost 1 atm = 101325 Pa (atm is an obsolete unit)
Gram mass g 0.001 kg A tolerated anomaly: the basic unit of mass (kg) has a prefix !
Tonne mass t 1000 kg Alternative names: ton, metric ton
Var reactive power var 1 VA, 1W Same as 1 Watt
Bit information bit   Dimensionless! The smallest quantum of information
Baud rate info flux Baud 1 s-1 Amount of information per unit of time, bits per second, bps
Neper ratio Np   Dimensionless! log10(A/B) for two quantities A,B of the same dimension
Bel power ratio B 0.1 Np Mostly used as decibel (dB): 1 dB = 0.1 B in power measurements

Table VII. Tolerated non-SI units with experimental values.

For the most recent values of these units, see Constants of Physics.

Unit of Symbol Equals Note
Electronvolt energy eV 1.602176487(40)x10-19 J Energy to move an electron across a potential difference of 1 V
Astronomical unit length au, AU 1.49597870700x10+11 m Mean Earth-to-Sun distance. Also denoted as ua
Dalton mass dalton, u, ua 1.66053906892(52)x10-27 kg 1/12 of the rest mass of an unbound 12C atom in ground state

Table VIII. Units deprecated by SI

which are still in current use in many countries.
Unit of Symbol Equals Note
Calory, frigory energy cal 4.1868 J The conversion factor is conventional
Convergence dioptry dioptry 1 m-1 Used in optics. Inverse of focal length
Curie (radio)activity Ci 3.7*10+10 Bq The conversion factor is conventional
Rad Radiation dose rad 0.01 Gy The term rad is also SI units for plane angles !
Rem Equivalent dose rem 0.01 Sv Dosimeters often display rems
Röntgen Radiation exposure R 0.000258 C/kg The conversion factor is conventional

Spelling differences

The symbols of various units are international. This, however, is not true about the names of the units. Thus, for example, the US meter corresponds to the French metre, Italian metro, Czech metr, etc. In particular, UK English uses the French spelling. Consequently, the US terms meter, gram and liter become, respectively, metre, gramm and litre in the British Commonwealth. Throughout this document, I have used the US spelling.

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Copyright ©2005 Sýkora S. Stan's Library, Vol 1, DOI: 10.3247/SL1Phys06.002 Designed by Stan Sýkora