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
Reference works | Physics Constants         SI Dimensions Science LINKS | Stan's Library | Stan's HUB
This is a unit-at-a-glance list. You can also download a PDF version of this document for off-line use.

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 6.62607015e-34 Planck constant
e C 1.602176634e-19 elementary charge
k J.K-1 1.380649e-23 Boltzmann constant
NA mol-1 6.02214076e23 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 M.L2.T-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, 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 1 rad is the angle subtended at the centre of a circle by an arc that is equal in length to the radius.
solid angle steradian sr 1 m2.m-2 1 sr is the solid angle subtended at the centre of a sphere by an area of the surface that is equal to the squared radius.
frequency hertz Hz 1 s-1 [number of events]/[time]. Use only for periodic phenomena.
Mechanics:
force newton N 1 kg.m.s-2 [mass].[acceleration]
pressure, stress 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 ...:
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 ...:
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.
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
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. Accepted non-SI units
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 VII. Some 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 1.602176487(40)x10-19 J Energy to move an electron across a potential difference of 1 V.
Astronomical unit length au, AU, ua 1.49597870700x10+11 m Mean Earth-to-Sun distance. Also denoted as ua.
Atomic mass unit mass u 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 the SI

which are still in current use in many 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).
Convergence dioptry dioptry 1 m-1 Used in optics. Inverse of focal length.
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

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.

References and Links
TOP | Reference works | Physics Constants SI Dimensions Stan's Library | Stan's LINKS | Stan's HUB
Copyright ©2005 Sýkora S. Stan's Library, Vol 1, DOI: 10.3247/SL1Phys06.002 Designed by Stan Sýkora