Constant | Value |
---|---|
= mol |
|
electric current; SI base unit. one A = one C ˣ s−1 = 6.241 509 744 607 ˣ 1018 e− ˣ s−1, based on the exact charge on the electron. |
|
Angular momentum (L) |
L = kg ˣ m2 ˣ s−1 ˣ rad |
length (not an SI unit); one au = 149,597,870,700 m (exactly); the mean distance from Earth to the Sun |
|
Atomic mass of 12C | 11.999 999 9958 g ˣ mol−1 |
Atomic mass unit (u, amu) | = Dalton (Da) = 12C atom/12 = 1.660 539 066 60 ˣ 10−27 kg [934] |
Atomic unit of action | = reduced Planck's constant = h/2π = ħ |
Atomic unit of charge | = elementary charge = e |
Atomic unit of energy | = Eh = Hartree |
Atomic unit of force | = Eh/a0 = 8.238 723 4983 ˣ 10−8 N |
Atomic unit of length | = a0 = Bohr radius |
Atomic unit of mass | = electron rest mass in the atomic units system. Not to be confused with the unified atomic mass unit, which is the Dalton |
Atomic unit of pressure | = Eh/a03 = 2.942 101 5697 ˣ 1013 Pa [934] |
Atomic unit of temperature | = Eh/kB = 3.157 750 248 ˣ105 K [934] |
Atomic unit of time | = h/2πEh = 2.418 884 326 5857 ˣ 10−17 s [934] |
Avogadro constant (NA) | NA = 6.022 140 76 ˣ 1023 mol−1 [2395] (exactly; a SI constant) |
Avogadro number | dimensionless; = 6.022 140 76 ˣ 1023 [2395] (exactly) |
Bar (not an SI unit) | pressure; 1 bar = 100,000 Pa (exactly) = 0.986 9233 atm = 14.695 9487 7551 psi |
Barrel (bbl) (not an SI unit) | volume of oil; 1 bbl = 42 US gallons = 158.987 294 928 L (exactly) |
Barn (b) | cross-sectional area; 1 b = 100 fm2 = 10−28 m2 |
Barye (Ba) (not an SI unit) | pressure; 1 Ba = 0.1 Pa (exactly) |
unit of radioactivity; = one nuclear disintegration ˣ s−1, 1 Bq = 27.0270 pCi |
|
number; = 1000,000,000; 109 (this is the US 'billion) |
|
length; a0 = 5.291 772 109 03 ˣ 10−11 m [934] (under revision) |
|
British Thermal Unit (Btu) (not an
SI
unit) |
|
energy; one cal = 4.1868 J (exactly) |
|
Calorie (nutrition only, Cal) |
|
luminous intensity; SI base unit; cd = lm/sr; A lit candle and a one-watt bulb both emit light of about one candela | |
Celsius ( °C) (centigrade) |
0ne °C = one kelvin by definition; TK = T°C + 273.15; T°C = TK - 273.15 |
Celsius zero point (0 °C exactly) |
273.15 K (exactly; one degree Celsius has the same size as one kelvin); |
Centipoise, (cP; not an SI unit) |
|
Charge density |
|
Compressibility |
1 GPa−1 = 100 ˣ 10−6 ˣ bar−1 |
Concentration (SI unit) |
= mol ˣ m−3 |
volume; = 10−6 m3 |
|
volume; = 0.028 316 846 592 m3 = 28.316 846 592 L (exactly) 1 m3 = 35.314 666 7215 cubic feet |
|
Cup | Culinary measure, liquid volume; American = 240 mL = 16 Tbsp; British= 250 mL |
Curie (Ci) (not an SI unit) |
unit of radioactivity; 1 Ci = 3.7 ˣ 1010 Bq (exactly) |
mass; = atomic mass unit, 12C atom/12 = 1.660 539 066 60 ˣ 10−27 kg [CODATA 2018] |
|
time: 86,400 seconds (exactly); the astronomical unit of time |
|
Debye (D) (not an
SI
unit) |
|
= mass/volume, kg ˣ m−3 |
|
Dyne (dyn) (not an
SI
unit) |
force; 1 dyn = 10−5 N |
Electric conductance | = 1/Ω; see siemens |
Electric field strength | V ˣ m−1 = N/C = kg ˣ m ˣ s−3 ˣ A−1 |
Electric resistance | = Ω; see ohm |
Electromagnetic radiation |
|
Energy conversion factors |
|
Enthalpy |
sensible heat; a measure of the heat content of the system; usual units = J ˣ mol−1 |
Entropy |
a measure of disorder and chaos of the system; usual units = J ˣ mol−1 ˣ K−1 |
Erg (erg) (not an
SI
unit) |
energy; 1 erg = 10−7 J |
Electrostatic unit of charge (esu) (not an
SI
unit) |
|
(Fahrenheit temperature) = (Kelvin temperature) ˣ 9⁄5 − 459.67 (Fahrenheit temperature) = (Celsius temperature) ˣ 9⁄5 + 32 One degree Fahrenheit has the same size as 5/9 kelvin and 5/9 °C |
|
Farad (F) | electric capacitance, = C/V = s4 ˣ A2 ˣ m−2 ˣ kg−1 |
Faraday | electrical charge: 96,485.332 123 310 0184 C (exactly) |
Faraday constant (F) | The amount of electric charge per mole of electrons = 96,485.332 123 310 0184 C ˣ mol−1 exactly [2395]; (96,485.332 123 310 0184 J ˣ V−1 ˣ mol−1) |
Foot (ft, ') (not an SI unit) | length; the international foot = 30.48 cm (exactly) |
Foot candle (fc) (not an SI unit) | illuminance; 1 fc = lm ˣ ft−2 (1 fc ≈ 10.763 91 lux) |
Frequency, 133Cs hyperfine transition frequency (ΔνCs) | Number of cycles per unit time (ν); ΔνCs = 9,192,631,770 s−1 exactly |
Gravitational constant (G) |
Relates energy to size; G = 6.674 30 ˣ 10−11 m3 ˣ kg−1 ˣ s−2; E = G ˣ m2/R |
Gallon (liquid volume; not an
SI
unit)) |
|
RT, Gas constant ˣ temperature (25 °C) |
|
RT/F, Gas constant ˣ temperature/Faraday (25 °C) |
|
Gauss (G) (not an SI unit) | magnetic flux density; 1 G = 1 Mx ˣ cm−2 = 10−4 T (this is about the strength of the Earth's magnetic field at its surface ) |
Giibbs, G |
Energy content; 1 G = 1 J ˣ mol−1 |
Giga anna (Ga, Gyr) (not an
SI
unit) |
time passed; 1000,000,000 years ago |
Gradient of , ∇ | mathematical symbol (inverted greek delta) = gradient of |
Gram (g) (gramme) | mass; g = 10−3 ˣ Kg |
Gram molecule | mass; relative molecular mass (molecular weight) in grams = mol |
Gray (Gy) | absorbed dose of radiation; Gy = J ˣ kg−1 = m2 ˣs−2 |
The golden
ratio (φ) |
|
g = 9.806 65 m ˣ s−2 (exactly) (≈ 32.1740 ft ˣ s−2 ) |
|
area, 1 ha = 1 hm2 = 104 m2 |
|
frequency (ν), cycles per second: = 1 s−1 (see also terahertz) |
|
Horsepower (hp) (not an
SI
unit) |
power; 745.7 W (imperial horsepower); 735.5 W (metric horsepower) |
time; 3600 s |
|
Standard Hydrogen Electrode |
Defined to have an electrode potential of 0 V (exactly) at all temperatures. |
Hyperfine splitting frequency of the caesium-133 atom |
Δν(133Cs)hfs is exactly 9,192,631,770 hertz (Hz) (defines the second) |
Inch (in, ") (not an
SI
unit) |
length; One inch = 2.54 cm (exactly) |
catalytic activity; one kat = one mol ˣ s−1 |
|
Thermodynamic temperature; SI base unit. It was defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of set isotopic composition water up until 20 May 2019; (Absolute zero, 0 K = −273.15 °C or −459.67 °F). Now it is defined in terms of the Boltzmann constant, the second, the meter, and the kilogram [934]. One kelvin is equal to a change in the thermodynamic temperature T resulting in a change of thermal energy kBT by 1.380 649 ˣ 10-23 J. 1 K = 0.086 173 332 621 4518 meV = 0.695 034 8004 cm1 ( ≡ 8.314 4626 J ˣ mol−1) |
|
Kilocalorie (kcal, not SI unit) |
thermochemical energy; one kcal = 4.184 kJ (exactly) |
force; 1 kgF = 9.806 65 N (exactly) |
|
work (= energy); 1 kWh = 3.6 MJ (exactly) |
|
Lightyear (ly) | length; one lightyear = 9,460,730,472,580.8 km (exactly; uses the speed of light and the Julian year) |
Liter, (L, l; alternatively spelled litre in the UK) | volume; commonly used but not an SI unit; one L = 1 dm3 = 103 ˣ cm3 = 10−3 ˣ m3 (exactly) |
Ln( ) |
Natural logarithm; = Loge( ). Sometimes referred to as log in the literature |
2.651 6462 ˣ 1025 molecules of ideal gas ˣ m−3 (= 44.615 036 mol ˣ m−3) |
|
Lumen, (lm) | luminous flux; one lm = one cd ˣ sr; (= cd ˣ m2 ˣ m−2) |
Luminous efficacy (Kcd) | The luminous efficacy (Kcd) of monochromatic radiation of frequency 540 ˣ 1012 Hz is exactly 683 lm ˣ W−1 (a SI constant) |
Luminance, (Lv) | one Lv = one cd ˣ m−2 |
Lux, (lx) | illuminance; 1 lx = 1 lm ˣ m−2 = 1 cd ˣ sr ˣ m−2 |
kg ˣ kg−1; but usually treated as dimensionless |
|
time passed; 1 Ma = 1,000,000 years ago |
|
Maxwell (Mx) (not an SI unit) | magnetic flux; 1 Mx = 1 G ˣ cm2 = 10−8 Wb |
Meter (m); alternatively spelled metre in the UK | length; SI base unit. defined by the speed of light below |
Micron (µ, µm), micrometer | length: = 1 µm = 10−6 m |
Mil (not an SI unit) | length; easily confused as may mean either, m ˣ 10−3 or in ˣ 10−3 |
Mile (international, not an SI unit) | length: 1,609.344 m (exactly) |
Mile per hour (mph) | velocity; 1 mph = 1.609 344 km ˣ hr−1 = 0.447 04 m ˣ s−1 (exactly) |
Millibar (mb) (not an SI unit) | pressure; 1 mb = 100 Pa = 1 hPa |
Minute | (min) time; 1 min = 60 s ('), plane angle; 1' = π/10800 rad (a circle ≡ 360° = 2π rad ); 1' = 60'' |
mm-Hg (not an
SI
unit) |
|
1 m = 1 mol ˣ (kg solvent)−1 (see for conversion to molarity) |
|
Molality, standard (m°) |
m° = 1 mol ˣ (kg solvent)−1 |
Molar concentration (M) |
1 mol ˣ (L solution)−1 = 103 mol ˣ m−3 (see for conversion to molality) |
Molar energy |
J ˣ mol−1 |
Molar mass of dry air | = 0.028 965 46 kg ˣ mol−1 (IAPWS) |
Molar volume of ideal gas (at STP) | |
Mole (mol) | amount; SI base unit. an Avogadro number of items; mol = (6.022 140 76 ˣ 1023)/NA A mole is the same as a gram-atom |
Moment of force | = N ˣ m |
Mya (not an SI unit) | time passed; 1 Mya = 1,000,000 years ago |
Nernst constant, = RT/F | 25.692 579 12 mV (25 °C); Loge(10) ˣ RT/F = 0.059 159 349 685 V (25 °C) |
Newton (N) | force; N = 1 kg ˣ m ˣ s−2 |
Oersted (Oe) | magnetic field; 1 Oe ≡ (103/4π) ˣ A ˣ m−1 |
Ohm (Ω) | electric resistance; = V/A, 1 Ω = 1 kg ˣ m2 ˣ s−3 ˣ A−2 |
Osmole (Osm) (not an SI unit) | amount; the number of moles of solute that contribute to the osmotic pressure of a solution. Ideally, 1 mole of NaCl in 1 Kg water is 2 Osm ˣ kg−1-water |
Pascal (Pa) | pressure, stress, 1 Pa = 1 N ˣ m−2 = 1 kg ˣ m−1 ˣ s−2 |
Parts per billion (ppb) | ppb = parts per 1000,000,000; for example, 1 µg ˣ kg−1 |
Parts per million (ppm) | ppm = parts per 1000,000; for example, 1 mg ˣ kg−1; also used in NMR |
Permeability |
the degree of magnetization of a material in response to a magnetic field; = H ˣ m−1 = m ˣ kg ˣ s−2 ˣ A−2 |
Permittivity (ε) |
the ability of a substance to store electrical energy in an electric field; ε = F ˣ m−1 = m−3 ˣ kg−1 ˣ s4 ˣ A2 |
Pint (liquid volume; not an SI unit) |
|
Poise (P) (not an
SI
unit) |
|
Pound (lb) (avoirdupois, US and UK) (not an SI unit) |
mass = 0.453 592 37 kg (exactly) |
Pound weight (lbf)
(not an
SI
unit) |
force = 4.448 221 615 2605 N |
parts per billion; 1 : 1000,000,000 |
|
Proton charge |
(+ve) 1.602 176 634 ˣ 10−19 C (exactly) |
Proton gyromagnetic ratio (γP) |
|
Proton mass |
= 1.672 621 923 69 ˣ 10−27 kg [934] |
Proton : electron mass ratio |
= 1836.152 673 43 [934] |
psu (practical salinity unit) (not an
SI
unit) |
The psu has no dimensional units; see elsewhere |
Quadrillion |
number; 1000,000,000,000,000; 1015 |
Rad (not an SI unit) | absorbed dose of radiation; 1 rad = 0.01 Gy |
Radiance | = W ˣ sr−1 ˣ m−2; kg ˣ s−3 |
Rem (not an SI unit) | Radiation dose equivalent; 1 rem = 0.01 Sv |
Roentgen (R) (not an SI unit) | Exposure of X-rays and gamma rays; 1 R = 2.580 ˣ 10−4 ˣ C ˣ kg−1 air at STP |
Second (s) | time; SI base unit. The second is equal to the duration of exactly 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the unperturbed ground state of the 133Cs atom. ΔνCs = 9,192,631,770 s−1 exactly. This definition of the second may be updated in 2026. |
Second ('') | (''), Plane angle; 1'' = π/648,000 rad (a circle ≡ 360° = 2π rad ) |
Siemens (S) | electric conductance; = A/V, 1 S = 1/Ω = 1 s3 ˣ A2 ˣ kg−1 ˣ m−2; The previously used units of mhos ˣ cm−1 are numerically equivalent to S ˣ cm−1. |
Sievert (Sv) | radiation dose equivalent; Sv = J ˣ kg−1 = m2 ˣs−2 |
Specific gravity |
relative density; for liquids, it is relative to water at 3.98 °C (~ 1000 kg ˣ m−3) for gases, it is relative to air at 20 °C, 101.325 kPa (~ 1.205 kg ˣ m−3) |
energy ˣ mol−1 ˣ K−1; = kg ˣ m2 ˣ s−2 ˣ mol−1 ˣ K−1 |
|
Spectral luminous efficiency (Kλ) |
|
101.325 kPa, 273.15 K, concentration 1 mol ˣ L−1 (1000 mol ˣ m−3) |
|
STP ≡ 100 kPa, 273.15 K (exactly) IUPAC |
|
Standard state (thermodynamic) |
varies as stated, but often; pressure, 100 kPa; temperature, 25.00 °C (77 °F) |
Stefan-Boltzmann constant (σ) |
|
The solid angle of the sphere surface (area A, radius r) from the center, = (A/r2 ) ˣ sr 4π ˣ sr = entire surface of a sphere radius r. The units of sr are m2 ˣ m−2 but usually treated as dimensionless. |
|
Stokes (St) (not an
SI
unit) |
kinematic viscosity; 1 St = 1 cm2 ˣ s−1 = 10−4 m2 ˣ s−1 |
Sun |
solar irradiation; measured in kWh ˣ m−2 |
Surface tension | = N ˣ m−1 ; kg ˣ s−2 |
Tablespoon (Tbsp) | Culinary measure, volume; standardised, Tbsp; 1 Tbsp = 15 mL = 3 tsp |
Teaspoon (tsp) | Culinary measure, volume; 1 tsp = 5 mL (4.93 mL US); = ⅓ tablespoon (Tbsp) |
frequency in cycles per second: one THz = 1012 s−1 = 2π ˣ 1012 radian ˣ s−1 = one ps−1 ≡ 33.355 640 951 981 52 cm−1 ≡ 299.799 365 702 µm (exactly) ≡ 0.004 135 667 516 eV ≡ 6.626 070 15 ˣ 10−22 J ≡ 399.031 271 289 J ˣ mol−1 |
|
Thermal conductivity |
= W ˣ m−1 ˣ K−1 = m ˣ kg ˣ s−3 ˣ K−1 |
Thermal energy (kT at 25 °C) |
|
Ton (metric ton) (tonne, t) | mass; one t = 103 kg (1 Mg) |
Ton (UK; long ton) | mass; = 2,240 lb avoirdupois, UK; = 1016.046 908 80 kg (exactly) |
Ton (US; short ton) | mass; = 2,000 lb avoirdupois, US; = 907.184 740 kg (exactly) |
Torr (not an SI unit) | historical pressure unit; one torr = 101325⁄760 Pa (exactly; ≈ 133.322 368 4211 Pa ) ≡ 1 mm Hg |
number; = 1000,000,000,000; 1012 |
|
Units; binary prefixes |
kibi, Ki, (210)1 ; mebi, Mi, (210)2 ; gibi, Gi, (210)3 ; tebi, Ti, (210)4 ; pebi, Pi, (210)5 ; exbi, Ei (210)6; zebi, Zi, (210)7 ; yobi, Yi (210)8 , NIST e.g., 1 gibibyte = 1 GiB = 1,073,741,824 byte; 1 gigabyte = 1 GB = 1,000,000,000 byte |
deci- d 10−1;centi- c 10−2; milli- m 10−3; micro- µ 10−6; nano- n 10−9; pico- p 10−12; femto- f 10−15; atto- a 10−18; zepto- z 10−21; yocto- y 10−24; ronto- r 10−27 a; quecto- q 10−30 a deca- da 101; hecto- h 102; kilo- k 103; mega- M 106; giga- G 109; tera- T 1012; peta- P 1015; exa- E 1018; zetta- Z 1021; yotta- Y 1024; ronna- R 1027 a; quetta- Q 1030 a |
|
Vacuum permeability | |
Viscosity, dynamic (η) | η = Pa ˣ s = kg ˣ m−1 ˣ s−1; |
Viscosity, kinematic (ν) | ν = η/ρ= m2 ˣ s−1 = dynamic viscosity divided by the density of the liquid; 10,000 stokes = 1 m2 ˣ s−1 |
Volt (V) | electrical potential; V = amps ˣ ohms (A ˣ Ω); V = power/current (W/A); V = energy/amount of charge (J/C); one V = one kg ˣ m2 ˣ s−3 ˣ A−1 |
Wavelength (nm) | The distance over which a wave's shape repeats; The wavelength in nm = 107/wavenumber (cm−1) (electromagnetic; one nm ≡ 3.335 640 95 attosecond) |
Wavenumber (m−1) (=1/wavelength) | Number of cycles per unit distance; one m−1 = 109/wavelength (nm), one m−1 ≡ 0.119 626 5656 J ˣ mol−1; 1000 cm−1 ≡ 11.962 656 56 kJ ˣ mol−1 (one cm−1 ≡ 0.029 979 2458 THz exactly) |
Wavenumber, wavelength. frequency conversions | |
Watt (W) | power, radiant flux; one W = one J ˣ s−1 = one kg ˣ m2 ˣ s−3 |
Weber (Wb) | magnetic flux; = T ˣ m2 = V ˣ s; one Wb = one kg ˣ m2 ˣ s−2 ˣ A−1 |
Weight | force which depends on terrestrial gravity = (1/9.806 65) ˣ kg ˣ m ˣ s−2, = N |
Year (a, yr) (not an SI unit) (varies from year to year) |
One Gregorian year = 365.2425 days = 31,556,952 seconds One Julian year (a) = 365.25 days = 31,557,600 seconds (exactly, fixed in astronomy and used in the light year) One mean Tropical year = 365.24219 days = 31,556,925.216 seconds |
Note that the SI unit definitions in terms of the natural Universal constants [1031] came into force on 20 May 2019 (see right). The Dimensions are given in red capitals; Mass, Length, Time, Substance, temperature as Theta Θ, luminosity in Joules per second, electric current Intensity.
s - second defined using the frequency of the cesium hyperfine transition (ΔvCs ≡ T−1),
kg - kilogram defined using the Planck constant (h ≡ L2MT−1),
mol -mole defined using the Avogadro constant (NA ≡ S−1),
cd -candela defined using the sensitivity of the human eye (luminous efficacy, Kcd ≡ J),
K - kelvin defined using the Boltzmann constant (kB ≡ L2MT−2Θ−1),
A - amp defined using the elementary electric charge (e− ≡ IT),
m - meter defined using the speed of light (c ≡ LT−1) .
[Back to Top ]
a Prefixes approved; Résolutions de la Conférence générale des poids et mesures (27 réunion) 15-18 novembre 2022[Back]
b The natural logarithm of x is the power to which e would have to be raised to equal x. [Back]
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