# Faraday constant
The **Faraday constant** (symbol F) is the electric charge carried by one mole of electrons, equal to the product of the [Avogadro constant](https://www.edgepedia.org/avogadro-constant) and the elementary charge, F = NAe, and expressed in coulombs per mole (C/mol)[1](https://www.bipm.org/documents/20126/41489679/SI-App2-mole.pdf/be4dea74-a526-e49b-f497-11b393665401?t=1637238198610&version=1.6). It is the conversion factor between charge measured in a circuit and amount of substance reacted at an electrode, and it is named for [Michael Faraday](https://www.edgepedia.org/michael-faraday) (1791–1867), whose quantitative laws of electrolysis first established the proportionality it expresses[2](https://www.sciencehistory.org/education/scientific-biographies/michael-faraday/).
| Key fact | Detail |
|---|---|
| Definition | F = NAe, the charge per mole of electrons; SI coherent unit C/mol[1](https://www.bipm.org/documents/20126/41489679/SI-App2-mole.pdf/be4dea74-a526-e49b-f497-11b393665401?t=1637238198610&version=1.6) |
| Exactness | Known exactly since the 2019 SI redefinition, because NA and e have fixed numerical values[1](https://www.bipm.org/documents/20126/41489679/SI-App2-mole.pdf/be4dea74-a526-e49b-f497-11b393665401?t=1637238198610&version=1.6) |
| Elementary charge | e = 1.602 176 634 × 10−19 C, exact by definition (CODATA 2022)[3](https://usephysics.com/reference) |
| Calculated value (IUPAC 2018) | 9.648 533 21 × 104 C mol−1 • [4](https://doi.org/10.1515/pac-2018-0109) |
| Last measured value (CODATA 2014) | 96 485.332 89(59) C mol−1, relative uncertainty 6.2 × 10−9 • [5](https://physics.nist.gov/cuu/pdf/CODATA_JPCRD2016.pdf) |
| Use | Converts charge Q to moles of electrons, n = Q/F, in electrolysis and battery calculations[6](https://openstax.org/books/chemistry/pages/17-7-electrolysis) |
## Definition and exact value
The constant is defined by the relation F = NAe: it is the magnitude of charge on one mole of electrons, about 9.65 × 104 coulombs[1](https://www.bipm.org/documents/20126/41489679/SI-App2-mole.pdf/be4dea74-a526-e49b-f497-11b393665401?t=1637238198610&version=1.6) • [4](https://doi.org/10.1515/pac-2018-0109). Before 2019 it was a measured quantity. The 2014 CODATA adjustment, the last in which F carried an uncertainty, gave 96 485.332 89(59) C mol−1 with a relative standard uncertainty of 6.2 × 10−9 • [5](https://physics.nist.gov/cuu/pdf/CODATA_JPCRD2016.pdf).
The 2019 revision of the SI fixed the numerical values of the Avogadro constant and the elementary charge (e = 1.602 176 634 × 10−19 C, exact by definition)[3](https://usephysics.com/reference). Because F is their product, it became a known exact constant, and the BIPM's mise en pratique for the mole lists electrolysis as one of the practical ways of realizing the mole using this relation[1](https://www.bipm.org/documents/20126/41489679/SI-App2-mole.pdf/be4dea74-a526-e49b-f497-11b393665401?t=1637238198610&version=1.6). An IUPAC terminology document on electrochemical methods of analysis tabulates the calculated value F = e·NA as 9.648 533 21 × 104 C mol−1 • [4](https://doi.org/10.1515/pac-2018-0109).
## Role in Faraday's laws of electrolysis
Faraday's laws connect the mass of substance transformed at an electrode to the charge passed. IUPAC's [Compendium of Chemical Terminology](https://www.edgepedia.org/compendium-of-chemical-terminology) formulates the combined relationship as m = MQ/zF, where M is the molar mass of the substance, z the number of electrons transferred in the reaction, and F the Faraday constant[7](https://goldbook.iupac.org/terms/view/09075). The second law states that when the same charge Q is passed through several electrolytes, the masses deposited are proportional to the chemical equivalent molar masses Mi/zi[7](https://goldbook.iupac.org/terms/view/09075).
In practical form, the total charge is Q = I × t = n × F, where I is current in amperes, t time in seconds, and n the number of moles of electrons[6](https://openstax.org/books/chemistry/pages/17-7-electrolysis). Dividing the measured charge by F gives moles of electrons, which stoichiometry then converts to moles of product; the BIPM writes the same realization of the mole as n = Q/(zF), noting that its uncertainty depends on the reaction efficiency of the ion of interest and on the absence of interfering ions[1](https://www.bipm.org/documents/20126/41489679/SI-App2-mole.pdf/be4dea74-a526-e49b-f497-11b393665401?t=1637238198610&version=1.6) • [6](https://openstax.org/books/chemistry/pages/17-7-electrolysis).
## Historical origin
The first exact quantitative study of electrolytic phenomena was made about 1830 by Michael Faraday (Experimental Researches, 1833)[8](https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Electrolysis). While demonstrating that electricity from different sources is one and the same, Faraday arrived at the two laws of electrolysis: the degree of chemical change matches the amount of electric charge flowing through a solution, and when equal charges deposit or dissolve various substances, the quantities involved relate to their chemical equivalent weights[2](https://www.sciencehistory.org/education/scientific-biographies/michael-faraday/). In 1833, together with the classicist [William Whewell](https://www.edgepedia.org/william-whewell), he devised a Greek-derived naming system for electrochemical concepts—terms such as ion and electrode—that remains largely in use today.[2](https://www.sciencehistory.org/education/scientific-biographies/michael-faraday/)
The 1911 [Encyclopædia Britannica](https://www.edgepedia.org/encyclop-dia-britannica) records that by that date Faraday's laws had been confirmed by modern research and, in favorable cases, shown to hold with an accuracy of at least one part in a thousand; the same account gives the electrochemical equivalent of an ion of unit chemical equivalent as 1.036 × 10−5 • [8](https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Electrolysis). The laws' success provided early quantitative evidence that electric charge comes in discrete units, since the fixed charge per mole of a given ion implies a fixed charge per entity[9](https://www.thequantumobserver.org/articles/1833/01/faraday-laws-of-electrolysis/).
## How the measured value changed
Because F is the product of two constants that were themselves measured, its recommended value was refined across the CODATA adjustments. A review of Faraday determinations at the National Bureau of Standards lists values such as 96485.4 ± 1.6 and 96487.0 ± 1.6 absolute C mol−1, at the level of parts per million[10](https://nvlpubs.nist.gov/nistpubs/jres/72A/jresv72An4p435_A1b.pdf). The 2014 CODATA value, 96 485.332 89(59) C mol−1, was uncertain only in its last two digits[5](https://physics.nist.gov/cuu/pdf/CODATA_JPCRD2016.pdf). After the 2019 SI redefinition the uncertainty disappeared entirely: F is now exact by construction, and the remaining uncertainty in electrolysis-based measurements lies in the chemistry, not in the constant[1](https://www.bipm.org/documents/20126/41489679/SI-App2-mole.pdf/be4dea74-a526-e49b-f497-11b393665401?t=1637238198610&version=1.6).
## Practical use
The constant appears wherever charge and amount of substance must be interconverted. In electroplating and electrolysis problems, the working steps are to compute Q = It, divide by F to obtain moles of electrons, and apply the reaction stoichiometry and molar mass to find the mass deposited or liberated[6](https://openstax.org/books/chemistry/pages/17-7-electrolysis). The same relation underlies coulometry as an analytical method and the BIPM's listed electrolysis realization of the mole[1](https://www.bipm.org/documents/20126/41489679/SI-App2-mole.pdf/be4dea74-a526-e49b-f497-11b393665401?t=1637238198610&version=1.6).
## References
1. [SI Brochure, 9th edition (2019), Appendix 2: Mise en pratique for the mole, BIPM](https://www.bipm.org/documents/20126/41489679/SI-App2-mole.pdf/be4dea74-a526-e49b-f497-11b393665401?t=1637238198610&version=1.6)
2. [Michael Faraday, Science History Institute](https://www.sciencehistory.org/education/scientific-biographies/michael-faraday/)
3. [Physics Constants: Exact CODATA 2022 (NIST) Values, UsePhysics](https://usephysics.com/reference)
4. [Terminology of electrochemical methods of analysis (IUPAC Recommendations 2019), Pure and Applied Chemistry](https://doi.org/10.1515/pac-2018-0109)
5. [CODATA Recommended Values of the Fundamental Physical Constants: 2014, NIST](https://physics.nist.gov/cuu/pdf/CODATA_JPCRD2016.pdf)
6. [17.7 Electrolysis, Chemistry 2e, OpenStax](https://openstax.org/books/chemistry/pages/17-7-electrolysis)
7. [Faraday's laws of electrolysis, IUPAC Compendium of Chemical Terminology (Gold Book)](https://goldbook.iupac.org/terms/view/09075)
8. [Electrolysis, Encyclopædia Britannica 11th ed. (1911), Wikisource](https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Electrolysis)
9. [1833: Michael Faraday Formulates Quantitative Laws of Electrolysis, The Quantum Observer](https://www.thequantumobserver.org/articles/1833/01/faraday-laws-of-electrolysis/)
10. [Resume of values of the Faraday, Journal of Research of the NBS (NVL Publications)](https://nvlpubs.nist.gov/nistpubs/jres/72A/jresv72An4p435_A1b.pdf)
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*Topic: Encyclopedia › Physical world and mathematics › Physics › Classical physics › Electromagnetism › Electromagnetic quantities and history › Electromagnetic quantities › Electric charge and current quantities*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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