Elementary charge (e)
e is the elementary charge: the charge of one proton, and the size of the charge of one electron. It is exactly 1.602 176 634 × 10⁻¹⁹ coulombs.
How to read it
OpenStax’s University Physics describes e as the smallest amount of charge an object can have. No free particle has less, the charge on any object is a whole-number multiple of it, and the smallest negative charge, −e, is exactly as large as the smallest positive one. Since the revised SI came into force in May 2019 its value has been fixed, and the Ampere (A) is defined from it (BIPM SI Brochure), and NIST’s 2022 CODATA table marks it exact. The IUPAC Green Book calls it the elementary charge or proton charge.
Also searched as: 1.602 × 10^-19 C, charge of an electron, charge of a proton, e, e in physics, elementary charge symbol.
Two e’s
The Green Book tells them apart by type. The number e, 2.718 281…, the base of natural logarithms, is a mathematical constant and printed upright, like i. When e stands for the elementary charge it is a physical quantity and printed in italic, “to distinguish them from the corresponding mathematical symbol”. Written by hand or on a calculator they look the same, and only the context says which is meant.
e and the electronvolt
The Electronvolt (eV) is built on e: the Green Book defines 1 eV as e times 1 V, the energy a charge e gains across one volt.
Read next
- Next in Physics and chemistry symbolsKelvin (K)K is the kelvin, the SI unit of temperature. It is written without a degree sign: 300 K, not 300 °K. A step of 1 K is the same size as a step of 1 °C; water freezes at 273.15 K.
- The same shape in Statistics symbolsCorrelation coefficient (r)r is the correlation coefficient, a number from −1 to 1 that measures how closely two variables follow a straight-line relationship, and in which direction. The correlation of a whole population is written with the Greek letter ρ (rho).
More physics and chemistry symbols
The same shape elsewhere
Browse symbols drawn with letters and numbers.
Sources
- CODATA Value: elementary charge, The NIST Reference on Constants, Units and Uncertainty (2022 CODATA), National Institute of Standards and Technology, elementary charge
- The International System of Units (SI), 9th edition, SI Brochure, 9th edition (2019), updated 2024, Bureau International des Poids et Mesures (BIPM), Sec. 2.2; Sec. 2.3.1 The ampere; Appendix 1
- University Physics Volume 2, 5.1 Electric Charge, OpenStax (Rice University), Properties of Electric Charge
- Quantities, Units and Symbols in Physical Chemistry (the Green Book), IUPAC Green Book, 3rd edition, 2nd printing (2008), International Union of Pure and Applied Chemistry (IUPAC), Sec. 2.4; Sec. 4.1; Sec. 4.2
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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