Electronvolt to Joule Converter
Convert energy between electronvolts (meV to TeV), joules and kJ or kcal per mole using the exact SI value of the elementary charge — plus the photon wavelength, frequency, wavenumber, temperature and mass equivalents, a full table of units and typical energies to compare with.
How it works
E (J) = E (eV) × 1.602176634 × 10⁻¹⁹ (exact) · E (eV) = E (J) ÷ 1.602176634 × 10⁻¹⁹ · per mole = E (J) × 6.02214076 × 10²³ ÷ 1000 (kJ/mol) · λ = hc/E · ν = E/h · T = E/k · m = E/c²
An electronvolt is the energy an electron gains crossing a one-volt potential difference — a natural unit for atoms, photons and particles. Because the 2019 redefinition of the SI fixed the elementary charge at exactly 1.602176634 × 10⁻¹⁹ coulombs, one electronvolt is exactly 1.602176634 × 10⁻¹⁹ joules. You can enter meV, keV, MeV, GeV or TeV, convert back from joules (scientific notation accepted) or start from a molar energy in kJ/mol or kcal/mol. Multiplying a per-particle energy in joules by Avogadro’s number scales it up to the per-mole energies chemists quote in kJ/mol: 1 eV per particle is 96.485 kJ/mol, or 23.06 kcal/mol. The calculator also gives the equivalents physicists use: the wavelength and frequency of a photon with that energy (1 eV ↔ 1,239.84 nm ↔ 241.8 THz), the wavenumber in cm⁻¹, the temperature at which kT equals the energy (1 eV ↔ 11,604.5 K) and the mass equivalent E/c². A comparison table puts the result next to familiar energies — room-temperature thermal energy, visible photons, a chemical bond, an electron’s rest energy.
Worked example
A photon of energy 2.5 eV carries 2.5 × 1.602176634 × 10⁻¹⁹ = 4.005441585 × 10⁻¹⁹ J. Across a full mole of such photons that is 4.005441585 × 10⁻¹⁹ × 6.02214076 × 10²³ ÷ 1000 = 241.21 kJ/mol (57.65 kcal/mol). Its wavelength is 1,239.84 nm ÷ 2.5 = 495.94 nm — blue-green visible light — and its frequency 604.5 THz. Going the other way, 3.2 × 10⁻¹⁹ J ÷ 1.602176634 × 10⁻¹⁹ = 1.997 eV.
Sources: Electronvolt — Wikipedia · NIST — elementary charge (exact, CODATA) · BIPM — The International System of Units (SI Brochure, 2019)
Frequently asked questions
Why is the electronvolt useful?
Energies at the atomic scale are tiny in joules — around 10⁻¹⁹ J — so the electronvolt gives conveniently sized numbers. Bond energies, photon energies and band gaps are quoted in eV, X-rays in keV, nuclear and particle physics in MeV, GeV and TeV.
Is 1 eV = 1.602176634 × 10⁻¹⁹ J exact?
Yes. Since 20 May 2019 the SI defines the elementary charge as exactly 1.602176634 × 10⁻¹⁹ C, and 1 eV is that charge times one volt, so the conversion has no uncertainty. Older textbooks give 1.602176565 × 10⁻¹⁹ or 1.60218 × 10⁻¹⁹ — close, but no longer the defined value.
How do eV relate to kJ/mol?
1 eV per particle equals 96.485 kJ/mol (23.061 kcal/mol) once multiplied by Avogadro’s number. Choose “Per mole” to convert a bond or activation energy in kJ/mol or kcal/mol into electronvolts per molecule.
How do I convert joules back to electronvolts?
Divide the energy in joules by 1.602176634 × 10⁻¹⁹, or multiply by 6.241509074 × 10¹⁸. For example, 3.2 × 10⁻¹⁹ J ÷ 1.602176634 × 10⁻¹⁹ ≈ 1.997 eV. Choose “Joules” and type the value in scientific notation.
How do I get a photon’s wavelength from its energy?
Use λ = hc/E. With hc = 1,239.84198 eV·nm, a photon’s wavelength in nanometres is 1,239.84 divided by its energy in eV — 2 eV is 619.9 nm (orange-red), 3 eV is 413.3 nm (violet).
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This calculator is for educational purposes. Double-check important results before acting on them.