Electronvolt Converter (eV, keV, MeV, J)

Understanding the electronvolt converter — concept and principle

This converter changes energies between electronvolts (eV), keV, MeV, GeV and joules. The electronvolt is the natural energy unit of atomic, nuclear and particle physics, X-ray imaging, radiation protection and semiconductor physics, while the joule is the SI unit used everywhere else. Students and technicians use the conversion when moving between textbook particle energies and SI calculations.

How it is calculated

• 1 eV = 1.602176634 × 10⁻¹⁹ J, exactly.
• 1 keV = 10³ eV, 1 MeV = 10⁶ eV, 1 GeV = 10⁹ eV.
• 1 J ≈ 6.241509 × 10¹⁸ eV.

The calculator multiplies your value by the size of the starting unit in eV and divides by the size of the target unit. Joule results are shown in scientific notation with seven significant figures.

The principle

One electronvolt is the kinetic energy an electron gains when it is accelerated through a potential difference of one volt. Energy = charge × voltage, so 1 eV = e × 1 V, where e is the elementary charge. Since the 2019 revision of the SI, e is fixed at exactly 1.602176634 × 10⁻¹⁹ C, which makes the eV-to-joule factor exact. The eV is convenient because energies at atomic scale are tiny in joules: visible light photons are about 1.6–3.3 eV, chemical bonds a few eV, the electron's rest energy about 0.511 MeV, and one uranium-235 fission releases roughly 200 MeV.

Worked example

Default input: 1 MeV to eV.

1. The prefix mega means 10⁶, so 1 MeV = 1,000,000 eV.
2. In keV: 1,000,000 / 1,000 = 1,000 keV; in GeV: 0.001 GeV.
3. In joules: 10⁶ × 1.602176634 × 10⁻¹⁹ = 1.602176634 × 10⁻¹³ J.

The calculator shows 1,000,000 eV and 1.602177 × 10⁻¹³ J (rounded to seven figures), matching these steps.

How to read the result

Because the scales differ by about 10¹⁹, any everyday energy in joules becomes an enormous number of eV, and any particle energy becomes a tiny number of joules. That is normal. Check that the result is in a sensible range: diagnostic X-rays are typically tens of keV, gamma rays from radioactive decay range from keV to several MeV, and accelerator energies are quoted in GeV or TeV.

Common mistakes

• Confusing the energy unit eV with mass units such as MeV/c², used for particle masses.
• Mixing up keV of photon energy with kV (kilovolts) of tube voltage on X-ray machines.
• Writing 1.6 × 10¹⁹ instead of 10⁻¹⁹, a sign error in the exponent.
• Treating eV as a unit of temperature or radiation dose; dose uses grays and sieverts.

Frequently asked questions

How many joules is 1 eV?

Exactly 1.602176634 × 10⁻¹⁹ J.

How do I convert MeV to joules?

Multiply by 1.602176634 × 10⁻¹³.

Why is the electron mass given as 0.511 MeV?

Particle physicists quote rest energy E = mc² in eV; the electron's rest energy is about 0.511 MeV.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition · NIST, CODATA recommended values of the fundamental physical constants