The mass of an electron is 9.1093837139 x 10-31 kg, which equals 0.51099895069 MeV/c2 and 5.485799090441 x 10-4 atomic mass units. Those are the 2022 CODATA recommended values published by the National Institute of Standards and Technology (NIST).
The kilogram figure has a relative standard uncertainty of 3.1 parts in 10 billion. No one weighs an electron on a scale, so the number is built from other measurements.
The electron mass in three units
Physicists quote the same quantity in three ways because each unit suits a different job. Kilograms are the SI unit, megaelectronvolts suit particle physics, and atomic mass units suit chemistry.
- 9.1093837139 x 10-31 kg (uncertainty 0.0000000028 x 10-31 kg)
- 0.51099895069 MeV/c2 (uncertainty 0.00000000016)
- 5.485799090441 x 10-4 u (uncertainty 0.000000000097 x 10-4 u)
The atomic-mass-unit value is known more tightly than the kilogram value, with a relative uncertainty of 1.8 parts in 100 billion. That is because masses of atoms and subatomic particles are compared against each other far more precisely than against the kilogram.
The MeV/c2 form comes from E = mc2: an electron at rest carries about half a million electron volts of energy. If you want to see how electron energies are used in chemistry, our explainer on ionization energy shows how much energy it takes to pull an electron off an atom.
How scientists worked out the mass
Nobody measured the electron mass directly at first. In 1897, J.J. Thomson showed that cathode rays are streams of particles, which the Nobel Prize site credits as the discovery that electrons conduct electricity and are part of atoms.
His experiment gave a ratio, charge divided by mass, not either number alone. The video below walks through that measurement.
Video: UNSW Physics
The missing half came from Robert Millikan. According to the Nobel Prize site, in 1910 he suspended tiny charged oil drops between two metal plates and found that each drop’s charge was always a multiple of one fixed value, the charge of the electron.
Put the two results together and the mass follows by division: the charge divided by the charge-to-mass ratio. The charge itself is now fixed by definition at exactly 1.602176634 x 10-19 coulombs in the SI, per NIST, so the electron’s mass is the quantity that carries the uncertainty.
Why modern values use ion traps
Today’s precision comes from trapping single charged particles. The 1989 Nobel Prize in Physics honored Hans Dehmelt and Wolfgang Paul for developing ion-trap techniques; the Nobel announcement notes that a Penning trap let Dehmelt’s group study a single electron with extreme precision.
The CODATA value is not one experiment. It is a least-squares adjustment that combines many measurements and the theory that links them, and NIST republishes it after each review.
That is also why the number changes in its later digits between releases. A value like 9.109 x 10-31 kg is safe for homework, but the full digits belong to a specific CODATA year.
Electron vs proton vs neutron
An electron is tiny next to the particles in the nucleus. The proton is about 1,836 times heavier, a ratio NIST lists as 1836.152673426.
| Particle | Mass (kg) | Mass (MeV/c2) | Mass (u) |
|---|---|---|---|
| Electron | 9.1093837139 x 10-31 | 0.51099895069 | 5.485799090441 x 10-4 |
| Proton | 1.67262192595 x 10-27 | 938.27208943 | 1.0072764665789 |
| Neutron | 1.67492750056 x 10-27 | 939.56542194 | 1.00866491606 |
Source: NIST Reference on Constants, Units, and Uncertainty, 2022 CODATA recommended values, physics.nist.gov/cuu/Constants, values opened October 28, 2026.
The table explains a habit chemists share: when they add up an atom’s mass, they count protons and neutrons and ignore the electrons. The electrons contribute less than 0.06 percent of a hydrogen atom’s mass in this table, since 5.4858 x 10-4 u against 1.0073 u for the proton is about 0.054 percent.
Where the electron mass matters
The electron mass sets the size of atoms and the energies of their spectral lines. It feeds directly into how tightly outer electrons are held, which is the starting point for the idea of a valence electron.
It also shows up in nuclear physics. In processes like those in fusion and fission, energy balances are written in MeV, and 0.511 MeV is the rest energy you account for whenever an electron or positron is created or destroyed.
One common slip is treating the mass as a measured property that varies with speed in everyday physics. The 9.109 x 10-31 kg figure is the rest mass; fast electrons carry more energy, but the rest mass stays the same.
Frequently asked questions
What is the mass of an electron in grams?
About 9.109 x 10-28 grams, which is the NIST kilogram value of 9.1093837139 x 10-31 kg multiplied by 1,000. Grams are rarely used in physics, so most references quote kilograms or MeV/c2.
Why is the electron mass given in MeV/c2?
Particle physics treats mass and energy as the same thing through E = mc2. NIST gives the electron’s energy equivalent as 0.51099895069 MeV, which is the same figure written as an energy.
Is the mass of an electron exact?
No. It is a measured value with a stated uncertainty, a relative standard uncertainty of 3.1 x 10-10 in the 2022 CODATA set. By contrast, the elementary charge is exact because the SI defines it.
