Electrons have a relative mass of 9.1 x 10^-31 kilograms, or 0.51 megaelectron volts. They are a lot smaller than protons and neutrons; and, an electron is roughly 0.054 percent of the mass of one neutron.
Protons, electrons and neutrons are subatomic particles that are found within an atom. Protons and neutrons have relatively the same mass, while electrons are almost half that size.
In terms of mass, electrons, protons and neutrons can all be measured in kilograms or electron volts. Typically, the masses of electrons, protons and neutrons are measured in electron volts, which is commonly used as a measurement of energy. However, the electron volt can be used to measure the mass of subatomic particles, since mass and energy are inter-related.