Why don't we count electrons in determining the atomic mass?

Electrons have such a small mass relative to protons and neutrons that they are typically not included in calculations of Atomic Mass. The mass of electrons is about 1/1836 of the mass of a proton or neutron, so their contribution to the overall mass of an atom is negligible. Instead, atomic mass is primarily determined by the sum of the number of protons and neutrons in the nucleus of an atom.