What is atomic mass?

Atomic mass is calculated as the total mass of the protons, neutrons, and electrons in a single atom when it is at rest. It is measured by mass spectrometry. Atomic mass is different from atomic weight. The formula used to calculate atomic mass is Mass of protons + Mass of neutrons + Mass of electrons = Atomic mass. Because electrons have almost negligible mass as compared to protons and neutrons, the mass of electrons does not influence the calculation.