Atomic mass unit (AMU or amu) is a unit used to measure an atom’s mass, called atomic mass. This is similar to how kilograms and pounds are used to measure an individual’s weight. AMU is defined as 1/12th of the isotope carbon-12’s mass. This makes the 1/12th unit approximately 1.66×10-27 kilograms in weight. In other words, a gram is equal to 600 sextillion amu. This is precisely the reason why an atom’s weight is not measured in grams. Different isotopes or variants of an atom won’t have the same atomic mass since the neutron number could vary.