It is defined as one twelfth of the mass of an unbound neutral atom of carbon\(-12\) in its nuclear and electronic ground state.

Mass of \(1\) mole \(C\) atoms \(= 12gms\)

\(1 \mathrm{amu}=\frac{12}{12 \times 6.023 \times 10^{23}}=1.66 \times 10^{-27} \mathrm{kg}\)

Energy equivalent, \(E=m c^{2}=1.66 \times 10^{-27} \mathrm{kg} \times\left(3 \times 10^{8}\right)^{2}=930 \mathrm{MeV}\)