The mass of the helium nucleus is 6.64 x 10^-27 kilograms. The mass is calculated by adding together the mass of two protons, each of which has a mass of 1.67 x 10^-27 kilograms and two neutrons, which each have a mass of 1.67 x 10^-27 kilograms.
The mass of a helium nucleus can also be expressed as "4 u," which stands for atomic mass units, or 3.72 GeV/c^2 gigaelectronvolts over the speed of light squared. The helium nucleus has a mass that is the same as that of an alpha particle. The number is used in the calculation for the mass defect of the helium nucleus that is needed for the equation E = mc^2.