The mass of an atom is incredibly small, so a special unit was created to allow mass numbers to be easily written down for atoms. This unit is the (a)tomic (m)ass (u)nit or amu.

1 amu = 1.7 x 10^-27 kg

So, a single carbon atom has a mass of approximately 12 amu, or 2.0 x 10^-26 kg.