C-12 or 'Carbon 12' is one of the three isotopes of the element Carbon. The other two are C-13 and C-14. The number 12 represents the mass number of the atom, or the sum of protons and neutrons. The mass of C-12 is 12 amu, while Carbon has a mass of 12.0107 amu.
Ratio = 12.0107/12 = 1.000891667
The ratio of their masses are approximately equal.
