Carbon 12 is assigned a mass of exactly twelve atomic mass:

1. Atomic masses are all quoted relative to one standard. 2. That standard is a particular carbon isotope. 3. Carbon 12 is assigned a mass of exactly 12 atomic mass unit. 4. Masses of all other atoms are given relative to this standard. _Source: syllabus Chemistry Part I, Unit 1 'Some Basic Concepts of Chemistry'_