What is the atomic mass unit (amu) standardized against?

The atomic mass unit (amu) is defined based on the carbon-12 isotope. Specifically, one atomic mass unit is defined as one twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, measured in kg. This standard was chosen primarily because carbon-12 is a stable isotope of carbon, and it is abundant in nature, which makes it a practical choice for establishing a consistent mass reference for all other elements.

Using carbon-12 provides a convenient baseline since it allows for comparative calculations of the masses of other isotopes and elements in relation to a commonly available and stable reference. Thus, when measuring the isotopic masses of other elements, those measurements are reported as a multiple of the mass of carbon-12, forming a consistent and universally understood system of atomic mass measurement.