The present atomic weight scale is based on

The modern unified atomic mass scale is internationally defined based on a reference standard of Carbon-12 (${\text{C}}^{12}$). By convention, one atomic mass unit ($\text{amu}$ or $\text{u}$) is defined as exactly $\frac{1}{12}\text{th}$ of the absolute mass of an unbound neutral atom of carbon-12 at rest.