The present atomic weight scale is based on:

The contemporary atomic mass scale is internationally standardized based on the stable carbon-12 (${}^{12}\text{C}$) isotope. By international agreement, a unified atomic mass unit ($\text{amu}$ or $\text{u}$) is designated as exactly $\frac{1}{12}\text{th}$ of the total mass of an unbound, isolated atom of carbon-12 in its electronic ground state.