Find the atomic masses and relative abundances (percent or fractional) of all isotopes of the element
Convert percent abundances to fractions if needed (divide each percent by 100)
Multiply each isotope’s atomic mass by its fractional abundance
Add the results to get the weighted average atomic mass
Use the formula: ( text{Average atomic mass}=sum (text{isotope mass} times text{fractional abundance}) )
Report the final value in atomic mass units (amu) typically to the precision given by the data source