List each isotope of the element
Note the atomic mass of each isotope
Note the natural abundance of each isotope
Convert each abundance percentage to a decimal
Multiply each isotope’s atomic mass by its decimal abundance
Add all of the products together
Use the formula: average atomic mass = (mass1 × abundance1) + (mass2 × abundance2) + (mass3 × abundance3) + …
Ensure the abundances add up to 1.00 or 100%
Report the final value in atomic mass units (amu)