List each isotope’s atomic mass
List each isotope’s percent abundance or fractional abundance
Convert percent abundance to decimal form if needed
Multiply each isotope’s atomic mass by its abundance
Add all the products together
Use the formula: average atomic mass = (m1 × a1) + (m2 × a2) + (m3 × a3) + …
Ensure all abundances add up to 1.00 or 100%
Report the result in atomic mass units (amu)