How to Calculate the AMU


The atomic mass unit (amu) is a relative unit of mass used in the metric system to describe atomic weight. It is equivalent to one-twelfth of the mass of an atom of carbon-12 (See References 1). The atomic mass unit for an element can be found on the periodic table, or it can be calculated.

  • Determine the atomic weight of an atom of a particular element by looking at the periodic table. The number located directly under the element symbol expresses the atomic weight of an element in atomic mass units.

  • Write down the number of grams of the substance.

  • Divide the number of grams of the substance by 0.60221415 x 10(-24).

  • Record the result of your calculation. This is the atomic weight of your substance in amus.


  • “General Chemistry, Seventh Edition;” Kenneth Whitten, Ph.D., Raymond Davis, Ph.D., M. Peck, Ph.D., & George Stanley, Ph.D.; 2004
  • Photo Credit Creatas Images/Creatas/Getty Images
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