Average Atomic Mass Calculator: Relative Atomic Mass from Isotope Masses and Abundances
The weighted average mass of an element's isotopes.
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How this calculator works
Choose how many isotopes.
Enter the mass and the abundance of each.
The calculator gives the average atomic mass.
Formulas used
- A_r = \frac{\sum m_ip_i}{\sum p_i}
Worked example
With the default numbers in the calculator above:
- Average atomic mass: 35.45293 u
- Total of the abundances: 100 %
- The sum of mass times abundance: 35.45293 u (per 100 percent)
Change the numbers in the calculator to see the full working, every step, for your own case.
A second worked example
A different case, with these numbers: Number of isotopes 3; Isotope 1: mass 23.985 u; Isotope 1: abundance 78.99 %; Isotope 2: mass 24.986 u; Isotope 2: abundance 10 %; Isotope 3: mass 25.983 u; Isotope 3: abundance 11.01 %.
- Average atomic mass: 24.30508 u
- Total of the abundances: 100 %
- The sum of mass times abundance: 24.30508 u (per 100 percent)
Common questions
How do you calculate average atomic mass?
Multiply the mass of each isotope by its abundance, add them up, and divide by 100 if the abundances are in percent.
Why is the atomic mass not a whole number?
Because it is an average over a mixture of isotopes, each of which has a different mass.
What is the average atomic mass of chlorine?
About 35.45 u, from chlorine-35 (75.76 percent) and chlorine-37 (24.24 percent).
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Got a question about this? Ask an engineering question and get a plain-language answer with references.
Further reading: Any chemistry text (isotopes and relative atomic mass).
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