How To Find Average Atomic Mass: 8 Steps (with Pictures) - WikiHow

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Terms of Use wikiHow is where trusted research and expert knowledge come together. Learn why people trust wikiHow How to Calculate Average Atomic Mass (and Use the Result) PDF download Download Article Making sense of the numbers and how to use them Co-authored by Meredith Juncker, PhD

Last Updated: January 24, 2026 Fact Checked

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  • Calculating Average Atomic Mass
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  • Using the Result
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This article was co-authored by Meredith Juncker, PhD. Meredith Juncker is a scientific researcher based in Brookline, Massachusetts. Currently working as a Senior Patent Technology Specialist at Dana-Farber Cancer Institute, Meredith is able to leverage her expertise in biochemistry and molecular biology. She received a PhD in Biochemistry and Molecular Biology from Louisiana State University Health Sciences Center, focusing on proteins and neurodegenerative diseases. She went on to work as a Postdoctoral Associate at the Jacobs School of Medicine and Biomedical Sciences at the State University of New York at Buffalo. There are 7 references cited in this article, which can be found at the bottom of the page. This article has been fact-checked, ensuring the accuracy of any cited facts and confirming the authority of its sources. This article has been viewed 511,943 times.

Average atomic mass is not a direct measurement of a single atom. Instead, it is the average mass per atom for a typical sample of a given element. If you could measure the mass of billions of individual atoms, you could calculate this value the same way you would find any average. Fortunately, there is a more practical method that relies on recorded information on the rarity of different isotopes.

Things You Should Know

  • Look up the mass of each isotope in atomic mass units, then take note of each isotope's abundance.
  • Convert the abundances into decimals by dividing by 100.
  • Find the weighted average of of the atomic mass of the stable isotopes using a weighted average calculation, and check your answer on the periodic table.

Steps

Part 1 Part 1 of 2:

Calculating Average Atomic Mass

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  1. Step 1 Understand isotopes and atomic masses. 1 Understand isotopes and atomic masses. Most elements can naturally occur in multiple forms, or isotopes. The mass number for each isotope is the sum of numbers of protons and neutrons in the nucleus. Each proton and each neutron weigh 1 atomic mass unit (amu). The only difference between two isotopes of the same element is the number of neutrons per atom, which affects the atom's mass. However, the element always has the same number of protons.[1]
    • The average atomic mass of the element takes the variations of the number of neutrons into account, and tells you the average mass per atom in a typical sample of that element.
    • For example, the element silver (Ag) has two naturally occurring isotopes: Ag-107 and Ag-109 (or 107Ag and 109Ag).[2] Isotopes are named after the "mass number," or the sum of protons and neutrons in one atom.[3] This means Ag-109 has two more neutrons per atom than Ag-107, giving it slightly more mass.
  2. Step 2 Look up the mass of each isotope. 2 Look up the mass of each isotope. You'll need two pieces of information for each isotope, which you can look up in a reference book or an online source such as webelements.com. The first is the atomic mass, or the mass of one atom of each isotope. Isotopes with more neutrons have more mass.
    • For example, the silver isotope Ag-107 has an atomic mass of 106.90509 amu (atomic mass units). The isotope Ag-109 is slightly heavier with a mass of 108.90470.
    • The last couple decimal places might be slightly different in different sources. Don't include any numbers in parentheses after the mass.
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  3. Step 3 Write down the abundance of each isotope. 3 Write down the abundance of each isotope. The abundance tells you how common the isotope is, as a percentage of all atoms of the element. Each isotope contributes proportionally to its abundance (the more abundant the isotope, the more it will contribute to the average atomic mass). You can find this in the same source you found the mass. The abundances of all isotopes should add up to 100% (though it might be slightly off due to rounding errors).
    • The isotope Ag-107 has an abundance of 51.86%. Ag-109 is slightly less common with an abundance of 48.14%. This means that a typical sample of silver is 51.86% Ag-107 and 48.14% Ag-109.
    • Ignore any isotopes that do not have an abundance listed. These isotopes do not occur naturally on Earth.
  4. Step 4 Turn your abundance percentages into decimals. 4 Turn your abundance percentages into decimals. Divide the abundance percentage by 100 to get the same value as a decimal.
    • In the sample problem, the abundance figures are 51.86 / 100 = 0.5186 and 48.14 / 100 = 0.4814.
  5. Step 5 Find the weighted... 5 Find the weighted average of the atomic mass of its stable isotopes. The average atomic mass of an element with n isotopes equals (massisotope 1 * abundanceisotope 1) + (massisotope 2 * abundanceisotope 2) + ... + (massisotope n * abundanceisotope n. This is an example of a "weighted average," meaning that more common (more abundant) masses have a greater effect on the result. Here's how to use this formula for silver:[4]
    • Average atomic massAg = (massAg-107 * abundanceAg-107) + (massAg-109 * abundanceAg-109)=(106.90509 * 0.5186) + (108.90470 * 0.4814)= 55.4410 + 52.4267= 107.8677 amu.
    • Look up the element on a periodic table to check your answer. The average atomic mass is usually written underneath the element symbol.
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Part 2 Part 2 of 2:

Using the Result

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  1. Step 1 Convert mass to number of atoms. 1 Convert mass to number of atoms. The average atomic mass tells you the relationship between mass and number of atoms in a typical sample of the element. This is useful in chemistry laboratories because it is almost impossible to count the number of atoms directly, but easy to measure mass. For example, you can weigh a sample of silver and predict that each 107.8677 amu of mass contains one silver atom.[5]
  2. Step 2 Convert to molar... 2 Convert to molar mass. Atomic mass units are very small, so chemists typically weigh samples in grams instead. Fortunately, these concepts are defined to make the conversion as easy as possible. Just multiply the average atomic mass by 1 g / mol (the molar mass constant) to get an answer in g / mol instead.[6] For example, 107.8677 grams of silver contain one mole of silver atoms on average.
  3. Step 3 Find average molecular mass. 3 Find average molecular mass. Since a molecule is just a collection of atoms, you can add the masses of the atoms together to find the mass of the molecule. If you use the average atomic masses (instead of the mass of a specific isotope), the answer is the average mass of the molecule as found in a naturally occurring sample.[7] Here's an example:
    • A molecule of water has the chemical formula H2O, so it contains two hydrogen (H) atoms and one oxygen (O) atom.
    • Hydrogen has an average atomic mass of 1.00794 amu. Oxygen atoms have an average mass of 15.9994 amu.
    • The average mass of a molecule of H2O equals (1.00794)(2) + 15.9994 = 18.01528 amu, equivalent to 18.01528 g/mol.
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Expert Q&A

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  • Question How do you find weighted average mass if you don't have one of the amu? Meredith Juncker, PhD Meredith Juncker, PhD Scientific Researcher Meredith Juncker is a scientific researcher based in Brookline, Massachusetts. Currently working as a Senior Patent Technology Specialist at Dana-Farber Cancer Institute, Meredith is able to leverage her expertise in biochemistry and molecular biology. She received a PhD in Biochemistry and Molecular Biology from Louisiana State University Health Sciences Center, focusing on proteins and neurodegenerative diseases. She went on to work as a Postdoctoral Associate at the Jacobs School of Medicine and Biomedical Sciences at the State University of New York at Buffalo. Meredith Juncker, PhD Meredith Juncker, PhD Scientific Researcher Expert Answer An atomic mass unit is the same thing as grams per mole (1 amu = 1 g/mol). It is also the same thing as a dalton (1 amu = 1 Da). so if you don't know the amu for one of your elements, you can search for this particular isotope online to find the amu and natural abundance specific to that particular isotope. Thanks! We're glad this was helpful. Thank you for your feedback. If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow Yes No Not Helpful 9 Helpful 15
  • Question What's the atomic weight of platinum? Meredith Juncker, PhD Meredith Juncker, PhD Scientific Researcher Meredith Juncker is a scientific researcher based in Brookline, Massachusetts. Currently working as a Senior Patent Technology Specialist at Dana-Farber Cancer Institute, Meredith is able to leverage her expertise in biochemistry and molecular biology. She received a PhD in Biochemistry and Molecular Biology from Louisiana State University Health Sciences Center, focusing on proteins and neurodegenerative diseases. She went on to work as a Postdoctoral Associate at the Jacobs School of Medicine and Biomedical Sciences at the State University of New York at Buffalo. Meredith Juncker, PhD Meredith Juncker, PhD Scientific Researcher Expert Answer The atomic weight of platinum is 195.078 amu. Check out "How to Calculate Atomic Mass" for more information. Thanks! We're glad this was helpful. Thank you for your feedback. If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow Yes No Not Helpful 12 Helpful 14
  • Question Why do we need to know average atomic mass? Meredith Juncker, PhD Meredith Juncker, PhD Scientific Researcher Meredith Juncker is a scientific researcher based in Brookline, Massachusetts. Currently working as a Senior Patent Technology Specialist at Dana-Farber Cancer Institute, Meredith is able to leverage her expertise in biochemistry and molecular biology. She received a PhD in Biochemistry and Molecular Biology from Louisiana State University Health Sciences Center, focusing on proteins and neurodegenerative diseases. She went on to work as a Postdoctoral Associate at the Jacobs School of Medicine and Biomedical Sciences at the State University of New York at Buffalo. Meredith Juncker, PhD Meredith Juncker, PhD Scientific Researcher Expert Answer It's important to know average atomic mass because different isotopes of an element exist at different abundances on Earth, so different isotopes contribute to the average atomic mass at different proportions. Thanks! We're glad this was helpful. Thank you for your feedback. If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow Yes No Not Helpful 15 Helpful 11
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Video

Tips

  • Use the average atomic mass when doing mass calculations involving elements and compounds.[8] Thanks Helpful 0 Not Helpful 0
  • The term relative atomic mass is sometimes used as a synonym for average atomic mass. There is a slight difference since the relative atomic mass has no units; it is a measure of mass relative to the carbon-12 atom. As long as you use atomic mass units in your average mass calculation, however, the two values are numerically identical. Thanks Helpful 0 Not Helpful 0
  • With rare exceptions, elements later on the periodic table have a higher average mass than the elements before it. This is a quick way to check whether your answers make sense. Thanks Helpful 0 Not Helpful 0
Show More Tips Submit a Tip All tip submissions are carefully reviewed before being published Name Please provide your name and last initial Submit Thanks for submitting a tip for review! Advertisement

Warnings

  • Atomic masses are almost always written in terms of atomic mass units (amu or u), sometimes called the dalton (Da). Never add a different unit of mass (such as kilograms) after the number without converting it. Thanks Helpful 0 Not Helpful 0
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Things You'll Need

  • Pencil
  • Paper
  • Calculator
  • Isotope abundance data
  • Isotope atomic mass unit data

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References

  1. ↑ https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Chemistry_for_Allied_Health_(Soult)/02%3A_Elements_and_Ions/2.01%3A_Isotopes_and_Atomic_Mass
  2. ↑ https://www.youtube.com/watch?v=wCuboqp86mI
  3. ↑ https://www.youtube.com/watch?v=65dDZulPhtg
  4. ↑ https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Chemistry_for_Allied_Health_(Soult)/02%3A_Elements_and_Ions/2.01%3A_Isotopes_and_Atomic_Mass
  5. ↑ https://www.khanacademy.org/science/biology/history-of-life-on-earth/radiometric-dating/a/atomic-number-atomic-mass-and-isotopes-article
  6. ↑ https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_and_Chemical_Reactivity_(Kotz_et_al.)/02%3A_Atoms_Molecules_and_Ions/2.03%3A_Isotope_Abundance_and_Atomic_Weight
  7. ↑ https://opentextbc.ca/introductorychemistry/chapter/masses-of-atoms-and-molecules/
  8. ↑ https://www.youtube.com/watch?v=J1XJpY2dxUI

About This Article

Meredith Juncker, PhD Co-authored by: Meredith Juncker, PhD Scientific Researcher This article was co-authored by Meredith Juncker, PhD. Meredith Juncker is a scientific researcher based in Brookline, Massachusetts. Currently working as a Senior Patent Technology Specialist at Dana-Farber Cancer Institute, Meredith is able to leverage her expertise in biochemistry and molecular biology. She received a PhD in Biochemistry and Molecular Biology from Louisiana State University Health Sciences Center, focusing on proteins and neurodegenerative diseases. She went on to work as a Postdoctoral Associate at the Jacobs School of Medicine and Biomedical Sciences at the State University of New York at Buffalo. This article has been viewed 511,943 times. 74 votes - 74% Co-authors: 18 Updated: January 24, 2026 Views: 511,943 Categories: Chemistry Calculations Article SummaryX

If you need to find the average atomic mass of an element, you will need to look up the atomic mass and the abundance of each isotope in that element. The abundance of all of the isotopes should add up to 100%. Multiply the mass times the abundance for each isotope, then add all of the results together to get the average atomic mass. For tips from our reviewer on how to convert the mass to the number of atoms, keep reading! Did this summary help you?YesNo

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