Average Atomic Mass Calculator
Calculate a weighted average atomic mass from isotope masses and abundances, then move cleanly into periodic-table interpretation, molar-mass work, or stoichiometry setup.
Edited by Gail Joyce
Gail Joyce edits core chemistry calculator pages for formula clarity, unit consistency, and practical classroom and lab-prep usability.
This calculator page is maintained by the Chemistry Calculators editorial team. The isotope-average workflow, reverse-abundance mode, worked examples, and reference notes on this page are reviewed against standard general chemistry references before major updates.
Average Atomic Mass Calculator
Compute a weighted average atomic mass or solve one missing isotope abundance from a target average.
Quick presets
How to Use the Average Atomic Mass Calculator
This page supports both the standard weighted-average setup and the reverse chemistry problem where one isotope abundance must be solved from a known target average.
Choose the problem mode first
Use the average-mass mode for a normal weighted-average calculation or switch to missing-abundance mode when one isotope percentage must be solved.
Enter isotope masses and abundance values
List one row per isotope, use atomic mass units for isotope masses, and enter abundances as percentages rather than decimals.
Add the target average only when reverse-solving
In missing-abundance mode, type the known average atomic mass and leave exactly one abundance field blank so the calculator knows which value to solve.
Check that the result stays within the isotope range
A valid average atomic mass should fall between the lightest and heaviest isotope values in your table.
Table of Contents
Quickly navigate to different sections of this guide.
Understanding Average Atomic Mass
Average atomic mass is the weighted average of an element’s naturally occurring isotopes. It is the value shown on the periodic table, not the mass number of one specific isotope.
More abundant isotopes contribute more strongly to the final value. That is why chlorine lands near `35.45` rather than exactly `35` or `37`, and why boron lands near `10.81` instead of one whole-number isotope mass.
| Element | Isotopes | Why it helps in class |
|---|---|---|
| Chlorine | Cl-35 and Cl-37 | Classic weighted-average example |
| Boron | B-10 and B-11 | Good reverse-abundance practice |
| Magnesium | Mg-24, Mg-25, Mg-26 | Three-isotope weighted-average setup |
Formulas and Calculations
Weighted Average Formula
Average Atomic Mass = Σ (Isotope Mass × Abundance Fraction)
If abundances are entered as percentages, divide each one by `100` before multiplying.
Result Check
Lightest Isotope Mass ≤ Average Atomic Mass ≤ Heaviest Isotope Mass
If your answer falls outside that range, the isotope masses or abundances were entered incorrectly.
Worked Examples
These examples mirror the most common chemistry homework patterns for isotope averages.
Chlorine
Given: Cl-35 = `34.969 u` at `75.77%`, Cl-37 = `36.966 u` at `24.23%`.
Solution: `(34.969 × 0.7577) + (36.966 × 0.2423) = 35.45 u`.
Answer: Average atomic mass of chlorine = `35.45 u`.
Copper
Given: Cu-63 = `62.930 u` at `69.15%`, Cu-65 = `64.928 u` at `30.85%`.
Solution: `(62.930 × 0.6915) + (64.928 × 0.3085) = 63.55 u`.
Answer: Average atomic mass of copper = `63.55 u`.
Magnesium
Given: Mg-24 = `23.985 u` at `78.99%`, Mg-25 = `24.986 u` at `10.00%`, Mg-26 = `25.983 u` at `11.01%`.
Solution: `(23.985 × 0.7899) + (24.986 × 0.1000) + (25.983 × 0.1101) = 24.30 u`.
Answer: Average atomic mass of magnesium = `24.30 u`.
Common Mistakes
Most weighted-average mistakes are input-format mistakes rather than chemistry mistakes.
Mixing percentages and decimals
If you enter `75.77`, do not also divide it again in your head as if the tool were expecting `0.7577`.
Using whole-number labels as masses
Use measured isotope masses like `34.969`, not just the label `35` unless the question explicitly tells you to approximate.
Ignoring missing abundance
A set of isotopes should usually total about `100%`. If it does not, the result may not reflect the true average.
Skipping the range check
The final average must land between the smallest and largest isotope masses every time.
Frequently Asked Questions (FAQs)
Short answers to the most common isotope-average questions.
What is average atomic mass?
It is the weighted average mass of all naturally occurring isotopes of an element.
Why is it not usually a whole number?
Because it blends multiple isotope masses according to abundance.
Do abundances need to sum to 100%?
Yes, or very close to it. Otherwise the weighted average is not based on a complete isotope set.
Can I use decimal abundances?
Yes in manual chemistry work, but this calculator flow is built around percentage input for clarity.
How do I verify my answer quickly?
Check that the average lies between the lightest and heaviest isotope masses and is pulled toward the most abundant isotope.
Where can I find reliable isotope data?
Use IUPAC, NIST, or a trusted chemistry text or teaching source.
References and Further Reading
| Resource | Description | Category |
|---|---|---|
| IUPAC Commission on Isotopic Abundances and Atomic Weights | Reference source for isotope abundance and standard atomic-weight data | Reference |
| ChemLibreTexts | General chemistry explanations of isotopes, abundance, and weighted-average atomic mass | Teaching Resource |