Average Atomic Mass Calculator

Calculate weighted average atomic mass from isotopes

Compute the average atomic mass of an element based on isotope masses and their natural abundances

Last updated: December 5, 2025

CreatorFrank Zhao

How many isotopes do you have?

Isotope 1

Percentage of isotope 1

Mass of isotope 1

amu

Isotope 2

Percentage of isotope 2

Mass of isotope 2

amu

Note: Ensure the percentages sum to 100% for an accurate result.

Introduction / Overview

The periodic table shows most atomic masses as decimals because many elements occur as a mixture of isotopes. The average atomic mass is a weighted average of those isotope masses.

Core idea: multiply each isotope’s mass by its fractional abundance, then add them up.

Related tools:

If you want the mass number from protons/neutrons, use Atomic Mass Calculator. If you’re solving for protons, neutrons, electrons, or ionic charge, try Atom Calculator.

Isotopes (Concept)

Isotopes are atoms of the same element (same proton count) with different neutron counts. They behave similarly in chemistry, but their masses differ slightly.

Example: Chlorine

Chlorine-35 and chlorine-37 both have $Z=17$ protons.
Their natural abundances are different, so the average mass falls between the two isotope masses.

How to Use (Quick Start)

Select the number of isotopes (up to 10).
Enter each isotope’s abundance (as % or as a decimal fraction).
Enter each isotope’s mass (in u / amu).
Read the weighted average; use the rounded value if needed.

Quick note

Abundances should sum to $100\%$ (or $1.0$ in fraction form). If they don’t, the calculator will flag it.

Calculation Method

The average atomic mass is a weighted sum:

\bar{m}=\sum_{i=1}^{n} f_i\,m_i

Here $m_i$ is the mass of isotope $i$ and $f_i$ is its fractional abundance. Fractions satisfy $\sum f_i = 1$ .

Converting % to fractions

f

=

\frac{\%}{100}

Worked Examples

Example: Chlorine

Use the two common isotopes and their natural abundances.

Chlorine-35

$\% = 75.78$ $m=34.96885\ \mathrm{u}$

Chlorine-37

$\% = 24.22$ $m=36.96590\ \mathrm{u}$

Compute the weighted sum

\bar{m}

=

0.7578\cdot 34.96885

+

0.2422\cdot 36.96590

\approx

35.452\ \mathrm{u}

Rounded to two decimals: 35.45 u.

Checks & Validation

Three quick checks

Abundances sum to $100\%$ (or fractions sum to $1$ ).
The average lies between the smallest and largest isotope masses.
Units are consistent: isotope masses should be in $\mathrm{u}$ (amu).

Common Scenarios

Chemistry homework

Verify weighted-average problems quickly and focus on the reasoning, not the arithmetic.

Molar mass / stoichiometry

Average atomic masses are the values typically used in molar-mass calculations.

Isotope mixtures

Compute how changing abundances shifts the measured average mass.

Lab write-ups

Sanity-check results when you’re comparing isotopic composition across samples.

Tips & Best Practices

Be consistent with formats

If you enter abundances as decimals (like $0.2422$ ), keep that format for every isotope.

Round at the end

Keep more digits during multiplication; round the final average to match your assignment’s requirement.

Use reliable data

Isotope masses and natural abundances are typically reported by standards organizations. If your textbook provides a table, use that for consistency.

FAQs

Why are periodic-table atomic masses decimals?

Because they represent a weighted average across naturally occurring isotopes, not the mass of a single isotope.

What’s the difference between average atomic mass and molar mass?

They use the same numerical value but different units: average atomic mass is in $\mathrm{u}$ , while molar mass is in $\mathrm{g/mol}$ .

What if my abundances don’t add to 100%?

The result won’t represent a true mixture. Adjust your inputs so the total is $100\%$ .

Can I calculate more than 10 isotopes?

This calculator supports up to 10 isotopes, which covers typical classroom and many real-world datasets.

Limitations & Sources

Results depend entirely on the isotope masses and abundances you provide. Natural isotopic abundances can vary by sample and measurement method.

For authoritative reference values, many courses and labs use published tables (for example, IUPAC and NIST datasets).

Related Calculators

Atom Calculator

Calculate atomic properties including protons, neutrons, electrons, atomic number and mass number

Try Now

Electron Configuration Calculator

Get the electron configuration for any element from the periodic table. View atomic number, atomic mass, and valence electrons.

Try Now

Atomic Mass Calculator

Calculate atomic mass from protons and neutrons. Get atomic mass in various units including atomic mass units (u), kilograms, and more.

Try Now

Electronegativity Calculator

Calculate electronegativity difference between two elements and determine bond type (ionic, polar covalent, or nonpolar covalent).

Try Now

BMI Calculator

Calculate your Body Mass Index and see your health category

Try Now

Age Calculator

Calculate your exact age in years, months, and days

Try Now