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Average Atomic Mass Calculator


This average atomic mass calculator determines the weighted average of a chemical element, by factoring in isotopic masses and their natural abundance in a sample. For each isotope, specify the mass and the percentage abundance in a given element.

What Is The Atomic Mass?

“Average atomic mass is the weighted average of the masses of all naturally occurring isotopes of an element” 

Average atomic mass is also known as average atomic weight. The mass of one mole of an atom of an element is equal to the average atomic mass of that element in grams. 

This average atomic mass calculator helps you to find the mass of a mole of an atom for a given element. 

Atomic Mass Units:

Average atomic mass is expressed in kg; generally, it is expressed in a non-SI unit known as amu. This unit is used for comparing the masses of various atoms and molecules. 

How Average Atomic Mass Calculator Function?

Our average atomic mass calculator operates well by considering some values with their respective abundance. By using these inputs, our tool performs vital calculations in just a click. 


  • Choose the number of isotopes 
  • Put the percentage and mass of isotope in amu


  • Average Atomic Mass in amu 
  • Complete calculation in steps 

Average Atomic Mass Formula:

The formula for calculating the average atomic mass (A) is:

AM = ( Mass of Isotope1 × Abundance1 ) + ( Mass of Isotope2 × Abundance2 ) + …

In the given formula, each term represents the mass of an isotope multiplied by its abundance, and the sum is taken over all isotopes of the element.

How to Calculate Average Atomic Mass?

By looking up the natural abundance and the mass of their isotopes, our average atomic mass calculator calculates fast results in a fraction of a second. 

Example # 1:

Chlorine Isotopes: -35 and -37

  • Chlorine-35 has a mass of 34.969 amu and a relative abundance of 75.77%
  • Chlorine-37 has a mass of 37.9627 amu and a relative abundance of 24.23%


The average atomic mass of chlorine is calculated as follows:

= (34.969 amu * 0.7577) + (37.9627 amu * 0.2423) 

= 35.45 amu

Example # 2:

Suppose, an element has three isotopes hydrogen, deuterium, and tritium. So, find their average atomic mass?

  • Isotope_1 has a mass of 1.007825 amu with a relative abundance of 99.9885%
  • Isotope_2 has a mass of 2.014101 amu with an abundance of 0.0115%
  • Isotope_3 has a mass of 3.01605 amu with an abundance of 11%


When expressing natural abundance (f) values as a percentage, divide by 100.

AM = ( Abundance1 x Mass ) + ( Abundance2 x Mass ) + … / 100

AM = [ (99.9885 x 1.007825) + (0.0115 x 2.014101) + (11 x 3.01605) ] / 100

AM = 1.33970622174


What Are Isotopes?

If two or more forms of the same element have the same number of protons but different numbers of neutrons in their nuclei is known as anisotope.

  • Isotopes differ in atomic mass but not in chemical properties
  • Some isotopes are stable, while others are radioactive

What Is amu?

AMU is the short form of atomic mass unit. “It is defined as 1/12 of the mass of an unbound atom of carbon-12”. 

One amu = 1.66053907015 × 10⁻²⁷ kilograms

This is a very small mass, but it is convenient for comparing the masses of atoms and molecules. This can help you in calculating average atomic mass.

What Is a Mass Number?

The sum of protons and the neutrons of an atom is known as the mass number. it is denoted by A

Average Atomic Mass Table:

Hydrogen 1 1.008
Helium 2 4.0026
Lithium 3 6.94
Beryllium 4 9.0122
Boron 5 10.81
Carbon 6 12.011
Nitrogen 7 14.007
Oxygen 8 15.999
Fluorine 9 18.998
Neon 10 20.180
Sodium 11 22.990
Magnesium 12 24.305
Aluminium 13 26.982
Silicon 14 28.085
Phosphorus 15 30.974
Sulfur 16 32.06
Chlorine 17 35.45
Argon 18 39.948
Potassium 19 39.098
Calcium 20 40.078
Scandium 21 44.956
Titanium 22 47.867
Vanadium 23 50.942
Chromium 24 51.996
Manganese 25 54.938
Iron 26 55.845
Cobalt 27 58.933
Nickel 28 58.693
Copper 29 63.546
Zinc 30 65.38



Bio.libretexts: Average Atomic Mass, Isotopes, Calculating Average Atomic Mass

Wikipedia: Atomic mass, Relative isotopic mass, Similar terms for different quantities, Mass defect, Measurement of atomic masses, Relationship between atomic and molecular masses. 

Chem.libretexts: Calculating Average Atomic Mass, Have you ever tried to move a boulder? Percent Natural Abundance, Example.