Enter the percentage abundance and mass of up to 5 different isotopes into the average atom mass calculator. The calculator will display the average atomic mass of the isotopes.

## Average Atomic Mass Formula

The following formula is used to calculate the average atomic mass of a substance.

AM = f_{1}M_{1} + f_{2}M_{2} +… + f_{n}M_{n}

- Where AM is the average atomic mass
- fn is the fractional percent of the isotope
- M is the mass of the isotope

## Average Atomic Mass Definition

An average atomic mass is defined as the average mass of all isotopes within a given substance.

## How to calculate average atomic mass?

How to calculate average atomic mass

**First, determine the fractional percent of each isotope in the substance**For example, chlorine has two major isotopes. 1 with 75.77 percent of atoms and 1 with 24.23 percent of atoms. These two percentages would be the fractional percents of those isotopes.

**Next, determine the masses of each isotope**Using the same example above, this would be 35 and 37 amu respectively.

**Finally, calculate the average atomic mass**Calculate the average atomic mass using the information from steps 1 and 2 and the formula above.

## FAQ

**What is average atomic mass?**

The average atomic mass is the average mass of all of the isotopes that make up a substance.

**What is the fractional percent?**

The fractional percent is the total percentage a particular isotope in a substance.