⚛️ Atomic Mass Calculator
Enter the mass and percent abundance of each isotope to find an element’s weighted average atomic mass, just like the value shown on the periodic table.
What is average atomic mass?
The atomic mass shown on the periodic table isn’t the mass of a single atom — it’s a weighted average of the masses of all naturally occurring isotopes of that element, weighted by how common each isotope actually is.
How this calculator works
For each isotope you enter, its mass is multiplied by its fractional abundance (percent abundance divided by 100). Those weighted values are added together to give the average atomic mass. The calculator divides by your total entered abundance so the result stays accurate even if your percentages don’t add up to exactly 100.
For example, chlorine’s two main isotopes are chlorine-35 (about 75.76%) and chlorine-37 (about 24.24%), which average out to the familiar atomic mass of about 35.45 amu.
This tool is intended for general chemistry education purposes.