⚛️ Percent Abundance Calculator
Find the natural percent abundance of two isotopes of an element from their individual masses and the element’s known average atomic mass.
What percent abundance means
Most elements exist in nature as a mix of isotopes, atoms with the same number of protons but different numbers of neutrons (and therefore different masses). The periodic table’s “average atomic mass” is a weighted average of these isotope masses, weighted by how common each isotope actually is.
The formula
For two isotopes with masses m1 and m2, and fractional abundances x1 and x2 (which add up to 1), the average atomic mass is: Average = x1·m1 + x2·m2. Solving for x1 given the average mass gives x1 = (Average − m2) / (m1 − m2), and x2 = 1 − x1. This calculator reports both as percentages.
Note: This method assumes exactly two naturally occurring isotopes contribute meaningfully to the average mass, which is true for many common elements (like chlorine) but not all. For general chemistry education.