Average Atomic Mass Calculator
Find the weighted average atomic mass of an element using its isotope masses and natural abundances. This is the number you see on the periodic table for each element.
Every element on the periodic table has an atomic mass that is not a whole number. That is because most elements exist as a mixture of isotopes, each with a slightly different mass. The value on the periodic table is the weighted average of all naturally occurring isotopes.
The Formula
Average atomic mass = (mass1 x fraction1) + (mass2 x fraction2) + ...
Fractions are the decimal form of the percent abundance. So 80.1% becomes 0.801.
Boron Example
Boron has two stable isotopes:
- Boron-10: mass = 10.0129 amu, abundance = 19.9%
- Boron-11: mass = 11.0093 amu, abundance = 80.1%
Average = (10.0129 x 0.199) + (11.0093 x 0.801) = 1.9926 + 8.8184 = 10.811 amu
That matches the periodic table value for boron.
Common Mistakes
- Using percentages instead of fractions (divide by 100 first)
- Forgetting that abundances must add up to 100%
- Confusing mass number (integer) with exact isotope mass (decimal)