Average Atomic Mass Calculator
Enter the mass in atomic mass units and the percent abundance of each isotope, up to five. The calculator returns the abundance-weighted average atomic mass, the matching molar mass in g/mol, and the mass of one average atom. Abundances must total 100%.
Last updated: September 26, 2026
Frequently asked questions
How do I calculate average atomic mass?
Multiply each isotope's mass by its fractional abundance (percent divided by 100) and add the results. For 10 u at 20% and 11 u at 80% the average is 0.2 × 10 + 0.8 × 11 = 10.8 u.
Is there a separate Atomic Mass Calculator?
This tool covers it. Enter one isotope at 100% to get the mass of a single atom in u, kg, and g, or several isotopes for the weighted average. It uses measured isotope masses, not mass numbers.
Why must abundances total 100%?
They describe fractions of the same natural sample. If they do not sum to 100, the calculator reports the total instead of quietly rescaling the numbers.
What is the difference between mass number and atomic mass?
Mass number is a whole-number count of protons and neutrons. Atomic mass is a measured mass in u that is slightly different because of nuclear binding energy, so it cannot be found by simply adding proton and neutron masses.
Why are g/mol and u numerically equal?
By the definition of the mole and the atomic mass unit, an element with an average atomic mass of X u has a molar mass of about X g/mol.
How does this relate to the Atom Calculator?
The Atom Calculator counts protons, neutrons, and electrons from Z, A, and charge. This calculator deals with measured masses and abundances.