Use our atomic mass calculator for quick and accurate calculations. Free online tool.
The atomic mass calculator computes the weighted average mass of an element based on the natural abundances and masses of its isotopes. Atomic mass is expressed in unified atomic mass units (u or Da), where 1 u equals one twelfth of the mass of a ¹²C atom.
The formula is: Atomic Mass = Σ (isotope mass × fractional abundance). For chlorine, with ³⁵Cl (75.77%, 34.969 u) and ³⁷Cl (24.23%, 36.966 u), the atomic mass = (34.969 × 0.7577) + (36.966 × 0.2423) ≈ 35.45 u, matching the periodic table value.
Enter individual isotope masses and their percent abundances to calculate the weighted average atomic mass. This tool is essential for stoichiometry, molar mass calculations, and understanding why periodic table masses are non-integers.
Isotopes, atomic mass, mass number, neutrons, and nuclear binding energy
Explore CategoryAtomic mass is the weighted average mass of all naturally occurring isotopes of an element, expressed in unified atomic mass units (u), where 1 u = 1/12 the mass of a ¹²C atom.
Because most elements exist as mixtures of isotopes with different masses and natural abundances; the periodic table value is a weighted average of all stable isotopes.
Mass number (A) is the integer count of protons plus neutrons in one specific isotope, while atomic mass is the fractional weighted average across all natural isotopes.
Numerically they are equal: the molar mass in g/mol has the same numerical value as the atomic mass in u (e.g., carbon ≈ 12.011 u and 12.011 g/mol).
Only if that element is monoisotopic (100% one isotope), such as fluorine (¹⁹F) or gold (¹⁹⁷Au); otherwise you need mass and abundance data for each isotope.