Average Atomic Mass Calculator — Isotope Weighted Average
Enter the mass (in atomic mass units) and natural abundance (%) for each isotope of an element to calculate the element's average (relative) atomic mass. Supports up to four isotopes. Default values show chlorine (³⁵Cl and ³⁷Cl), giving the well-known average of ≈ 35.45 u.
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Weighted average mass across all entered isotopes
- 1
Isotope 1 weighted term
35.969 × (75.77 ÷ 100) = 27.253711 - 2
Isotope 2 weighted term
37.966 × (24.23 ÷ 100) = 9.199162 - 3
Average atomic mass
27.253711 + 9.199162 = 36.4529Weighted average mass across all entered isotopes in unified atomic mass units.
How does this calculator work?
Average atomic mass = Σ (isotope mass × fractional abundance). Multiply each isotope's mass in u by its natural abundance fraction, then sum across all isotopes. For chlorine: 35.969 × 0.7577 + 37.966 × 0.2423 ≈ 35.45 u. The numerical value is the same as the molar mass in g/mol.
Formula
How this is calculated
Elements in nature occur as mixtures of isotopes — atoms with the same number of protons but different numbers of neutrons. Each isotope has a slightly different mass (measured in unified atomic mass units, u, also called daltons, Da). The average atomic mass reported on a periodic table is the abundance-weighted mean of all naturally occurring isotopes.
For chlorine: ³⁵Cl has mass 35.969 u and natural abundance 75.77%; ³⁷Cl has mass 37.966 u and abundance 24.23%. Average = 35.969 × 0.7577 + 37.966 × 0.2423 ≈ 35.45 u — matching the periodic table value.
If your entered abundances do not sum to exactly 100% (due to rounding in textbook data), the calculator normalises the result automatically and flags it with an info notice. Atomic mass units are numerically equal to g/mol, so the average atomic mass is the same as the molar mass of the element in grams per mole — useful for stoichiometry calculations.
Frequently asked questions
In modern IUPAC usage, 'atomic weight' is the preferred term for the dimensionless weighted mean of isotope masses relative to 1/12 of carbon-12. In practice, atomic weight and average atomic mass (in u) have the same numerical value. This calculator computes the value in u (= g/mol), which is what is listed on the periodic table.
By definition, 1 u equals 1/12 of the mass of a carbon-12 atom. Because Avogadro's number links atomic scale to molar scale, an atom weighing 1 u in the atomic mass scale corresponds to a molar mass of exactly 1 g/mol. So numerically the average atomic mass in u equals the molar mass in g/mol.
The NIST WebBook (webbook.nist.gov) and the Commission on Isotopic Abundances and Atomic Weights (CIAAW, ciaaw.org) publish authoritative values. The values used in the default example for chlorine come from NIST/IUPAC standard atomic weights (2021 table).
TG we-Calculate Editorial Team. (2026). Average Atomic Mass Calculator — Isotope Weighted Average [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/average-atomic-mass-calculator
TG we-Calculate Editorial Team. "Average Atomic Mass Calculator — Isotope Weighted Average." TG we-Calculate. 2026. https://we-calculate.com/calculator/average-atomic-mass-calculator.
TG we-Calculate Editorial Team, "Average Atomic Mass Calculator — Isotope Weighted Average," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/average-atomic-mass-calculator
@misc{wecalculate_average_atomic_mass_calculator, title = {Average Atomic Mass Calculator — Isotope Weighted Average}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/average-atomic-mass-calculator}}, year = {2026}, note = {TG we-Calculate} }
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