Average Atomic Mass Calculator (from isotope masses and abundances)

Understanding the average atomic mass calculator — concept and principle

The atomic mass shown on the periodic table is a weighted average of the masses of an element's naturally occurring isotopes. This calculator computes that average from isotope masses and their abundances.

Each isotope mass counts in proportion to its abundance, so the average 35.45 u sits close to the common Cl-35.

How it is calculated

Average atomic mass = Σ (massᵢ × abundanceᵢ) ÷ Σ abundanceᵢ.
With abundances in per cent that add up to 100, this is Σ massᵢ × (abundanceᵢ ÷ 100).
Masses are in unified atomic mass units (u), where 1 u is 1/12 of the mass of a carbon-12 atom.

The principle

A sample of an element contains a mixture of isotopes. Each isotope contributes to the average in proportion to how common it is, just as a class average is weighted by the number of students with each score. That is why chlorine's atomic mass, about 35.45 u, is not a whole number: it lies between its two main isotopes, closer to the more abundant one.

Worked example

Default inputs: chlorine, ³⁵Cl 34.96885 u at 75.76% and ³⁷Cl 36.96590 u at 24.24%.

Abundances add up to 100%; 2 isotopes entered.
Contribution of ³⁵Cl = 34.96885 × 0.7576 ≈ 26.4924 u.
Contribution of ³⁷Cl = 36.96590 × 0.2424 ≈ 8.9605 u.
Average atomic mass ≈ 26.4924 + 8.9605 = 35.4529 u.
Most abundant isotope = 34.9689 u; the average is 0.4841 u higher.

How to read the result

The closer the average lies to one isotope's mass, the more that isotope dominates. Natural abundances vary slightly between sources, so IUPAC publishes some atomic weights as intervals. For precise work, use the isotopic composition of your actual sample.

Common mistakes

Using mass numbers (35, 37) instead of exact isotope masses.
Entering abundances that do not add up to 100% without noticing.
Using fractional and percentage abundances in the same list.
Mixing up the order of masses and abundances.

Keep in mind: Isotopic abundances vary in nature and in enriched materials. Use measured compositions for precise applications.

Frequently asked questions

Why is chlorine's atomic mass 35.45?

Because natural chlorine is about 76% chlorine-35 and 24% chlorine-37.

Is atomic mass the same as mass number?

No. Mass number counts protons and neutrons; atomic mass is the measured mass in u, averaged over isotopes.

Where can I find isotope masses and abundances?

In IUPAC and NIST tables of atomic weights and isotopic compositions.

Sources: NIST, Atomic Weights and Isotopic Compositions (Coursey et al.) · IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), Standard Atomic Weights

Checked 9 Nov 2026. Weighted-average atomic mass: OpenStax, Chemistry 2e (2019), §2.3 “Atomic Structure and Symbolism” (standard method). Chlorine example: isotope masses and abundances as tabulated by NIST (Atomic Weights and Isotopic Compositions) — standard values.

Your numbers