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Average Atomic Mass Calculator

Work out average atomic mass instantly with clear inputs, formula shown and shareable results.

About Average Atomic Mass Calculator

The atomic mass on the periodic table is an abundance-weighted mean of the isotopes, not any single nuclide's mass. Chlorine is 75.76% Cl-35 (34.96885 u) and 24.24% Cl-37 (36.96590 u), which averages to 35.45 u. Because the weighting is by number of atoms, the mean always sits nearer the more abundant isotope.

Formulas

Weighted mean

average = (m1 x a1 + m2 x a2) / 100

Abundance closure

a2 = 100 - a1

Worked example

With the default values below, Average Atomic Mass Calculator returns Average Atomic Mass of 35.4529. Change any input above and every figure here is recalculated in your browser.

Inputs used

Mass of the first isotopeminimum 0 u
34.96885 u
Abundance of the first isotopeaccepts 0–100 %
75.76 %
Mass of the second isotopeminimum 0 u
36.9659 u

Results

Average Atomic Mass
35.4529 u
Abundance Of The Second Isotope
24.24%
Mass Gap Between Isotopes
1.9971 u
Which Isotope Dominates
Closer to the first isotope

Step by step

  1. 1Values used
  2. 2Weighted mean
  3. 3Abundance closure
  4. 4Average Atomic Mass= 35.4529 u
  5. 5Abundance Of The Second Isotope= 24.24
  6. 6Mass Gap Between Isotopes= 1.9971 u
  7. 7Which Isotope Dominates= Closer to the first isotope

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is chlorine's mass 35.45 and not a whole number?

Because it is a mixture. No chlorine atom has a mass of 35.45 u - three quarters are 35 and one quarter is 37.

What about elements with three or more isotopes?

The same weighted mean extends to any number of terms. This two-isotope form covers the common exam cases such as chlorine, boron and copper.

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