Average Atomic Mass Calculator
Weighted mass from isotope abundances, with a normalisation check.
Work out Average Atomic Mass. Weighted mass from isotope abundances, with a normalisation check. Free, with no account and nothing to download.
Comma separated, in atomic mass units.
Must line up with the masses above. They should total 100.
Average atomic mass (u)
35.45274
2 isotopes, abundances total 100
This is why periodic table masses are rarely close to whole numbers. Chlorine reads 35.45 not because any chlorine atom weighs that, but because natural chlorine is roughly three parts Cl-35 to one part Cl-37. The weighting is by number of atoms, not by mass, so the abundance figures go in directly without conversion.
How the Average Atomic Mass Calculator works
Enter each isotope's mass and natural abundance to get the weighted average that appears on the periodic table. If your abundances do not total 100 the page normalises them and says so, rather than quietly returning a mass that is low by whatever is missing.
Also known as: isotope abundance calculator · weighted average atomic mass · relative atomic mass calculator · atomic mass from isotopes
Frequently asked questions
How do I calculate average atomic mass?
Multiply each isotope's mass by its fractional abundance and add the results. Chlorine is 75.77% Cl-35 and 24.23% Cl-37, which averages to 35.45 u.
Why is the periodic table mass not a whole number?
Because it is an average over isotopes, weighted by how common each one is. No individual chlorine atom weighs 35.45 u — they weigh about 35 or about 37, in roughly a 3:1 ratio.
Is it weighted by mass or by number of atoms?
By number of atoms. Natural abundances are already expressed as percentages of atoms, so they go straight into the calculation without conversion.
Why do some elements have a mass in brackets?
Because they have no stable isotope and no fixed natural abundance. For those, IUPAC lists the mass number of the longest-lived isotope instead of a weighted average.
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