Isotope masses and their abundances, weighted into one average atomic mass — the number behind every periodic-table weight.
average atomic mass (amu)
How to use this tool
The exact mass of that isotope, in atomic mass units.
The percentage of natural atoms that are this isotope.
Leave the third row at zero to ignore it.
Abundances must add to 100% — the calculator refuses anything else.
Formula & worked example
Chlorine: 34.96885 × 75.76% + 36.96590 × 24.24% = 35.4529 amu — which is why the periodic table lists chlorine as 35.45.
Limitations — read this before you rely on it
This is the average over natural isotopic abundances; a single atom of one isotope has that isotope’s exact mass, not this average. Abundances are natural values — enriched or synthetic samples differ.
Frequently asked questions
The abundance-weighted average mass of an element’s atoms as found in nature — the figure printed on the periodic table.
Because natural chlorine is ~76% chlorine-35 and ~24% chlorine-37; the weighted average lands between them.
You have missed an isotope or mistyped a figure. The calculator refuses totals outside 99.5–100.5%.
For an element, yes numerically: the average atomic mass in amu equals the molar mass in g/mol.
Isotopes of one element have the same protons but different neutron counts, so their masses differ while their chemistry stays the same.