Atomic Mass Calculator
This atomic mass calculator works out the average atomic mass of an element from the masses of its isotopes and how common each one is. The atomic mass printed on the periodic table is not the mass of a single atom: it is the weighted average of every naturally occurring isotope, each counted in proportion to its abundance. That is why chlorine reads 35.45 rather than a tidy whole number, even though every individual chlorine atom has a whole-number mass number. Enter the mass of each isotope in unified atomic mass units (u) alongside its percentage abundance, for up to three isotopes, and the calculator multiplies each mass by its share, adds the results and divides by the total abundance to give the average. It is built for chemistry students and teachers checking homework, working through isotope problems, or confirming a value before using it in a molar mass calculation. The tool copes with abundances that do not add up to exactly 100, dividing by whatever total you enter, though for real elements the natural abundances should sum to 100. Use it to see why atomic masses sit where they do, to compare elements with one dominant isotope against those with several, or simply to speed up a calculation you would otherwise do by hand.
Enter isotope mass in unified atomic mass units (u) and abundance as a percentage. Leave unused rows blank. Defaults are the two isotopes of chlorine.
Weighting each isotope mass by its abundance gives an average atomic mass of 35.4527 u across 2 isotopes, which rounds to 35.45 u.
For real elements the abundances should add up to 100%. If yours do not, the calculator still divides by the total you entered.
How it works
The average atomic mass is a weighted mean. Each isotope mass is multiplied by its abundance, the products are added, and the total is divided by the sum of the abundances. Written as a formula, average = (m1 x a1 + m2 x a2 + m3 x a3) / (a1 + a2 + a3), where the m values are isotope masses in u and the a values are percentage abundances. Dividing by the total abundance rather than assuming 100 means the result stays sensible even if your figures are slightly off or you are only entering a subset of isotopes.
Worked example
Chlorine has two stable isotopes: chlorine-35 with a mass of 34.96885 u at 75.77% abundance, and chlorine-37 with a mass of 36.96590 u at 24.23%. The weighted total is 34.96885 x 75.77 + 36.96590 x 24.23 = 3545.27, and dividing by the total abundance of 100 gives an average atomic mass of 35.4527 u. Rounded to two decimal places that is 35.45 u, exactly the value shown for chlorine on the periodic table. Because chlorine-35 is three times as common as chlorine-37, the average sits much closer to 35 than to 37.
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