Look at the periodic table and carbon's atomic mass reads 12.011 — not the tidy 12 you might expect. That decimal exists because natural carbon is a mixture: about 99% carbon-12 atoms and 1% carbon-13 atoms, and the listed mass is the weighted average of the isotopes. Atomic mass is computed as the sum of (isotope mass × fractional abundance) across every naturally occurring isotope of the element. Chlorine is the dramatic example: roughly 76% chlorine-35 and 24% chlorine-37 average out to about 35.45. This weighted-average idea is why atomic masses are almost never whole numbers, and it is also why the mass of a single atom (the mass number) differs from the element's tabulated atomic mass. Chemists need these values for mole conversions — grams to moles and back — which underpin every stoichiometry calculation. This calculator performs the underlying math: enter your values in the Variable A and Variable B fields and read the Result.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Atomic Mass Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.