Calculate a weighted isotope average
Average atomic mass combines the relative mass of each isotope with the fraction of atoms represented by that isotope. Enter one isotope per line as mass, abundance percent. The calculator converts every percent to a fraction, multiplies it by the matching isotope mass, and adds the contributions. The result is therefore pulled toward the more abundant isotope rather than sitting automatically halfway between the lightest and heaviest masses.
The default chlorine example uses two isotope masses and abundances that total 100 percent. Its answer is close to 35.45 u, between the two isotope masses and nearer chlorine-35 because that isotope is more common. This demonstrates why the decimal on a periodic table is not a fractional neutron count: it summarizes a population of atoms containing several isotopes.
Why the abundance total is checked
A weighted average assumes the listed fractions describe the complete population. The calculator accepts a small tolerance of 0.05 percentage point for published values that have already been rounded, then normalizes that accepted total to 100 percent before calculating. It rejects a materially incomplete or excessive total. Normalizing 80 percent would conceal a missing isotope, so totals outside the narrow rounding tolerance are never repaired silently.
Each mass must be positive and each abundance must lie from zero to 100 percent. Two to twelve isotope rows are accepted. A row uses the final comma as the separator, so values such as “34.96885268, 75.78” remain easy to paste and audit. A malformed row is identified by number, allowing the source list to be corrected without guessing which value was ignored.
Follow the weighted-sum method
Convert 75.78 percent to 0.7578, then multiply by 34.96885268 u. Convert 24.22 percent to 0.2422, multiply by 36.96590260 u, and add the two contributions. The result panel prints each multiplication before the sum. A manual solution can use the same sequence, keeping enough digits until the final addition to avoid accumulating rounding error.
Use measured or supplied isotope masses when a problem asks for average atomic mass. Mass numbers such as 35 and 37 can be used for a rough demonstration, but they cannot reproduce a precise standard atomic weight. Likewise, use fractional abundance values between zero and one only after converting them to percentages for this input format. Entering 0.7578 as though it were 75.78 percent would underweight the isotope by a factor of 100.
Ways to check the average
The weighted result should fall between the smallest and largest entered isotope masses when all abundances are non-negative. It should equal one isotope mass if that isotope has 100 percent abundance. Swapping the two abundances should move the result toward the isotope that receives the larger share. These checks expose misplaced percent signs and transposed rows without requiring a reference table.
Also inspect the contribution list. The contributions should add to the reported average, and the abundance total should be approximately 100 percent. If published abundance values are intervals or vary by sample origin, one single average may not describe every material. The output reflects exactly the figures supplied; it does not claim that isotope abundance is universal or unchanged across all sources.
Data limits and privacy
Natural abundance can vary, and radioactive or synthetic elements may not have a conventional standard atomic weight. This calculator performs the weighted arithmetic but does not select isotopes, verify a specimen, propagate measurement uncertainty, or replace the latest IUPAC element data. Cite the source and date of isotope masses and abundances when the result is used in formal work.
The values are calculated in the current browser tab. NexaCurrent does not upload the quantities, formulas, chemical names, element choices, or other information entered here, and no account is needed. Copying is a deliberate action after the answer and working are visible. Refreshing or leaving the page clears the current calculation, so keep a copied result if it is needed for later study.