Calculate the weighted average atomic mass from two isotopes and their percent abundances.
| Average atomic mass | – |
Use this calculator when a problem gives two isotope masses and how common each is. That weighted mean is the value shown on the periodic table for a two-isotope element.
Average = (m₁·p₁ + m₂·p₂) / (p₁ + p₂). Abundances are percents, not fractions (19.9, not 0.199). If they sum to 100 the formula matches the usual textbook line; if they do not, the calculator still weights by the abundances you entered rather than forcing a divisor of 100.
Isotope 1 mass: Mass of the first isotope, in u, for example 10.013 for boron-10.
Isotope 1 abundance (%): Abundance of that isotope as a percent.
Isotope 2 mass: Mass of the second isotope, in u, for example 11.009 for boron-11.
Isotope 2 abundance (%): Abundance of the second isotope as a percent.
Average atomic mass: Abundance-weighted mean mass in u.
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