About this tool
The Average Atomic Mass Calculator is a free, in-browser chemistry tool that finds the weighted-average atomic mass of an element from the mass and natural abundance of each of its isotopes. Enter two to four isotopes, each with its exact mass in unified atomic mass units (u) and its percent abundance, and the calculator returns the element's standard atomic weight along with each isotope's contribution.
The average is a weighted mean: average = Σ(massᵢ × abundanceᵢ ⁄ 100), summed over every isotope, with abundances expressed as percentages that should total 100%. In "solve missing abundance" mode you leave one abundance blank and it is found from the constraint that all abundances add to 100% (missing = 100 − Σ of the others); for a two-isotope element you can instead leave both abundances blank and supply the target average, and the tool solves the pair from x₁ = 100·(avg − m₂) ⁄ (m₁ − m₂).
Everything runs locally in your browser with plain JavaScript — nothing you type is uploaded or stored. A worked check: chlorine's isotopes 34.969 u (75.77%) and 36.966 u (24.23%) give 34.969 × 0.7577 + 36.966 × 0.2423 ≈ 35.45 u, the familiar atomic weight of chlorine.