Estimate atomic mass from proton and neutron counts, convert u to kg, and understand why mass number is not measured isotope mass.
Method, examples and limits
Enter proton count Z and neutron count N. The mass number is A = Z + N. This tool estimates one atom’s mass as A u and converts that estimate to kg using the CODATA 2022 atomic mass constant, approximately 1.66053906892 × 10⁻²⁷ kg/u.
Six protons and six neutrons give A = 12 and an estimate of 12 u. Carbon-12 defines the unified atomic mass unit; for other isotopes, A u is generally an approximation.
The tool does not calculate binding energy or retrieve measured isotope masses. For isotope mixtures, use the average atomic mass tool instead. A decimal output in kg does not make the underlying estimate more precise.
Worked learning guide
Mass Number vs Atomic Mass: Why Carbon-12 Is a Special Case
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Frequently asked questions
Is A u a measured isotope mass?
No. This tool uses a labelled classroom approximation; precise isotope masses require evaluated data.
How is mass converted from u to kg?
Multiply the value in u by approximately 1.66053906892 × 10⁻²⁷ kg/u.