Use our mass number calculator for quick and accurate calculations. Free online tool.
The mass number calculator determines A, the total count of nucleons (protons + neutrons) in an atomic nucleus using the simple formula A = Z + N. Unlike atomic mass, the mass number is always a whole integer and uniquely identifies a specific isotope when paired with the element symbol.
For example, uranium-235 has Z = 92 protons and N = 143 neutrons, so A = 92 + 143 = 235. It is written as ²³⁵U or U-235. Uranium-238, the more abundant isotope, has A = 238 (92 protons + 146 neutrons).
Enter the number of protons (Z) and neutrons (N) to calculate the mass number, or enter Z and A to find N. Useful for nuclear physics homework, identifying isotopes, writing nuclear symbols, and balancing nuclear equations.
Isotopes, atomic mass, mass number, neutrons, and nuclear binding energy
Explore CategoryThe mass number (A) is the total number of protons (Z) and neutrons (N) in an atomic nucleus: A = Z + N. It is always a positive integer.
Mass number is an integer count of nucleons in a specific isotope, while atomic mass is the fractional weighted average of all natural isotopes of an element in unified atomic mass units.
Yes — atoms with the same mass number but different atomic numbers are called isobars. For example, ¹⁴C (Z=6) and ¹⁴N (Z=7) both have A = 14.
It depends on the decay type: alpha decay decreases A by 4; beta decay keeps A unchanged; gamma decay also keeps A unchanged.
Oganesson (Og, Z=118) has the highest mass number of confirmed elements, with its most stable isotope being ²⁹⁴Og (A = 294).