Use our neutron number calculator for quick and accurate calculations. Free online tool.
The neutron number calculator computes N, the number of neutrons in an atomic nucleus, using the formula N = A − Z, where A is the mass number and Z is the atomic number (number of protons). The neutron number is the key factor that distinguishes one isotope from another of the same element.
For example, iron-56 (the most abundant iron isotope) has Z = 26 protons and A = 56, giving N = 56 − 26 = 30 neutrons. Iron-58 has N = 58 − 26 = 32 neutrons. Same element, different isotopes — defined by their neutron count.
Input the mass number (A) and atomic number (Z) to find N instantly. This calculator is useful for nuclear physics, radiochemistry, and building a thorough understanding of nuclide notation and the neutron-to-proton ratio that governs nuclear stability.
Isotopes, atomic mass, mass number, neutrons, and nuclear binding energy
Explore CategoryThe neutron number (N) is the count of neutrons in an atomic nucleus: N = A − Z, where A is the mass number and Z is the atomic number.
Because two atoms of the same element (same Z) but different N are different isotopes; the number of neutrons controls the nucleus's stability and radioactive properties.
The N/Z ratio indicates nuclear stability; stable nuclei tend to have N/Z ≈ 1 for light elements and N/Z up to ~1.5 for heavy elements. Deviations lead to radioactive decay.
Yes — the protium isotope of hydrogen (¹H) has Z = 1 and A = 1, so N = 0; its nucleus is a single proton.
Very heavy elements like lead-208 (N=126, Z=82, N/Z ≈ 1.54) and bismuth-209 (N=126, Z=83) have among the highest neutron-to-proton ratios in stable nuclides.