Atomic Mass Calculator

How to use this atomic mass calculator

What this calculator does

This calculator estimates the mass number and approximate atomic mass of a single atom from the number of protons and neutrons in its nucleus. It uses the standard introductory approximation that one proton and one neutron each contribute about one unified atomic mass unit, written u. It also converts that approximate mass into kilograms using the unified atomic mass constant. The result is useful for learning nuclear composition, checking isotope notation, and moving between atomic-scale and SI mass units. It is not a high-precision nuclear-mass model: it does not subtract the mass equivalent of nuclear binding energy, and it does not include the tiny contribution from electrons.

When to use it

Use the calculator when identifying an isotope from its nuclear composition, checking a chemistry or physics exercise, comparing neutron-to-proton balance, or converting a mass number into an approximate kilogram value for an order-of-magnitude calculation. For precise isotope masses or standards work, consult evaluated atomic-mass data rather than this nucleon-count approximation.

How to calculate

1. The calculator opens with a complete carbon-12 demonstration: 6 protons and 6 neutrons. Its results and a validated example Excel workbook are available immediately.
2. Replace Number of protons (Z) with the element's atomic number. Enter a whole number from 1 through 118.
3. Replace Number of neutrons (N) with the isotope's neutron count. Enter a whole number from 0 through 177.
4. Read Approximate atomic mass, Atomic mass (SI), Mass number (A), and Neutron-to-proton ratio. The composition table shows each nucleon type's share of the nucleus.
5. Select Download Excel to create a current-state .xlsx workbook. Select Reset to clear the demonstration and all results; export then remains unavailable until both required fields contain a complete valid state again.

Input guide

Number of protons (Z) is required and accepts an integer count only. It identifies the element: for example, 6 means carbon. Increasing Z increases the mass number and approximate mass by one unit per added proton. Do not enter an element symbol, a decimal, a sign, scientific notation, or a grouped number. Values above 118 are rejected because 118 is the highest currently recognized atomic number.

Number of neutrons (N) is required and accepts an integer count from 0 to 177. For carbon-12, enter 6. Increasing N adds one to the mass number and about one u to the approximate mass. Zero is valid for hydrogen-1. Do not confuse neutron count with mass number: neutron count is obtained from A – Z when isotope notation gives A and Z.

Output guide

Approximate atomic mass is shown in unified atomic mass units and is the exact sum Z + N within this simplified model. Atomic mass (SI) multiplies that total by 1.66053906660 × 10 ^{– 27} kg/u. Mass number (A) is the whole-number nucleon count. Neutron-to-proton ratio is N/Z; a value of 1 means equal counts, while a higher value means more neutrons than protons. The proton, neutron, and nucleon summary pills repeat the current canonical counts. In the composition table, Count is the integer number of each nucleon type and Share of nucleons is that count divided by A. These outputs are identities or approximations, not recommendations.

Worked example

With the startup values Z = 6 and N = 6, the mass number is A = Z + N = 12. The approximate atomic mass is therefore 12 u. Using the unified atomic mass constant, 12 × 1.66053906660 × 10 ^{– 27} kg gives 1.99264687992 × 10 ^{– 26} kg, displayed as 1.99265 × 10 ^{– 26} kg. The neutron-to-proton ratio is 6/6 = 1.000, and protons and neutrons each make up 50.00% of the 12 nucleons.

Learn more

The NIST value for the unified atomic mass unit provides the conversion constant used here. IUPAC explains the role of atomic weights and nuclide masses in its periodic-table resources, while the U.S. Department of Energy gives an accessible overview of atomic nuclei and nucleons.