Chemistry

Instant, private, and free

Atomic Mass Number Calculator.

Calculate mass number from proton and neutron counts.

On-device calculationNo signup
01

Set your values

Results update as you type.

Mass number: 12

Mass number

12.000000

Educational formula calculator. Keep units consistent; chemistry procedures, concentrations, and safety decisions require qualified laboratory guidance. Never mix cleaning products.

Use Cases

Determine isotope mass numbers

Quickly find the mass number of a specific isotope by entering its proton and neutron counts. Useful for chemistry homework or lab work.

Example: For an isotope with 20 protons and 22 neutrons, the mass number is 42.

Verify nuclear notation

Check the mass number for nuclear symbols or equations. Enter the known proton and neutron numbers to confirm the superscript in nuclear notation.

Example: If you have 92 protons and 146 neutrons, the mass number is 238 (as in Uranium-238).

Frequently Asked Questions

What is the atomic mass number?
The atomic mass number (A) is the total number of protons and neutrons in an atom's nucleus. It is calculated by adding the number of protons (Z) and neutrons (N). For example, carbon-12 has 6 protons and 6 neutrons, so its mass number is 12.
How do I use this calculator?
Enter the number of protons and neutrons in the respective fields. The calculator will automatically add them to give the mass number. Ensure you enter positive integers for accurate results.
Is the mass number the same as atomic mass?
No. The mass number is the total count of protons and neutrons, an integer. Atomic mass (or atomic weight) is the weighted average mass of an element's isotopes, including electrons, and is usually a decimal. This calculator provides the mass number, not atomic mass.

Tips & Common Mistakes

Tips

  • Always double-check your proton and neutron counts from a reliable source, such as the periodic table or isotope data.
  • Remember that the number of protons defines the element, while the number of neutrons can vary (isotopes).
  • For neutral atoms, the number of protons equals the number of electrons, but this calculator only needs protons and neutrons.
  • Use this tool to quickly compare mass numbers of different isotopes for the same element.

Common Mistakes to Avoid

  • Entering the atomic number (protons) as the mass number. The mass number is the sum of protons and neutrons, not just protons.
  • Using the atomic weight from the periodic table (e.g., 12.011 for carbon) as the mass number. The mass number is a whole number for a specific isotope.
  • Forgetting to include neutrons when calculating the mass number. Neutrons contribute significantly to the mass.

Last updated: August 13, 2026