Atomic Mass Calculator

By Steven Wooding
Last updated: Jun 18, 2020

This atomic mass calculator shows you how to find the atomic mass of an atom using the atomic mass formula, and explains the atomic mass definition. It also describes the atomic mass unit, and explores why an atom needs neutrons.

Atomic mass definition and atomic mass formula

The atomic mass is the mass of an atom, including its protons, neutrons, and electrons. However, because an electron is 1,836 times less massive than a proton, electrons account for an insignificant amount of mass of the atom in most atoms.

While you can state the atomic mass in kilograms, it is more usual to express the atomic mass in atomic mass units (u), which are also called daltons (Da). One atomic mass unit (one dalton) is equal to 1/12 the mass of a carbon-12 atom. That makes one atomic mass unit equal to 1.66 * 10-27 kg. How to calculate atomic mass? To find a reasonable approximation of the atomic mass of an atom in atomic mass units, we can simply add the number of protons and neutrons. So the atomic mass formula is:

atomic mass (u) ≈ number of protons + number of neutrons

This formula is very similar to the mass number formula:

A = Z + N


  • A is the mass number (total number of protons and neutrons);
  • Z is the number of protons in the atom; and
  • N is the number of neutrons in the atom.

Note, that the mass of an atom will be slightly less than its parts due to binding energy mass loss, which you can calculate using E = mc². This mass loss is a tiny amount for most atoms, and this calculator doesn't take this effect into account.

How to calculate atomic mass using this calculator?

If you're asking yourself how to find atomic mass, then our atomic mass calculator is a perfect tool for you. You only need to follow the below steps:

  1. Enter the number of protons in the atom. The calculator will warn you if you enter a value larger than 118, as this is the highest number of protons so far observed in an atom.
  2. Enter the number of neutrons in the atom. The calculator will warn you if you enter a value larger than 177, as this is the highest number of neutrons observed in an atom so far.
  3. The calculator then displays the atomic mass in the default unit of atomic mass units. You can change the units by clicking on the unit.
  4. It also displays the atomic mass in kilograms, in case you need to use the atomic mass in some equation.
  5. Finally, it shows the mass number for the atom.

Why do atoms need neutrons?

You might be thinking that as the neutron has no electric charge, why does the atom need them? Overall, an atom has a neutral charge. However, all of the positive charge is concentrated in the center with the protons. Like charges repel, so there is a force trying to break the nucleus apart. Countering this force is the attractive strong nuclear force that exists between protons and neutrons. Therefore, an atom can stop itself from breaking up by adding neutrons to increase the total amount of attractive strong nuclear force.

Some atoms are unstable, and might only last for a limited time before ejecting some components and turning into something else. You can learn more about the decay of radioactive atoms in our half-life calculator.

Steven Wooding
Number of protons (Z)
Number of neutrons (N)
Atomic mass
Atomic mass (SI)
Mass number
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