Atomic Mass Calculator
Estimate an isotope’s mass from its proton and neutron counts, with the mass number shown separately.
01 Set up your calculation
- Enter the whole-number proton and neutron counts for one isotope.
- Read the mass number and the approximate mass in atomic mass units.
- Use a measured isotope mass when a precise mass is needed.
02 The relationship
For the assumptions behind these results, read the explanation below. You can also explore Atom and Average Atomic Mass.
Follow the numbers
Six protons plus six neutrons give mass number 12. The estimate is 12 u, or about 1.99265 × 10⁻²⁶ kg.
03 A useful first estimate
Most of an atom’s mass is concentrated in its nucleus. Counting its nucleons gives a convenient approximate mass in atomic mass units. This is a classroom estimate, not a sum of individually measured free-particle masses.
The numerical equality between mass number and mass in u is exact for carbon-12 by definition, but it is only approximate for other isotopes. Proton and neutron masses are not each exactly one u.
04 When you need a measured mass
Binding energy changes the mass of a bound nucleus relative to its separated constituents. Electrons also contribute a small amount. This tool intentionally does not reconstruct these corrections.
Use isotope-resolved data for mass spectrometry, nuclear energy calculations, or isotope labeling. For an ordinary chemical mixture of isotopes, an average atomic weight is usually the more relevant quantity.
Questions from the notebook
Is this the periodic-table atomic weight?
No. This estimates one specified isotope. The table normally reports an average over a defined isotope distribution.
Does a valid answer mean the isotope is stable?
No. The calculator counts particles; it does not assess nuclear stability.
References & reading
- OpenStax, Rice University — Chemistry 2e: Atomic Structure and Symbolism ↗
Defines atomic number, mass number, ions and isotope-weighted atomic masses.
- NIST — Atomic Weights and Isotopic Compositions ↗
Lists isotope masses and standard atomic weights. Formula calculations use rounded conventional values, not isotope-specific exact masses.
- NIST — 2022 CODATA Fundamental Physical Constants ↗
Lists the exact Avogadro constant and the measured atomic mass constant in kilograms.
Our explanations are written for ChemistryCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.