Mass defect (Δm) is the difference between the total mass of the free protons and neutrons that make up a nucleus and the actual measured mass of that nucleus. The "missing" mass is converted to the binding energy that holds the nucleus together.

Should I enter the nuclear mass or the atomic mass?

Enter the nuclear mass (nucleus only, without electron masses) for the most accurate result. If you only have the atomic mass, subtract Z × 0.000549 u (electron mass) to convert it to nuclear mass before entering.

What are the proton and neutron masses used?

This calculator uses mp = 1.007276 u (proton rest mass) and mn = 1.008665 u (neutron rest mass), which are the standard CODATA values in atomic mass units.

Why is the mass defect always positive?

Bound nuclei are lower-energy states than free nucleons. The nucleus always has less mass than the sum of its parts, so Δm is always positive (for a stable nucleus). A negative result usually indicates an incorrect mass value was entered.

What is 931.494 MeV/u?

This is the energy equivalent of one atomic mass unit: 1 u × c² = 931.494 MeV. It comes directly from E = mc² with 1 u = 1.66054 × 10⁻²⁷ kg and c = 2.998 × 10⁸ m/s.

Mass Defect Calculator

Name: Mass Defect Calculator
Availability: InStock
Author: Nham Vu

Enter proton count, neutron count, and nuclear mass to get the mass defect in atomic mass units and the equivalent binding energy in MeV.

Nucleus Parameters

Z — Number of Protons

N — Number of Neutrons

M — Nuclear Mass (u)

Enter nuclear mass in atomic mass units (u). For atomic mass, subtract Z × 0.000549 u.

Quick Examples

Enter Z, N, and nuclear mass, then click Calculate.

Summary

Enter proton count, neutron count, and nuclear mass to get the mass defect in atomic mass units and the equivalent binding energy in MeV.

How it works

Enter Z (number of protons) and N (number of neutrons) for the nucleus.
Enter M, the measured nuclear (or atomic) mass in atomic mass units (u).
The calculator sums Z × 1.007276 u (proton mass) and N × 1.008665 u (neutron mass).
Mass defect Δm = (Z × mp + N × mn) − M is computed.
Binding energy E = Δm × 931.494 MeV/u converts the defect to energy.
Results show Δm to 6 decimal places and E to 3 decimal places.

Use cases

Verify nuclear physics homework involving mass defect and binding energy.
Compare binding energies across isotopes to assess nuclear stability.
Understand why fusion of light nuclei and fission of heavy nuclei release energy.
Cross-check published nuclear data tables against calculated values.
Teach or learn the connection between special relativity and nuclear structure.
Explore why iron-56 has the highest binding energy per nucleon.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu