What Is A Neutron’s Mass? | Numbers You Can Plug In

Guide / By

A neutron has a rest mass of 1.67492750056×10−27 kg, which equals 939.56542194 MeV/c2.

You’ll see “neutron mass” in textbooks, lab sheets, and problem sets, yet the number can feel slippery because it shows up in several unit systems. Let’s pin it down, then make it usable. By the time you reach the end, you’ll know which value to use, how it’s written in real references, and what changes (and what doesn’t) when the neutron is moving or bound inside a nucleus.

What Is A Neutron’s Mass? In Real Numbers

When physicists say “the neutron’s mass,” they mean its rest mass: the mass of a free neutron measured in its own rest frame. The latest CODATA recommended value expresses that rest mass as:

  • mn = 1.67492750056×10−27 kg
  • mnc2 = 939.56542194 MeV (same fact in energy units)
  • mn = 1.00866491606 u (atomic mass unit form used in chemistry-style tables)

Those lines are all the same statement, just written in the unit system that fits the task. The kilogram version is handy for mechanics and SI-based formulas. The MeV/c2 version is handy for particle and nuclear work. The u version is handy when you’re adding up nuclear masses or using mass defects.

What “Mass” Means In Particle Physics

In everyday life, mass feels like “how much stuff.” For a neutron, mass is better thought of as a property that sets its rest energy and how it responds to forces. In modern physics language, the rest mass is an invariant: every observer agrees on it, even if they disagree on the neutron’s speed.

Rest Mass Versus “Relativistic Mass”

You might still run into the term “relativistic mass,” which grows with speed. Many courses skip that term because it can blur the clean separation between mass (invariant) and energy/momentum (which change with motion). A safer way to say it is:

  • The neutron’s rest mass stays the same.
  • Its total energy and momentum rise as it speeds up.

So if a neutron is flying through a beamline at high speed, you still use the same mn in formulas like E2 = (pc)2 + (mnc2)2. What changes is p and E, not the rest mass term.

Mass And Weight Aren’t The Same

Mass is intrinsic. Weight is a force from gravity. A neutron in deep space still has the same rest mass, even where “weight” is close to zero.

Why You See MeV/c2 For Neutron Mass

Particle and nuclear physics often treat energy as the common currency. Using Einstein’s relation, rest energy equals mass times c2. If you divide both sides by c2, energy units can stand in for mass units. That’s why you’ll see “MeV/c2” as a mass unit, and why many references drop the “/c2” in casual writing when c = 1 is used in natural units.

Conversion Steps Without Hand-Waving

If you have a mass in kilograms and want the energy equivalent, multiply by c2 to get joules, then convert joules to electronvolts. If you have the energy equivalent in MeV and want kilograms, go the other way. In practice, you rarely need to do the full chain because CODATA tables already list the neutron’s mass energy equivalent directly.

How Neutron Mass Is Measured

A free neutron is neutral, which makes direct “trap-and-weigh” methods trickier than for charged particles. The best values come from combining high-precision measurements and consistency checks across related constants. Here are the main ideas behind the number you quote in class or in a lab report.

Atomic Mass Routes

One route is to measure atomic masses with Penning traps and related techniques, then infer nuclear masses. Since atoms include electrons and binding energies, careful bookkeeping is needed. This route is one reason you’ll often see neutron mass expressed in atomic mass units: it fits naturally into atomic-mass tables.

Reaction And Decay Energy Balances

Another route uses reactions where neutrons appear in the initial or final state. If you measure the energies of reaction products with tight precision, the mass differences fall out from energy conservation. Measurements of the neutron–proton mass difference are a classic case.

Least-Squares Adjustments In CODATA

CODATA recommended values are not a single measurement. They are a self-consistent set built by fitting many inputs together, checking correlations and uncertainties. That’s why a NIST CODATA entry gives both a value and a standard uncertainty in one tidy line.

For the exact values used in most coursework and reference tables, see the NIST pages for the 2022 CODATA neutron mass and the neutron mass energy equivalent in MeV. These are the two numbers you’ll see echoed across modern references.

Neutron Mass In Context With Nearby Particles

A raw number sticks better when it has neighbors. The neutron is slightly heavier than the proton, and vastly heavier than the electron. That small neutron–proton gap is tied to why free neutrons decay, while protons in ordinary matter are stable on observable time scales.

Still, “slightly heavier” can mislead if you don’t attach a scale. The neutron–proton difference is a tiny fraction of a nucleon’s mass, yet it drives real nuclear outcomes, from beta decay to which isotopes can exist.

Reference Values And Handy Comparisons

Quantity Value Notes
Neutron rest mass (SI) 1.67492750056×10−27 kg Use in SI mechanics and gravitation problems.
Neutron rest mass (u) 1.00866491606 u Handy for mass defects and nuclear mass sums.
Energy equivalent 939.56542194 MeV Same fact as mnc2.
Mass in grams 1.67492750056×10−24 g Just the kg value shifted by 103.
Compared with proton Neutron is heavier The gap is small but matters for beta decay.
Compared with electron Neutron is far heavier Electrons are lighter by about three orders of magnitude in MeV/c2.
Use in E–p relation E2 = (pc)2 + (mnc2)2 Pick MeV units and keep c consistent.
Free-neutron lifetime link Mass enables decay channel Decay depends on mass differences and weak interaction.

What Changes Inside A Nucleus

Here’s a common point of confusion: “Does a neutron weigh less inside a nucleus?” The rest mass of a neutron as a particle is the same property. What changes is the mass of the whole nucleus compared with the sum of its parts.

Binding Energy And Mass Defect

Nuclei have binding energy. When nucleons bind, the nucleus ends up with less total mass-energy than the separate nucleons would have. That missing mass shows up as released energy when the nucleus forms. In reverse, you must supply that energy to split the nucleus apart.

So if you add up Z protons and N neutrons using their rest masses and compare that sum to the measured nuclear mass, you’ll get a difference tied to binding energy. The neutron hasn’t “lost mass”; the bound system has a lower total mass-energy because energy left the system during binding.

Effective Mass In Models

In some condensed-matter topics, you’ll hear about “effective mass.” That is a model parameter for how electrons act inside a crystal, not a statement about a neutron’s intrinsic rest mass. In nuclear models, you might also see effective masses used to fit data. Those are modeling tools, not replacements for the CODATA neutron mass.

When You Need Which Neutron Mass Value

Most problems fall into a few patterns. If you pick the unit system that matches the rest of the equation, you’ll avoid a pile of conversion mistakes.

Use Kilograms For Classical-Style Problems

Use 1.67492750056×10−27 kg when you’re plugging into Newton’s laws, gravitational formulas, or any SI-only setup. Typical examples include neutron beam momentum at low speed, thermal motion calculations that already use joules and kelvin, or density estimates in simplified neutron-star exercises where the rest of the setup is already SI.

Use MeV/c2 For Nuclear And Particle Problems

Use 939.56542194 MeV/c2 when energies are in MeV and you’re working with reactions, Q-values, or relativistic energy–momentum relations. If you’re using natural units (c = 1), keep track of what your course expects: many solutions write the neutron mass as 939.565 MeV and treat it as “mass in energy units.”

Use Atomic Mass Units For Mass Defects

Use 1.00866491606 u when you’re adding atomic or nuclear masses in a table that already uses u. This shows up in isotope mass comparisons and binding-energy calculations where u-to-MeV conversions are built into the workflow.

Common Calculations That Use Neutron Mass

Task Plug in Small tip
Thermal speed estimate mn in kg Keep energy in joules when using kB.
De Broglie wavelength mn and momentum p Pick one unit system for p and stick with it.
Relativistic beam energy mnc2 in MeV Use E2 − (pc)2 = (mnc2)2.
Q-value of a reaction Masses in u or MeV/c2 Convert once, at the start, not mid-stream.
Binding energy per nucleon Mass defect in u Multiply by the u→MeV factor your table uses.

Precision, Uncertainty, And Rounding

In real work, you rarely need every digit shown in CODATA tables. Your calculator, your measurement error, or your model will usually dominate well before the last few digits matter. Still, it’s smart to round in a way that matches the precision of the rest of the problem.

How Many Digits Should You Keep?

If a homework problem gives energies to three decimal places in MeV, writing the neutron mass as 939.565 MeV/c2 fits that style. If you’re doing a unit conversion demo, keep more digits so rounding doesn’t swamp the conversion. If you’re writing a lab report with uncertainties, match your reported digits to the uncertainty you can defend.

A Note About Updates

CODATA values get refreshed when new measurements and theory inputs shift the least-squares adjustment. The changes from one release to the next tend to be tiny for the neutron mass, but they can still matter in precision metrology. When a task is high-precision, cite the specific CODATA release you used and copy the value from an official constants page.

Checklist For Students

  • If your equation is SI-only, use the neutron mass in kilograms.
  • If your equation uses MeV, use the neutron mass energy equivalent in MeV/c2.
  • If you’re adding isotope masses from a u table, use the neutron mass in u and keep conversions consistent.
  • When in doubt, write the unit next to every number on your scratch work; most mistakes show up right away.

References & Sources