What units should I enter for atomic mass?

Enter masses in unified atomic mass units (u), also called daltons (Da). One u = 1.66054 × 10⁻²⁷ kg. Most nuclear data tables (NUBASE, AME) list masses in u or MeV/c². Divide an MeV/c² value by 931.494 to convert to u.

Can I add more than two reactants or products?

Yes. Use the "Add reactant" and "Add product" buttons to enter as many particles as needed. The calculator sums all masses on each side before computing the difference.

What does a negative Q-value mean?

A negative Q-value means the products are heavier than the reactants. The reaction is endothermic and will not proceed spontaneously — external energy (at least |Q|) must be supplied, for example as kinetic energy of an incident particle.

Should I use nuclear masses or atomic masses?

Atomic masses are more convenient because electron masses cancel when the same number of electrons appears on both sides of the reaction (as in most nuclear reactions). If you use nuclear masses, add the electron mass (0.000548580 u) times the proton number for each nucleus manually.

How accurate is the 931.494 MeV/u conversion?

The 2018 CODATA value is 931.49410242 MeV/u. The value used here (931.494 MeV/u) gives accuracy better than 1 ppm, which is sufficient for all practical nuclear-physics calculations.

Q Value Nuclear Reaction Calculator

Name: Q Value Nuclear Reaction Calculator
Availability: InStock
Author: Nham Vu

Enter reactant and product atomic masses to calculate the Q-value (energy released or absorbed) in a nuclear reaction using E=Δmc².

Reaction Masses

Reactants

Products

Enter reactant and product masses,
then click Calculate Q-Value.

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Summary

Enter reactant and product atomic masses to calculate the Q-value (energy released or absorbed) in a nuclear reaction using E=Δmc².

How it works

Enter the atomic masses of all reactant nuclei in atomic mass units (u).
Enter the atomic masses of all product nuclei in atomic mass units (u).
The calculator sums the total reactant mass and total product mass.
It computes the mass difference: Δm = M_reactants − M_products.
Q is obtained by multiplying Δm by the conversion factor 931.494 MeV/u.
Results are shown in MeV, keV, and joules alongside an exo/endothermic label.

Use cases

Verify whether a proposed nuclear fusion or fission reaction releases energy.
Determine the threshold energy needed to drive an endothermic nuclear reaction.
Cross-check textbook Q-value problems in nuclear physics courses.
Evaluate the energy yield of alpha, beta, or gamma decay processes.
Design nuclear reaction chains in reactor physics studies.
Confirm mass balance in particle physics reaction equations.

Frequently Asked Questions

Last updated: 2026-07-24 · Reviewed by Nham Vu