Nuclear Q-Value Calculator
Calculates the energy released or absorbed in a nuclear reaction from the mass difference between reactants and products in atomic mass units. Used in nuclear physics to determine whether a reaction is exothermic or endothermic.
Last updated: October 2026
Q-Value
17.59 MeV
Formula below · 3 sources (physics.nist.gov, NIST, ph.qmul.ac.uk) · Updated Oct 2026
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About this calculator
The Q-value of a nuclear reaction is the mass that disappears multiplied by c²: Q = (total reactant mass − total product mass) × 931.494 MeV per atomic mass unit. For deuterium–tritium fusion, 5.030151 u of reactants become 5.011268 u of helium-4 and a neutron, so Q ≈ 17.59 MeV. A positive Q means the reaction releases energy; a negative Q means energy must be supplied.
The conversion factor
One atomic mass unit is equivalent to 931.494 MeV; the CODATA 2018 value is 931.49410242 MeV. Enter masses in u and the result is in MeV directly.
Use atomic masses consistently
Mass tables, such as NIST's Atomic Weights and Isotopic Compositions (from the Atomic Mass Evaluation), list neutral-atom masses, which include the electrons. For reactions where the electron count balances on both sides, including fusion, alpha decay and beta-minus decay, atomic masses can be used as they are. For positron emission, two extra electron masses (about 1.022 MeV) must be subtracted.
Familiar values
D + T → ⁴He + n releases 17.6 MeV, shared as 3.5 MeV for the alpha particle and 14.1 MeV for the neutron. D + D gives either T + p (4.03 MeV) or ³He + n (3.27 MeV), with the two branches about equally likely, and D + ³He → ⁴He + p releases 18.3 MeV. See the table below.
| Reaction | Q-value | Energy shared as |
|---|---|---|
| D + T → ⁴He + n | 17.6 MeV | ⁴He 3.5 MeV, neutron 14.1 MeV |
| D + D → T + p | 4.03 MeV | About half of D–D reactions |
| D + D → ³He + n | 3.27 MeV | About half of D–D reactions |
| D + ³He → ⁴He + p | 18.3 MeV | Charged particles only |
How to use
Enter the total mass of the reactants and the total mass of the products in atomic mass units. The default is D + T → ⁴He + n: reactants 2.014102 + 3.016049 = 5.030151 u and products 4.002602 + 1.008665 = 5.011268 u, giving about 17.59 MeV. A negative result means the reaction is endothermic and needs at least that much energy supplied.
Frequently asked questions
What does a negative Q-value mean?
The products are heavier than the reactants, so the reaction absorbs energy and cannot happen spontaneously. In the lab frame the projectile needs somewhat more kinetic energy than |Q| because momentum must also be conserved; the threshold is about |Q| × (1 + m_projectile ÷ m_target).
Should I use atomic or nuclear masses?
Use neutral-atom masses from a standard table such as NIST's, which come from the Atomic Mass Evaluation. When the number of electrons balances on both sides, the electron masses cancel. Positron-emission decays are the main exception and need a correction of two electron masses.
How is Q-value different from binding energy?
Binding energy is the energy needed to split one nucleus into its separate protons and neutrons. Q-value is the net energy released or absorbed by a particular reaction. A reaction releases energy when its products are more tightly bound than its reactants.
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Sources & references
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