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: May 2026
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About this calculator
The Q-value of a nuclear reaction is the mass-energy released (positive Q, exothermic) or absorbed (negative Q, endothermic) when reactants convert to products. It comes straight out of Einstein's E = mc² applied to the mass defect between the two sides of the equation. This calculator handles fission, fusion, alpha decay, beta decay, and generic (a,b) reactions.
The Q-value formula
Q = (Σ m_reactants − Σ m_products) × c². Use atomic mass units (amu, where 1 amu ≈ 931.494 MeV/c²) and the result comes out directly in MeV. For the classic D+T → He-4 + n fusion reaction, the mass defect is 0.01888 amu, giving Q ≈ 17.59 MeV per reaction — the fusion energy source used in ITER-class tokamaks and thermonuclear weapons.
Fission Q-values
A typical U-235 fission with a thermal neutron produces two fragments and 2-3 neutrons; total Q ≈ 200 MeV per reaction. Most of that energy (~85%) shows up as kinetic energy of the fission fragments. The calculator handles the two-fragment approximation — for the full fission-product spectrum you'd need a decay-chain table, which is beyond a single-shot Q calculation.
Fusion Q-values
D+T → He + n: Q ≈ 17.6 MeV. D+D → T + p: Q ≈ 4.0 MeV. D+D → He-3 + n: Q ≈ 3.3 MeV. D+He-3 → He + p: Q ≈ 18.4 MeV. The last one is the "aneutronic fusion" reaction pursued for cleaner-fuel concepts — it releases most of its energy as charged particles rather than neutrons.
Alpha and beta decay Q-values
Alpha decay Q = m_parent − (m_daughter + m_He4), typically 4-9 MeV for heavy nuclei. Beta-minus decay Q includes the neutrino, but for a Q-value calculation you use the atomic masses (which handle the electron count implicitly): Q = m_parent − m_daughter. Positive Q is required for a decay to happen spontaneously.
How to use
Consider the fusion reaction D + T → He-4 + n. Masses: deuterium (²H) = 2.014102 u, tritium (³H) = 3.016049 u, helium-4 = 4.002602 u, neutron = 1.008665 u. Step 1: Total reactant mass = 2.014102 + 3.016049 = 5.030151 u. Step 2: Total product mass = 4.002602 + 1.008665 = 5.011267 u. Step 3: Mass difference = 5.030151 − 5.011267 = 0.018884 u. Step 4: Q = 0.018884 × 931.494 ≈ 17.59 MeV. This confirms the D-T fusion reaction is strongly exothermic, releasing about 17.6 MeV per event.
Frequently asked questions
What does a negative Q-value mean for a nuclear reaction?
A negative Q-value means the reaction is endothermic—the products have more total mass than the reactants, so energy must be supplied for the reaction to occur. This energy comes from the kinetic energy of the bombarding particle. The minimum kinetic energy required in the lab frame (called the threshold energy) is slightly greater than |Q| due to conservation of momentum, given by E_threshold = |Q| × (1 + m_projectile/m_target). Endothermic reactions are important in accelerator-based experiments and in producing certain radioisotopes that cannot be made any other way. They cannot occur spontaneously and require a particle accelerator or high-energy neutron source.
How accurate are Q-value calculations and what limits their precision?
Q-value calculations are extremely precise when based on high-quality atomic mass data, with modern AME tables providing masses accurate to better than 1 keV/c² for most stable and near-stable nuclides. The main source of uncertainty is the atomic mass values themselves, particularly for exotic or short-lived nuclides far from the valley of stability where masses may be estimated by nuclear models rather than measured directly. Binding energy corrections for electron masses are also important when using atomic rather than nuclear masses. For well-characterized reactions like D-T fusion, Q-values are known to six or more significant figures and match experimental measurements very closely.
How is Q-value different from binding energy in nuclear physics?
Binding energy describes the energy required to completely disassemble a single nucleus into its constituent protons and neutrons, characterizing nuclear stability. Q-value, by contrast, describes the net energy released or absorbed in a specific reaction involving two or more nuclei. While both use the mass-energy equivalence principle, they answer different questions: binding energy tells you how tightly a nucleus is bound, while Q-value tells you the energy budget for a particular nuclear transformation. A reaction can have a large positive Q-value even between nuclei with moderate binding energies, as long as the products are more tightly bound than the reactants—which is the underlying principle behind both fission and fusion energy release.
How do I calculate the Q-value of a nuclear reaction?
Q = (mass of reactants − mass of products) × c². Using atomic mass units, 1 amu ≈ 931.494 MeV, so the Q-value drops out directly in MeV. Positive Q means the reaction releases energy (exothermic); negative Q means it absorbs energy (endothermic).