In nuclear physics and chemistry, the **Q value** for a reaction is the amount of energy absorbed or released during the nuclear reaction. The value relates to the enthalpy of a chemical reaction or the energy of radioactive decay products. It can be determined from the masses of reactants and products. Q values affect reaction rates. In general, the larger the positive Q value for the reaction, the faster the reaction proceeds, and the more likely the reaction is to "favor" the products.

where the masses are in atomic mass units. Also both and are the sums of the reactant and product masses respectively.

The conservation of energy, between the initial and final energy of a nuclear process enables the general definition of Q based on the mass–energy equivalence. For any radioactive particle decay, the kinetic energy difference will be given by:

where K denotes the kinetic energy of the mass m .
A reaction with a positive Q value is exothermic, i.e. has a net release of energy, since the kinetic energy of the final state is greater than the kinetic energy of the initial state.
A reaction with a negative Q value is endothermic, i.e. requires a net energy input, since the kinetic energy of the final state is less than the kinetic energy of the initial state.^{[1]} Observe that a chemical reaction is exothermic when it has a *negative* enthalpy of reaction, in contrast a positive Q value in a nuclear reaction.

The Q value can also be expressed in terms of the Mass excess of the nuclear species as:

- Proof
- The mass of a nucleus can be written as where is the mass number (sum of number of protons and neutrons) and MeV/c. Note that the count of nucleons is conserved in a nuclear reaction. Hence, and .

Chemical Q values are measurement in calorimetry. Exothermic chemical reactions tend to be more spontaneous and can emit light or heat, resulting in runaway feedback(i.e. explosions).

Q values are also featured in particle physics. For example, **Sargent's rule** states that weak reaction rates are proportional to Q^{5}. The Q value is the kinetic energy released in the decay at rest. For neutron decay, some mass disappears as neutrons convert to a proton, electron and antineutrino:^{[2]}

where *m*_{n} is the mass of the neutron, m_{p} is the mass of the proton, m_{ν} is the mass of the electron antineutrino, and m_{e} is the mass of the electron; and the K are the corresponding kinetic energies. The neutron has no initial kinetic energy since it is at rest. In beta decay, a typical Q is around 1 MeV.

The decay energy is divided among the products in a continuous distribution for more than two products. Measuring this spectrum allows one to find the mass of a product. Experiments are studying emission spectrums to search for neutrinoless decay and neutrino mass; this is the principle of the ongoing KATRIN experiment.