The six-factor formula is used in nuclear engineering to determine the multiplication of a nuclear chain reaction in a non-infinite medium.
The symbols are defined as:
*,  and  are the average number of neutrons produced per fission in the medium (2.43 for uranium-235).
* and  are the microscopic fission and absorption cross sections for fuel, respectively.
* and  are the macroscopic absorption cross sections in fuel and in total, respectively.
* is the number density of atoms of a specific nuclide.
* is the resonance integral for absorption of a specific nuclide.
* is the average lethargy gain per scattering event.
**Lethargy is defined as decrease in neutron energy.
* (fast utilization) is the probability that a fast neutron is absorbed in fuel.
* is the probability that a fast neutron absorption in fuel causes fission.
* is the probability that a thermal neutron absorption in fuel causes fission.
* is the geometric buckling.
* is the diffusion length of thermal neutrons.
* is the age to thermal.
** is the evaluation of  where  is the energy of the neutron at birth.