There are five equations available when determining these five variables:

- Continuity equation (preservation of mass).

- 3 momentum / motion equations (preservation of momentum).

- Energy equation (preservation of energy, 1. theorem of thermodynamics).

For the isotropic Newton fluids that are regarded here, the 5 balance equations contain the following material variables that generally depend on temperature and pressure:

- Isobaric specific heat capacity: | c_{p.} |

- Viscosity: | |

- Heat conductivity: | |

- Density: | |