There are various definitions of the Péclet number. The most typical are as follows:

Pe = l * v / α

Pe = l * v * ρ * c_{p} / λ

Pe = l^{2} * ρ * c_{p} / λ / t

With:

- α - Thermal diffusivity
- λ - Thermal conductivity
- ρ - Density
- c
_{p}- Heat capacity - l - Characteristic length
- t - Characteristic time
- v - Velocity