The Chandrasekhar number is usually denoted by the letter , and is motivated by a dimensionless form of the Navier-Stokes equation in the presence of a magnetic force in the equations of magnetohydrodynamics:
where  is the Prandtl number, and  is the magnetic Prandtl number. 
The Chandrasekhar number is thus defined as:
where  is the magnetic permeability,  is the density of the fluid,  is the kinematic viscosity, and  is the magnetic diffusivity.  and  are a characteristic magnetic field and a length scale of the system respectively.
It is related to the Hartmann number, , by the relation: