In magnetohydrodynamics, the magnetic Reynolds number (R_m) is a dimensionless quantity that estimates the relative effects of advection or induction of a magnetic field by the motion of a conducting medium to the magnetic diffusion. It is the magnetic analogue of the Reynolds number in fluid mechanics and is typically defined by:

${\displaystyle \mathrm {R} _{\mathrm {m} }={\frac {UL}{\eta }}~~\sim {\frac {\mathrm {induction} }{\mathrm {diffusion} }}}$

where

• ${\displaystyle U}$ is a typical velocity scale of the flow,
• ${\displaystyle L}$ is a typical length scale of the flow,
• ${\displaystyle \eta }$ is the magnetic diffusivity.

The mechanism by which the motion of a conducting fluid generates a magnetic field is the subject of dynamo theory. When the magnetic Reynolds number is very large, however, diffusion and the dynamo are less of a concern, and in this case focus instead often rests on the influence of the magnetic field on the flow.