Permeability (μ), also called magnetic permeability, is the ratio of the magnetic flux density, B, in a substance to the external magnetic field strength, H, i.e.:


   μ = B / H.


The permeability of free space, μ0 is also called the magnetic constant and has the value 4π × 10-7 H m-1 in SI units. The relative permeability of a substance, μr, is given by μ / μ0, and is therefore dimensionless.


Materials showing diamagnetism and paramagnetism have permeabilities just below free space value; materials showing ferromagnetism have permabilities a thousand times greater. A material with a particularly high permeability is permalloy.