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, μ_r, 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.