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In electromagnetism, permeability is the degree of magnetisation of a material in response to a magnetic field. Absolute permeability is represented by the symbol μ. In SI units, permeability is measured in henrys per metre.

where

μ is the permeability, measured in henrys per metre

B is the magnetic flux density (also called the magnetic induction) in the material, measured in teslas

H is the magnetic field strength, measured in amperes per metre

Absolute permeability

Absolute permeability is represented by the symbol μ₀ and is the permeability of the vacuum, where μ₀ = 4π × 10⁻⁷ N A⁻² (exactly).

Together with permittivity, permeability defines the speed of light.

Relative permeability

Relative permeability, sometimes denoted by the symbol μ_r, is the ratio of the permeability of a specific medium to the permeability of free space μ₀:

Relative permeability for some materials
Medium
Hydrogen	0.008 × 10-6
Copper	−6.4 × 10-6
Water	−8.0 × 10-6
Aluminium	22.2 × 10-6
Platinum	265 × 10-6

SI magnetism units

SI electromagnetism units
Name	Symbol	Dimensions	Quantity
ampere (SI base unit)	A	C / s	Current
coulomb	C	A · s	Electric charge, Quantity of electricity
volt	V	J / C = kg m2 / (s2 C)	Potential difference
ohm	Ω	V / A = kg m2 / (C2)	Resistance, Impedance, Reactance
ohm &middot metre	Ω &middot m	kg m3 / (C2)	Resistivity
farad	F	C / V = C2 s / (kg m2)	Capacitance
farad per metre	F / m	C2 s / (kg m3)	Permittivity
reciprocal farad	F−1	kg m2 / (C2 s)	Elastance
siemens	S	Ω−1 = C2 / (kg m2)	Conductance, Admittance, Susceptance
siemens per metre	S / m	C2 / (kg m3)	Conductivity
weber	Wb	V &middot s = kg m2 / (C s)	Magnetic flux
tesla	T	Wb / m2 = kg / (C s)	Magnetic flux density
ampere / metre	A / m	C / (s m)	magnetic induction
ampere-turns / weber	A / Wb	C2 / (kg m2)	Reluctance
henry	H	V &middot s / A = kg m2 s / (C2)	Inductance
henry per metre	H / m	kg m s / (C2)	Permeability
(dimensionless)	-	-	Magnetic susceptibility