The vacuum magnetic permeability is the magnetic permeability in a classical vacuum. It is a physical constant, conventionally written as μ0, approximately equal to 4π × 10−7 H/m. It quantifies the strength of the magnetic field induced by an electric current. Expressed in terms of SI base units, it has the unit kg⋅m⋅s−2⋅A−2. It can be also expressed in terms of SI derived units, N⋅A−2, H·m−1, or T·m·A−1, which are all equivalent.