**Permeance**, in general, is the degree to which a material admits a flow of matter or energy. Permeance is usually represented by a curly capital P: P.

##
Electromagnetism

In electromagnetism, **permeance** is the inverse of reluctance. In a magnetic circuit, permeance is a measure of the quantity of magnetic flux for a number of current-turns. A magnetic circuit almost acts as though the flux is conducted, therefore permeance is larger for large cross-sections of a material and smaller for smaller cross section lengths. This concept is analogous to electrical conductance in the electric circuit.

Magnetic permeance P is defined as the reciprocal of magnetic reluctance R (in analogy with the reciprocity between electric conductance and resistance):
${\mathcal {P))={\frac {1}{\mathcal {R))))$

which can also be re-written:
${\mathcal {P))={\frac {\Phi _{\mathrm {B} )){NI))$

using Hopkinson's law (magnetic circuit analogue of Ohm's law for electric circuits) and the definition of magnetomotive force (magnetic analogue of electromotive force):
${\mathcal {F))=\Phi _{\mathrm {B} }{\mathcal {R))=NI$

where:

Alternatively in terms of magnetic permeability (analogous to electric conductivity):
${\mathcal {P))={\frac {\mu A}{\ell ))$

where:

- μ, permeability of material,
- A, cross-sectional area,
- ℓ, magnetic path length.

The SI unit of magnetic permeance is the henry (H), equivalently, webers per ampere.^{[a]}

##
Materials science

In materials science, **permeance** is the degree to which a material transmits another substance.