Magnetic reluctance is the resistance of a material to a magnetic field. It is defined as the ratio of magnetomotive force (SI unit: ampere-turns) to magnetic flux (SI unit: weber).

The reluctance R of a uniform magnetic circuit can be calculated as:

where

R is the reluctance, measured in ampere-turns per weber. This unit is equivalent to the reciprocal henry (H ^{- 1}) multiplied by the number of turns

l is the length of the circuit, measured in metres

μ is the magnetic permeability of the material, measured in henry per metre (H.m ^{- 1})

A is the cross-sectional area of the circuit, measured in square metres

Applications

The variation in the reluctance of a magnetic circuit, in which part of the circuit is moving, is used in measuring instruments called reluctance sensors.

Increasing the reluctance of a magnetic circuit enables it to store more energy. This effect is used in the flyback transformer.