A value that represents the force of attraction exerted by a magnet on a ferromagnetic object at a particular distance. As concerns attraction, it is the extent to which a magnetic field is inhomogeneous that determines its strength.
The force index is calculated by multiplying the local flux density in a particular direction by the amount of change of flux density per unit of length in that direction.
As a formula: FI = B (ΔB/Δx).