Magnetic flux passing through a surface of vector area S is

where B is the magnitude of the magnetic field (the magnetic flux density) having the unit of Wb/m² (tesla), S is the area of the surface, and θ is the angle between the magnetic field lines and the normal (perpendicular) to S.

The magnetic flux through some surface, in this simplified picture, is proportional to the number of field lines passing through that surface (in some contexts, the flux may be defined to be precisely the number of field lines passing through that surface; although technically misleading, this distinction is not important). Note that the magnetic flux is the net number of field lines passing through that surface; that is, the number passing through in one direction minus the number passing through in the other direction. Thus magnetic flux may have positive or negative signs.