Magnetic flux (denoted as Φ), is the amount of magnetic field (also called "magnetic flux density") passing through a surface (such as a conducting coil). The SI unit of magnetic flux is the weber (Wb)
The magnetic flux Φ= is defined as:
Φ=BAcosθ
or
Φ=BAsinθ
Where A is an area which is perpendicular to the field lines.
Around the bar magnet, the flux is greatest around the poles. Similarly the greatest flux in a solenoid is found through the inner core.
For multiple loops of wire like in a solenoid, we use the quantity N φ. Nφ is defined as the flux linkage between each of the coils.