# Which area is considered while calculating the magnetic flux?

We have a situation in which we deliberately let the magnetic field lines pass through only half of the area of the loop. So, while calculating magnetic flux do we only use that area from where field lines actually pass or just the total area of the loop?

At this instant, is the area to be used 160 cm sq or 80 cm sq?

See the definition of flux is $$d(\phi) = B . d(S)$$
So wherever $$B = 0$$ $$\implies d(\phi) = 0$$