The electric flux Φ_e of a uniform electric field E through a loop of area A is defined as

 Φ_e = E A cosθ where θ is the angle between the field lines and the normal vector, n, to the plane of the loop.

If E is uniform that means it is not a function of space (x, y, or z), the calculation of flux Φ_E = E A cosθ is easy.  If E is not uniform, such as the electric field around a point charge, then since E varies with x, y, or z, differentials and integrals must be used.

The above figure  shows the flux of a uniform field E through a ring of area A.

Φ_E means electric flux