Faraday's Law states that the line integral of electric field around a closed curve (= the voltage you'd measure in a loop of wire following that curve) is equal to (negative) the rate of change of flux with time. Flux = (magnetic field strength x area enclosed by curve). Negative just has to do with definition of positive voltage and magnetic North or South.

Let's say F=flux=B x A. With N turns you have F = B * A * N. If your loop is fixed and the field fluctuates, you have

V = - dF/dt = - A * N * (dB/dt)

V: volts
A: area, square meters
N: number of turns
B: magnetic field, Tesla (10000 gauss = 1 Tesla)

If B = B0 sin(wt) then dB/dt = w B0 cos(wt) where w=2*pi*f and f=50 Hz. The RMS value of field strength if you have a sine wave is Brms = B0 / 1.414 where B0 is the zero-peak value of magnetic field.

Substituting in from above,

|V| = | A N (w B0 cos(wt)) |

or (V)rms = A * N * 2 * pi * 50 Hz * (B)rms