Using Fc=1/(2*pi*R*C) to find out the cut-off frequency of a low pass filter of say a filter with values of 10K and 500pF, you get an answer of 31,831Hz. This gives you the -3dB point I believe. When I try and prove this by working out the reactance of the capacitor at that frequency, then using the potential divider rule to find the voltage loss, my sums don't add up:

Xc=1/(2*pi*f*c)=10,000R

A potential divider with a resistance of 10K and 10K gives you half the voltage you put in to it, not the Vin*0.707 I was expecting. Any idea on where I am going wrong here?

Thanks a lot!