Bandwidth of lowpass filter?
If you consider an ideal low-pass filter with cut-off frequency of $f_c$, all frequencies greater than $f_c$ will be removed. Then it's bandwidth is equal to $f_c\;\mathrm{Hz}$ (from $0$ up to $f_c$). 

The total bandwidth $B_T$ is simply twice that: $B_T=2f_c$, since we are also considering negative frequencies, from $-f_c$ up to $f_c$.

Here is a picture of that: