$Q$ factor refers to what on a lowpass filter? (EE kind of definition) For what I'm learning, the bandwidth (of a BPF, for example) refers to the $\Delta$ between $f_L$ and $f_H$ at (usually) $-3\textrm{ db}$ from the $f_0$ (from wiki). For a peaking filter instead, $f_L$ and $f_H$ are situated (usually) at the peak $\textrm{dB Gain}/2$.

Q Factor become $f_0 /\Delta$. It's clear till here. 


Now, if I take a lowpass filter, how is calculated $Q$ Factor? 
I have $f_0$, but how would I calculate the $\Delta$ of a lowpass? 


There is no a "peak" as reference, so $-3\textrm{ dB}$ (or $\textrm{dB Gain/2}$) make no sense as references points for the "created" bandwidth.
For lowpass and highpass filters, Q factor is not well defined. What is defined is the cutoff frequency and bandwidth. 

Q factor is useful for bandpass or bandstop (notch) filters as it shows the selectivity of the BPS. Quantitatively Qf is the ratio of the center frequency of the pass-band to the bandwidth of the pass-band. Assuming the system has only a single pass band.

For a highpass filter, since the bandwidth is infinite, this Q will be zero.
And for a lowpass filter it will always be 1/2.
As a result, Q is not used for HighPass or Lowpass analog filters.