RC circuits

I->---+---------->-O 
      |     
     (cap)
      |
     (resistor)
      |
I<----+----------<-O

Similarly, in an RC circuit composed of a single resistor and capacitor, the time constant τ (in seconds) is: [ τ = R * C ] where R is the resistance (in ohms) and C is the capacitance (in farads).
; it is inversely proportional to the cutoff frequency f, that is, [ f = 1 / 2 *π *τ = 1 / 2 *π *R *C ], where f is in hertz, τ is in seconds, R is in ohms, and C is in farads.
In digital electronic circuits another measure, the FO4 is often used. This can be converted to time constant units via the equation 5 τ = FO4