An apparatus and method for calibrating a cutoff frequency of a tunable filter, such as a tunable filter coupled to a read channel of a data storage system is disclosed. A calibration circuit includes a variable gain amplifier (VGA), an envelope detector, and a tunable filter coupled therebetween. A calibration signal having a preestablished frequency representing a specified cutoff frequency associated with the tunable filter is applied to the VGA. The gain of the amplifier is adjusted until an amplitude of an output signal provided by the filter is substantially equal to a first predetermined amplitude. A cutoff frequency of the filter is then adjusted until an amplitude of an output signal provided by the filter is substantially equal to a second predetermined amplitude, at which time the desired cutoff frequency of the tunable filter is precisely established. A single cutoff frequency or the low and high cutoff freqencies of a range of cutoff frequencies may be calibrated. The envelope detector includes a number of analog comparators which are used to compare an output signal provided by the filter with predetermined voltage amplitudes provided by first and second reference voltage sources. A control circuit controls amplifier gain and adjusts the cutoff frequency of the filter. A cutoff frequency calibration circuit according to the present invention may be advantageously implemented in-situ a read channel of a disk drive system.