Disclosed is a measuring circuit for quantizing variations in the operating speed of a target circuit. The measuring circuit includes: a signal generator configured to generate a predetermined signal; an adjustable delay circuit configured to generate a first and second delay signals according to the predetermined signal respectively; a signal detector configured to detect the first and second delay signals respectively and thereby generate a first and second detection results respectively; and a calibrating circuit configured to enable a first and second numbers of delay units of the adjustable delay circuit according to the first and second detection results respectively so as to make each of the delays respectively caused by the first and second numbers of delay units be less than a delay threshold, in which the first and second numbers relate to the operating speed of the target circuit operating in the first and second conditions respectively.