Precise measurement techniques of instantaneous acoustic pressure using the frequency response of hydrophone sensitivity have been investigated to evaluate the safety of diagnostic ultrasound. This technique requires the frequency range of sensitivities broader than bandwidth of the ultrasound. But available frequency range of sensitivities on a calibration certificate is limited. Extrapolation to the frequency range of certificated sensitivities might be an effective approach; however, the effectiveness of such extrapolation has not been sufficiently studied. In this paper, we propose a quantitative method for evaluating the effectiveness of such extrapolation, which is characterized by showing the narrowest frequency range of certificated hydrophone sensitivity necessary for precise measurement of instantaneous acoustic pressure in diagnostic ultrasound. The evaluation results showed that the extrapolation was generally effective for precise measurement using the certificated hydrophone sensitivity with a narrower frequency range than the bandwidth of the measured ultrasound.