Abstract: Embodiments of a system and method of this disclosure continually determine a mass balance of a drilling or circulating system, using mass flow and density data adjusted for temperature and pressure. The system and method may be used to continually monitor and control wellbore pressures to achieve over-balanced managed pressure drilling (“MPD”). In some embodiments, the system and method may be used as a fluid tracking engine. In other embodiments, the system and method may be used to accurately track a fluid caliper intended to survey the well to determine string volumes or borehole volumes; an example is determining the volume of cement needed to circulate the cement or bring the cement up to desired height. The system and method may be used to track the effects of mixing. The system and method may also be used as a viscometer.