A method and a system for determining reservoir pressure in a wellbore during a falloff test are provided. An exemplary method includes shutting in a wellbore, measuring pressure versus time using a wellhead sensor, calculating a reservoir pressure versus time from the measured pressure, and determining if the measured pressure has dropped to zero. If the measured pressure has dropped to zero, measuring distance from the wellhead sensor to a liquid level versus time, calculating a vertical depth versus time from the distance versus time, and calculating the reservoir pressure versus time based, at least in part, on the distance and the vertical depth of the well. The method also includes creating a falloff curve from the reservoir pressure versus time and providing the falloff curve.