Volume of a concrete mix load in a rotatable mixer drum is determined using an in-and-out sensor probe system wherein the probe submerges into and exits from the concrete during mixer drum rotation and provides data to a processor used in the system for calculating volume of the concrete mix load based on the data. To take into consideration any concavity, convexity, and/or cascading surface flow effects that can hinder accurate determination of the concrete load volume, the processor is configured to compare original batch volume and rheology of the concrete load monitored during drum rotation. The calibration of load volume involves a comparison between real-time data and historic data stored in processor-accessible memory, and further take into account the speed and tilt of the mixer drum (such as caused by roadway conditions), the concrete mix design, and other factors.