An implantable medical device (IMD) includes a drug reservoir located within a reservoir chamber of the IMD that includes a first side and a second side directly opposite the first side defining a reservoir volume there between. The IMD further includes and a volume sensor system including an ultrasound transmitter within the reservoir chamber and positioned to transmit an ultrasound signal toward the second side at an angle relative to the second side and a plurality of ultrasound sensors adjacent to at least one of the first side or second side of the drug reservoir with each sensor positioned to selectively receive the signal from the transmitter at different reservoir volume levels to indicate a current volume capacity of the drug reservoir.