An implantable medical device (IMD) including a housing defining a propellant chamber, a drug reservoir located within the propellant chamber of the housing configured to receive a therapeutic fluid, a propellant gas within the propellant chamber; and a volume measurement system that includes a temperature sensor configured to measure a temperature of the propellant gas within in the propellant chamber and a pressure sensor configured to measure a pressure of the propellant gas within the propellant chamber. The volume measurement system is configured to measure the pressure and the temperature of the propellant gas to provide current volume information of the therapeutic fluid in the drug reservoir.