Abstract: Disclosed herein are methods, systems, and devices for determining a recommended magnet for an external unit of an implantable medical device. An example diagnostic device includes a primary sensor configured to measure a characteristic of a magnetic field generated by a magnet included in an implantable unit of an implantable medical device. The diagnostic device also includes an output component and a processor component. The processor component is configured to receive one or more measurement signals from the primary sensor. Each measurement signal includes information indicative of the measured characteristic. The processor is also configured to, based on the measured characteristic included in each of the one or more measurement signals, determine a magnet characteristic. The processor component is further configured to cause the output component to output information indicative of the magnet characteristic.