The KM3NeT/ARCA calibration unit is a dedicated calibration system designed to improve the accuracy of the acoustic positioning system of the detector optical modules and monitor the water column physical properties. The calibration unit is composed of a calibration base and an instrumentation unit, connected with an electrical inter-link cable. The deployment of one calibration unit for each of the two building-blocks, in which the ARCA detector is subdivided, is foreseen, with the first one to be deployed in 2024. The calibration base is made of an anchoring structure, connected for power supply and communication to a junction-box, where an acoustic beacon and a hydrophone used for the positioning system are mounted and which hosts a pressure vessel containing the required electronics. The instrumentation unit consists of an anchoring base and a 750m-long inductive line, kept vertical by a top buoy and equipped with oceanographic sensors. The base is linked to the calibration base for power and readout and hosts an absolute pressure gauge used as depth reference and a vessel containing the electronics for managing sensor communication. The line hosts two sound velocimeters and two conductivity-temperature-depth probes equipped with dissolved oxygen sensors, to measure sound velocity and allow for the determination of acoustic wave speed, and two Doppler current sensors to provide information on sea current speed and direction, further improving the accuracy of the positioning system.