Sonar transducers are usually calibrated prior to installation on a ship. They are not, however, recalibrated after installation even though their receiving (and transmitting) properties can change significantly. This report presents a procedure for theinsitucalibration of sonar transducers at any desired time after installation when the ship is in open water. The procedure provides an up‐to‐date knowledge of the complex free‐field sensitivity (or response) of each of the array transducers. This knowledge can be used to minimize, perhaps even eliminate, the adverse effects on array performance due to changes in the properties of one or more transducers. Theinsitucalibration procedure is based on a form of three‐transducer reciprocity calibration that uses sound propagation factors to account for the influence of the ship’s structure. The report describes how to obtain numerical values for the required sound propagation factors. It also describes how to use two‐transducer comparison calibration to simplify theinsitucalibration process, at least for highly reliable sonars.