Microwave breast imaging requires a calibration process, which extracts the tumor response from the received signal. Existing calibration methods require more than two measurements. In this letter, we propose a calibration method called "in-place calibration" for time-domain microwave imaging. The method can obtain the received signal and the reference signal in a single measurement with the antenna array in place. It can minimize the measurement errors that occur due to environmental changes during calibration. The method also minimizes the error due to random clock jitter. The proposed in-place calibration is compared to array rotation calibration and differential calibration using homogeneous and heterogeneous breast phantoms. In most cases, in-place calibration provided better results than array rotation calibration.