Navigation systems based on intra-operative ultrasound have been introduced for different surgical procedures and interventions. The accuracy of the ultrasound probe calibration is a main contribution to the overall accuracy of the navigation system. Different calibration methods based on phantoms have been introduced. The challenge of the calibration procedure is to identify the phantom structures accurately and possibly automatically in ultrasound images. In the majority of cases 2D ultrasound probes have been calibrated. The advantage of 3D probes is the acquisition of 3D ultrasound volumes out of the box enabling fast calibration approaches based on only one image acquisition. We introduce a new inexpensive and easily to manufacture planes phantom and automatic calibration algorithm for 3D ultrasound probes.