Freehand 3D ultrasound allows intra-operative imaging of volumes of interest in a fast and flexible way. However, the ultrasound device must be calibrated before it can be registered with other imaging modalities. We present a needle-fiducial based electromagnetic localization approach for calibrating freehand 3D ultrasound as a prerequisite for creating an intra-operative navigation system. Although most existing calibration methods require a complex and tedious experiment using a customized calibration phantom, our method does not. The calibration set-up requires only a container of water and only several frames (three to nine) to detect an electromagnetically tracked needle tip in a 2D ultrasound image. The tracked needle is dipped into the water and moved freehand to locate the tip in the ultrasound imaging plane. The images that show the needle tip are recorded and the coordinates are manually or automatically identified. For each frame, the pixel indices, as well as the discrete coordinates of the tracker and the needle, are used as the inputs, and the calibration matrix is reconstructed. Three group positions, each with nine frames, are recorded for calibration and validation. Despite the lower accuracy of the electromagnetic tracking device compared to optical tracking devices, the maximum RMS error for calibration is 1.22mm with six or more frames, which shows that our proposed approach is accurate and feasible.