Method and apparatus for grading food items, such as shrimps and chicken parts, with an automatically calibrated imaging system. Singulated food items are individually imaged. An estimated weight for each food item is computed from its image and an image-to-weight function. Calibration weighers weigh the food item individually or in batches before or after imaging. The actual weights are compared to the estimated weights to fine-tune the image-to-weight function and improve weight estimation.