1. A method for grading a food item comprising: singulating a supply of individual food items with a singulating conveyor; imaging each of the food items with an imaging system to produce an image of each of the food items; computing in a controller an estimated weight of each of the food items using an image-to-weight function; weighing a sample of the food items with a calibration weigher to produce an actual weight of the weighed food items in the sample; comparing the estimated weights to the actual weights in the controller; adjusting the image-to-weight function in the controller based on the comparison of estimated weights to actual weights; grading the food item into a plurality of weight grades with a sorter.