How much corned beef do you need for your St. Patrick's Day dinner? I would recommend buying at least 3/4 of a lb. to 1 lb per person. When you purchase corned beef there is going to be extra juices in the package which are included in the total weight. There will also be a good deal of shrinkage during the cooking process. If you want leftovers plan on buying over 1 lb. per person.