There are three consumer grades for eggs: U.S. Grade AA, A, and B. The grade is determined by the interior quality of the egg and the appearance and condition of the egg shell. Eggs of any quality grade may differ in weight (size).
U.S. Grade AA eggs have whites that are thick and firm; yolks that are high, round, and practically free from defects; and clean, unbroken shells. Grade AA and Grade A eggs are best for frying and poaching where appearance is important.
U.S. Grade A eggs have characteristics of Grade AA eggs except that the whites are “reasonably” firm. This is the quality most often sold in stores.
U.S. Grade B eggs have whites that may be thinner and yolks that may be wider and flatter than eggs of higher grades. The shells must be unbroken, but may show slight stains. This quality is seldom found in retail stores because they are usually used to make liquid, frozen, and dried egg products.
Sizing of Eggs
Size tells you the minimum required net weight per dozen eggs. It does not refer to the dimensions of an egg or how big it looks. While some eggs in the carton may look slightly larger or smaller than the rest, it is the total weight of the dozen eggs that puts them in one of the following classes:
Jumbo 30 ounces
Extra Large 27 ounces
Large 24 ounces
Medium 21 ounces
Small 18 ounces
Peewee 15 ounces
Source: United States Dept. of Agriculture, USDA
Food Safety and Inspection Service