Egg sizes and grades: large, extra large, AA and A

Egg size is a weight class, not a measure of how big any single egg looks. USDA defines each size by the minimum net weight of a dozen, so the label on the carton is really a promise about the pack weight. The ladder climbs by three ounces a step: peewee at 15 ounces per dozen, small at 18, medium at 21, large at 24, extra large at 27 and the biggest class at 30. A dozen large eggs must weigh at least 24 ounces, which is where the standard everyone cooks by comes from.

Size is minimum weight per dozen

Because size is a dozen-weight, individual eggs inside a carton vary while the pack still makes weight. Two eggs from the same hen can fall on either side of a size line as she ages, since hens tend to lay larger eggs later in the laying cycle. A packing line grades by weight and drops each egg into the class it fills. That is also why a recipe calls for large rather than a number of ounces. The size class is the standard unit, and large is the one recipes assume.

What the grades mean

Grade is a separate axis from size, and it describes interior quality rather than weight. USDA grades are AA, A and B, set by candling, which means shining a light through the shell to judge the air cell, the white and the yolk without cracking the egg. Grade AA has a small air cell, a thick firm white and a high round yolk. Grade A is a small step down, still very good, and it is the bulk of what sits on the shelf. Grade B, with a larger air cell and a thinner white, rarely reaches a carton. It goes to breakers to be turned into liquid, frozen or dried egg products, where interior structure no longer matters. So a carton in the store is almost always AA or A, in whatever size class it was packed.

Why large is the benchmark

Large white is the size the market quotes, and the chart above sets extra large against it. Large is the highest-volume retail size and the recipe default, so its price is the reference the whole trade anchors to. The other sizes trade at their own prices around it, extra large usually a bit above large, medium a bit below. Watch how the extra large line tracks the large benchmark. They move together, because they come off the same hens, but the spread between them widens and narrows with how the current flock is laying: more big eggs from an older flock, more mediums from a young one.

The Scramble reports USDA prices by size as published, in cents per dozen. Sizes and grades follow USDA's published standards.