When measuring by the egg only, a single medium egg is equal to a large egg for recipe use, according to About Food. The ratio changes as larger amounts of eggs are required, with five medium eggs needed to equal four large eggs.
If the recipe requires eggs by volume, then one cup of large eggs is equal to six medium eggs, and there are five large eggs in one cup. A cup of large egg whites requires seven eggs, which can be replaced with eight medium egg whites. For egg yolks, a cup requires 14 large egg yolks, which is equal to 16 medium ones.