The amount of lemon juice that can be extracted from a single lemon is dependent on many factors. The most obvious factor is the size of the lemon. Generally, the larger the lemon, the more juice it will yield. The variety of lemons also makes a difference. "Meyer" lemons are juicy, sweet, and thin-skinned (they can't take a joke)! "Eureka" lemons, on the other hand, have thicker skin and may contain more seeds. Still, another determining factor is the ripeness of the fruit. A riper lemon will yield more juice than a lemon that was picked too early. The juicer used (a hand-held juicer or an electric juicer ) can also affect the amount of the juice output. Keeping all these facts in mind, a medium-sized lemon will yield approximately 1/4 - 1/3 cup of lemon juice. So, four medium-sized lemons should yield approximately one cup of lemon juice.