Fruit juices tend to lose their brightness and flavor quickly, so it's always best to work with fresh juice when possible. Recipes calling for lemon juice are often based on that notion and suggest that you squeeze a specific amount of juice before starting. That's all well and good, but unless you're an experienced cook or baker, it can be difficult to judge how many lemons you'll need.
A Simple Average
As a rule of thumb, a single medium-sized lemon yields 2 1/2 to 3 tablespoons of juice. That means you'll usually need 5 to 6 lemons to make up a full cup of juice. These are only averages though, so in practical terms you'll find there's a lot of variation between them. Size is the most visible difference -- a tiny lemon might give 1 tablespoon or less, while a jumbo can yield up to 5 or more -- but other factors can determine how much juice you get from each lemon.
The Lemon Twins
The main lemon varieties grown in the United States are the Eureka and the Lisbon. They resemble each other closely, but there are discernible differences between them.
- Eureka lemons tend to have a thicker, stiffer skin and a characteristically thick neck at one end.
- Lisbon lemons are smoother and sleeker, tapering gently to either end. If you pick up a lemon and squeeze it gently, you'll find that a Lisbon lemon yields more readily to your grip.
The two varieties taste the same and are equally useful, but you'll get slightly more juice from a Lisbon lemon of the same size. The thicker peel of a Eureka lemon means there's less actual flesh inside, so it's smaller than it appears.
Age and Method
Age can be a factor as well. Any citrus fruit gradually becomes less juicy as it ages, because moisture evaporates through its porous skin. That's why lemons from lower-volume retailers may give less juice. You'll also find that your juicing method makes a big difference. If you squeeze your lemon by hand, you might struggle to get 2 tablespoons of juice from it, especially if you're working with a thicker-skinned Eureka lemon. With a handheld reamer or a countertop citrus juicer, you're likely to get more juice from each fruit.