Understanding Custom Function Rules

A custom function must start with a Function statement and end with an End Function statement. In addition to the function name, the Function statement usually specifies one or more arguments. You can, however, create a function with no arguments. Excel includes several built-in functions-RAND and NOW, for example-that don't use arguments. As you'll see later in this chapter, you can also create functions with optional arguments, which are arguments that you can either include or omit when you call the function.

Following the Function statement, a function procedure includes one or more VBA statements that make decisions and perform calculations using the arguments passed to the function. Finally, somewhere in the function procedure, you must include a statement that assigns a value to a variable with the same name as the function. This value is returned to the formula that calls the function.