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, all enclosed within a set of parentheses. You may include as many as 29 arguments, using commas to separate them. You can also 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—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.