To create a user-defined function in Excel VBA, you write a Function procedure in the Visual Basic Editor. 1. Open the VBA Editor by pressing Alt F11 and insert a standard module. 2. Write the Function Procedure starting with the Function keyword, defining arguments and their data types, and assigning the result to the function name, such as Function AddNumbers(a As Double, b As Double) As Double followed by AddNumbers = a b. 3. Use the function in Excel by typing =FunctionName(arguments) in a cell, ensuring the workbook is saved as .xlsm. 4. Optional: Add help descriptions via Tools → Macro → Options and include comments for clarity. Ensure functions are in standard modules and test them thoroughly for robustness.

To create a user-defined function in Excel VBA, you write a Function procedure in the Visual Basic Editor. This lets you build custom calculations that work like built-in Excel functions.

Open the VBA Editor and Insert a Module

Before writing your function, you need to access the VBA environment:

• Press Alt F11 to open the Visual Basic Editor.
• In the Project window, right-click on any existing module (or your workbook name) → select Insert → choose Module.

This is where you’ll write your function code. Functions should be placed in standard modules for best results — not in sheet or workbook code areas.

Write the Function Procedure

A user-defined function starts with the Function keyword and ends with End Function. Here's a basic structure:

Function MyFunctionName(Argument1 As DataType, Argument2 As DataType) As ReturnType
    ' Your code here
End Function

For example, if you want a function that adds two numbers:

Function AddNumbers(a As Double, b As Double) As Double
    AddNumbers = a   b
End Function

• The function name becomes the name you use in Excel formulas.
• Each argument should have a defined data type.
• The final result is assigned to the function name itself (AddNumbers = ...).

You can now use =AddNumbers(2,3) in a worksheet cell just like any other formula.

Use the Function in Excel

Once written, your function appears in the formula bar when you start typing = in a cell. It won’t show up in the function list like built-in functions, but it will autocomplete once you know the name.

Some tips:

• Save your workbook as Excel Macro-Enabled Workbook (.xlsm) to keep the VBA code.
• If the function doesn’t appear, make sure it’s in a standard module and not inside a class or sheet module.
• You can add comments and error handling later to make it more robust.

Optional: Add Help and Descriptions

If you want others (or your future self) to understand what the function does without reading the code, you can add a description:

• In the VBA editor, go to Tools → Macro → Macros.
• Select your function → click Options → add a description.

This description shows up in the formula wizard under “Insert Function” in Excel.

Also, consider adding comments inside your function explaining what it does, especially if the logic gets complex.

That’s basically how you do it. Writing a UDF in VBA isn’t hard, but it’s powerful once you get the hang of it. Just remember to test your function with different inputs and handle possible errors, like text input when numbers are expected.

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