### Excel Recalculates UDFs Only When Their Input Changes

You've created a User-Defined Function (UDF). Excel only runs the code of a UDF when the cell(s) that serve as input to the function change.

For example, suppose I have this UDF:

```
Public Function MyFunction(Target As Range)
MsgBox "The target cell's address is " & Target.Address
End Function
```

And in cell A1 on my worksheet I reference the cell with the formula:

```
=MyFunction(A2)
```

During normal use of my workbook, the code of my UDF will only be called if there's a change to the contents of cell A2 or another cell referenced by a formula in A2.

### Solutions

You can work around this behavior in several ways:

- Force Excel to manual recalculate all of the formulas in the workbook even if they have not changed since the last calculation by pressing
`Ctrl`+`Alt`+`F9` on Windows and (I assume on Mac) `Cmd`+`Alt`+`F9`. If that doesn't work you can also try adding `Shift` which additionally rechecks dependent formulas before recalculating.
Include a volatile function in one of the cells referenced by your UDF. Do this by modifying the definition of your VBA function to accept an additional parameter:

```
Public Function MyFunction(Target As Range, Optional VolatileParameter As Variant)
```

Then edit the cell referencing your UDF to pass the result of a volatile function such as `Now()`

to the UDF:

```
=MyFunction(A2,Now())
```

The net result will be that Excel will deem the cell containing the reference to your UDF as needing to be recaculated every time the worksheet is changed because it references a volatile function.

Edit the cell containing the UDF. Simply entering the cell then pressing Enter without making changes should be sufficient to trigger an update.

Create macro button that runs the following line of code:

```
Application.CalculateFull
```

Place the following code in your Workbook's Open event:

```
ActiveWorkbook.ForceFullCalculation = True
```

This causes Excel to *always* recalculate all formulas, regardless of whether it believes recalculation is required. This setting remains in effect until Excel is restarted.

### Why Doesn't Excel Always Recalculate My UDF?

When Excel performs an automatic recalculation, it does not recalculate every single formula in the workbook. Instead it only updates those cells which contain formulas that refer to the most recently modified cell. This method avoids the much lengthier process of needlessly performing many calculations across the entire workbook only to come up with the same result for the vast majority of them.

With a UDF, the only references in the workbook of which Excel's calculation engine is aware are those identified in the function's inputs. Excel cannot examine the VBA code within the UDF and identify *other* cells that may influence the function's output. So while the function's author may construct the function to return different results based on any number of changes made in the workbook, the only cells Excel knows will change the function's result are the declared inputs. Therefore, changes to these cells are the only ones Excel pays attention to when deciding whether the UDF needs to be recalculated.