Forcing a Macro to Run when a Worksheet is Recalculated

When you write a macro, it is designed to be run whenever you choose to run it. What if you need to develop a macro that will run whenever something changes in your worksheet? This is particularly necessary if you are creating a custom function that you want to use within the cells of the worksheet.

This is where it is important to understand what is meant by function volatility. Excel considers a function to be volatile if its value cannot be assumed to be the same from one moment to the next. The following worksheet functions are considered to be volatile in nature:

• CELL
• INDIRECT
• INFO
• NOW
• OFFSET
• TODAY
• RANDBETWEEN
SUMIF

Even though the list looks short, there is some controversy over what worksheet functions are actually volatile and which ones aren't. The following short article is helpful in understanding this:

https://www.decisionmodels.com/calcsecretsi.htm

What do worksheet functions have to do with macros? Well, you can create a macro that is a UDF (user-defined function) that may fit into the category of not having a value that "can be assumed to be the same from one moment to the next." By default, Excel assumes that any macro you create is non-volatile. This may not be the case, however. Consider the following single-line UDF:

Function FutureDay(D As Integer) As Double
    FutureDay = Now() + D
End Function

All this does is determine the time, right now, and add an integer value to that time in order to deternine a future (or past) date and time. You might call this UDF in a worksheet in this manner:

=FutureDay(7)

That returns a date/time value 7 days in the future. When your worksheet recalculates, though, the UDF doesn't recalculate. This means that the date/time value in the cell is no longer accurate.

The solution is to tell Excel that your UDF is volatile. You do this by adding a single line to the macro:

Function FutureDay(D As Integer) As Double
    Application.Volatile
    FutureDay = Now() + D
End Function

Now when Excel recalculates the worksheet, it will force a recalculation of your UDF, as well.

You should note that you should only use the Volatile method sparingly in your macros. It doesn't hurt anything to use it everywhere, except it adds additional work to the recalculation of your worksheets. To make sure things run as quickly as possible, just use Volatile where you determine that it is really needed.

ExcelTips is your source for cost-effective Microsoft Excel training. This tip (10598) applies to Microsoft Excel 2007, 2010, 2013, 2016, 2019, and Excel in Office 365.