I'm having trouble understanding your explanation. There is no
ActiveSheet.Calculate in the code you've quoted.

> I'm changing  the ColumnWidth value of a cell  in a loop.
> Every time  I change  that value I do an ActiveSheet.Calculate.
> In another cell I have the function  below.

Do you mean the value of the cell or the width of the cell? Changing the
width of a cell will not make it "dirty". Therefore it won't necessarily
participate in a calculate. (Calculate only recalculates what needs it.)

In your function, one of the Select legs has

Range(Application.Caller.Address).Width *
Sheets("Conversion").Range("C2").Value

and the other legs are similar, but simpler.

These will do what you expect them to do - i.e. they will use the current
values of those items when the function runs.

But is the function running?

A change to either of those two cells will not trigger the running of the
function. It will only run if the cell that includes the function call is
directly referencing a cell whose value has changed - e.g. the cell that
provides spUnits.

We probably need more information, though.

You can work around this:

add another optional parameter to your function  - you don't need to use it!

where the function is referenced in the spreadsheet enter a cell  reference
which will change  as the second parameter - like

fncColumnWidth(MM,A1:Z999)

then the function will fire every time  a cell in the range changes.

True but ...

Changing the width of a column isn't a change  to the cell, so the function
won't be called merely for that.

OK.

So...

we need to have a cell  with the formula =CELL("width",F1) (assuming col F is
the one being altered). Then we make the formula additional parameter refer
to this cell.

The cell can have white text if you Lisa does not want the width shown.