If you include ALL references to the spreadsheet data in the UDF parameter list, Excel will recalculate your function whenever the referenced data changes:
Public Function doubleMe(d As Variant)
doubleMe = d * 2
End Function
You can also use Application.Volatile
, but this has the disadvantage of making your UDF always recalculate - even when it does not need to because the referenced data has not changed.
Public Function doubleMe()
Application.Volatile
doubleMe = Worksheets("Fred").Range("A1") * 2
End Function