Private Sub Worksheet_Change(ByVal Target As Range)
Const TargetRange = "F:F"
Dim CellsSet As Range, Cell As Range
Application.EnableEvents = False
On Error Resume Next
Set CellsSet = Intersect(Target, Range(TargetRange).Cells.SpecialCells(xlCellTypeConstants))
If Err = 0 Then
    For Each Cell In CellsSet
        If IsNumeric(Cell) Then Cell = Cell * 1000
    Next
End If
Err.Clear
Application.EnableEvents = True
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
    If Target.Cells.CountLarge = 1 Then
        If Not IsEmpty(Target.Value) Then
            If IsNumeric(Target.Value) Then
                Application.EnableEvents = False
                Target.Value = 1000 * Target.Value
                Application.EnableEvents = True
            End If
        End If
    End If
End Sub