Display or actually become a negative number?
Display...........Format>Cells>Custom -#,##0.00 but they would still be
positive numbers.
To actually become negative would require VBA.
You could use event code to change them to negative as you enter.
Private Sub Worksheet_Change(ByVal Target As Range)
On Error GoTo ws_exit:
Application.EnableEvents = False
If Not Intersect(Target, Me.Range("A1:A10")) Is Nothing Then
With Target
If .Value <> "" Then
.Value = .Value * -1
End If
End With
End If
ws_exit:
Application.EnableEvents = True
End Sub
Right-click on the sheet tab and "View Code"
Copy/paste the above into that module.
Edit the range of A1:A10 to suit.
Gord Dibben MS Excel MVP