Private Sub Worksheet_Change(ByVal Target As Range)
If (Target.Count = 1 And IsNumeric(Target.Value) And Target.Value > 0) And (Target.Address = "$A$2" Or Target.Address = "$B$2") Then
Application.EnableEvents = False
If Target.Column = 1 Then
Cells(2, 2) = CDbl(Target.Value) * 2.54
Else
Cells(2, 1) = CDbl(Target.Value) / 2.54
End If
Application.EnableEvents = True
End If
End Sub