Use it like this :-In that case You maybe would like to get this:
Code:aw_y_max = Sheets("Sheet1").UsedRange.Rows.Count
Sub AW()
aw_y_max = Sheets("Sheet1").UsedRange.Rows.Count
With Worksheets("Sheet1")
.Cells.Item(aw_y_max, "AW").Interior.ColorIndex = 3
End With
End Sub