Things are this are usually a little touchy depending on the data layout, so check it out
Option Explicit
Const iColToSum As Long = 5
Sub test()
Dim rArea As Range, rEnd As Range
Set rEnd = ActiveSheet.Cells(ActiveSheet.Rows.Count, iColToSum).End(xlUp)
Do While rEnd.Row > 1
Set rArea = rEnd.CurrentRegion
rEnd.Offset(1, 0).Value = Application.WorksheetFunction.Sum(rArea.Columns(iColToSum))
rEnd.Offset(1, 0).Interior.Color = vbRed ' marker
Set rEnd = rEnd.End(xlUp)
Set rEnd = rEnd.End(xlUp)
Loop
End Sub
Paul