how to add thick line as divider automatically

I have serial number column in an excel and would like to add thick border when user goes next number.

Please see attached file. When user goes to next level of number I want to see think border as partition between to levels. Can a macro do this?

Appreciate your help
var

So what are you looking for?

For example, do you want a macro so that every time a number is added in column A, a line is placed above it covering A:K?

Or something else?

Exactly Steven. that is what i'm looking for..

I assumed that you would be adding the number manually, and not copy and pasting the numbers.

Inside the Visual Basic Editor, click on the folder: Microsoft Excel Objects
Click on the worksheet of your choice.
Copy the code below into your worksheet module.

Private Sub Worksheet_Change(ByVal Target As Range)
    If Not Intersect(Range("A:A"), Target) Is Nothing Then
        If IsNumeric(Target.Cells(1, 1)) Then
            Range("A" & Target.Row & ":K" & Target.Row).Borders(xlEdgeTop).Weight = xlMedium
        End If
    End If
End Sub

You can change the weight of the line to any of the following values:

xlHairline
xlMedium
xlThick
xlThin