Sub testb()
Dim MaPlage As Range
Dim DernLigne As Long, DernColonne As Integer
'dernière ligne colonne A
DernLigne = Range("A" & Rows.Count).End(xlUp).Row
'dernière colonne ligne 1
DernColonne = Cells(1, Cells.Columns.Count).End(xlToLeft).Column
Set MaPlage = Range(Cells(1, 1), Cells(DernLigne, DernColonne))
Range("M13").Formula = "=HLookup(M12, Ma_Plage ,2,0)"
End Sub