Variables doesn't null after step of loop

I want to count chars of data in columns, so I'm using for loop to do it. Here is an example of my code:

Sub CharCount()
    Dim Counter As Long

    For Counter = 1 To 257 Step 1
        Dim SumA As Variant
        Dim SumB As Variant
        Dim SumC As Variant
        SumA = 0
        SumB = 0
        SumC = 0
        Dim j As Long
        For j = Counter To Counter * 257 Step 1
            SumA = SumA + Len(Cells(j, 1))
            SumB = SumB + Len(Cells(j, 2))
            SumC = SumC + Len(Cells(j, 3))
        Next j
        Range("E" & Counter) = SumA
        Range("F" & Counter) = SumB
        Range("G" & Counter) = SumC
        SumA = Null
        SumB = Null
        SumC = Null
    Next Counter
End Sub

As you can see, I'm trying to Null variables for sum, but it doesn't seem to work and after each step of loop these variables just stack values of previous steps. How can I null variables?