You could use some code to do this. Here is one way.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)

If Range("A2").Value = Range("AA1").Value Then End
Range("A1").Value = Range("A1").Value + Range("A2").Value
Range("AA1").Value = Range("A2").Value

End Sub


This uses cell AA1 to store the number in A2. If A2 changes, then the new
number is added to A1, otherwise it is left alone.

HTH,
Paul

"Rod" wrote in message
...
If I have a Number in Cell A1 and then want to add say 10 to that number,
I
then want to replace the Number in A1 with the new results.

If A1 has 10
Then A2 has 20

I want A1 to then say 30.

If I change A2 to then 10 again
Then A1 changes to 40.

I need like a running SUM in a Cell.