fredonic,

You need to do a lot for that, and it must be equation specific: Copy the code below, right click
on the sheet tab, select "View Code", and paste the code in the window that appears.

HTH,
Bernie
MS Excel MVP

Private Sub Worksheet_Change(ByVal Target As Range)

If Target.Cells.Count 1 Then Exit Sub
If Intersect(Target, Range("A1:C1")) Is Nothing Then Exit Sub
If Target.Value = "" Then Exit Sub
If Application.WorksheetFunction.CountBlank(Range("A1 :C1")) < 1 Then Exit Sub

Application.EnableEvents = False
If Range("A1").Value = "" Then
Range("A1").Value = Range("C1").Value - Range("B1").Value
End If
If Range("B1").Value = "" Then
Range("B1").Value = Range("C1").Value - Range("A1").Value
End If
If Range("C1").Value = "" Then
Range("C1").Value = Range("A1").Value + Range("B1").Value
End If
Application.EnableEvents = True
End Sub



"fredonic" wrote in message
...
Could you be more specific, as I am still learning to program in excel.

"Gary''s Student" wrote:

Use a worksheet change event macro. It can detect two cells entered and
calculate the third, See:

http://www.mvps.org/dmcritchie/excel...m#wb_shtChange

--
Gary''s Student


"fredonic" wrote:

If I input a formula e.g. 'A1+B1=C1'; how do I tell excel to calculate B1 if
I have A1 and C1's values or A1 if I have the other 2 values - doing all this
using only the 3 original cells