On Mar 18, 8:22*am, Jagaude wrote:
The problem is that the amount of extra pmt is not necessarely the
same as the regular monthly payments. If I have a mortgage, the pmts
are set but if I want to calculate the impact of adding an extra 200$
every 3 months, how do I come up with the formula?

You keep changing the requirements: Now it's every 3 months instead
of every 2 months. Do you really want a solution for "every N
months", where N is variable (i.e. a cell value)?

That's hard. But for "every 3 months", one solution might be: Put
the following formula into E3 and copy down:

=if(and(n(E1)=0,n(E2)=0), 200, 0)

Of course, you could put 200 into a cell and replace "200" in IF()
with an absolute reference to the cell, for example $Z$1.

NOTE: I had an error in the formula in my first response. The use of
N() allows cells in column E to contain text (including "") or a
number or to be empty (no formula or constant). The IF() results in
zero instead of "" because "" might adversely affect your formula to
compute the balance, unless you were careful.