Don't know what the function might be, but why don't you just create an
amortization table and plug in the extra payments?
Col A - Period
Col B - Pymt()
Col C - Interest
Col D - Principal
Col E - ADDITIONAL PRINCIPAL PAYMENTS
Col F - New Mortgage Balance

HTH,
--
Gary Brown

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"Dan Patrick" wrote:

I'm working on a plan to pay off my mortgage early. Each year I plan to
increase the amount I am paying by a fixed amount. For example, this year I
will pay $800. Starting next year I will pay $850, the year after that, $900,
and so on.

I know the NPER function to calculate the number of periods given a rate,
fixed payment, and present value.

How can I calculate the number of periods required to pay off the loan when
the payment is changing, assuming the timing and amount of the change are
known?

Thanks for your help.