I would calculate it as follows:

Payment for the first XX months = loan amount / loan term in months
Payment for the remaining term = original loan less payments during XX months = current balance
Calculate Payment() on current balance for remaining term.

Example:
A $100,000 loan taken out on January 15. Term 10 years, interest 12% per year, no interest first three months.

First 3 payments would be due February, March and April 15 and would be $100,000 / 120 months = $833.33 per month

On April 16th the loan balance is $97,500 ($100,000 -($833.33 * 3)

Now calculate th payment on the $97,500 for 117 months (120 - 3) at 12% interest.

>Anyone know how to calculate the payment on a loan where the first XX months are interest free and the remaining months are charged interest? PAYMENT() doesn't work in that situation. I found a good site on loan calculations at http://oakroadsystems.com/math/loan.htm, but it doesn't cover that scenario.
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>Thanks.