>What I am trying to calculate is the interest rate if a person pays there mortgage every other week which gives 1 extra payment. I want to know the equivalent. So I do know the number of payments. I calculate that first to show a person the difference between the original loan and paying it biweekly. I will look at the payment calculation and see if that helps.
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>Thanks everyone for your input...

If they make a payments every other week they are actually making on average 1.17 extra payments per month.

Making a payment every other week does not change the interest rate. And "Equivalent rate" isn't really what you are looking for here either. It is used to compare two rates based on different compounding periods and you are not affecting the compounding periods or the interest rate by making extra payments.

What you are affecting is total interest paid, and the length of the loan.

I am not sure that you can represent this as some sort of APR number as the value would vary over the term of the loan.