Found this thread.

I started back on Jul 1, 2009 with a mortgage principal of $255K. So a total of 7 years of payment now. I am still paying for my "sin" of having a 5.5% interest during the first 21 months before pulling the trigger on refinancing to 4%.

The cost of this "sin" right now is $5,240. This was arrived at by calculating if I had a fixed 4% on 15 years. Without any overpayment and just straight fixed payment monthly, the interest projects to a total of $59,325 for 7 years.

Since I had a "sin" of 21 months, my actual total interest paid the past 7 years was $64,565. So the difference between actual and projected interest payment stands at an overpayment of $5,240.

I have been paying additional principal but was only able to start late due to me prioritizing other higher interest debts (credit card, car loan, etc.). I currently stand $35K ahead of the fixed projection for 15 years. So, I am excited to see in the coming months and years the fruit of the additional payments in the form of payment for my "sin" and eventually, an actual savings on interest payments.

My principal balance is $122K.