How are the penalties calculated if I break my mortgage early? Specifically, ask what rate they use to calculate the “interest rate differential”. Typically, if the lender has “posted rates” they use these to calculate the penalty. If this is the case, the penalty can be 3, 4 or even 5 times higher than a mortgage lender that does not have posted rates and uses them in their early payout penalty calculation. This one question can save you thousands of dollars!