Negative amortization loans calculate two interest rates. The first is called the payment rate the second is the actual interest rate. The true interest rate is calculated as simply the index plus the margin without periodic caps. Borrowers are given a choice of which rate to pay. Thus advertisers of negative amortization loans often refer to these loans as “payment option” loans.

A loan that allows negative amortization means the borrower is allowed to make a monthly mortgage payment that is less than the interest actually owed during that month. For example, let’s say we have a $200,000 loan with an adjustable rate that’s currently sitting at five percent. Simple interest on this loan is easy to calculate. Multiply the interest rate by the loan amount and you have the annual interest of $10,000. Divide $10,000 by 12 months and the monthly “interest only” payment is $833.33 or simply here is the formula for your monthly payment for interest only loans: loan balance x interest rates / 12 = monthly payment.

Now, let’s say that there’s a provision in the loan documents that allow the borrower to make a minimum payment based on a “payment rate” of four percent. So your lowest payment would be $666.67 because the “payment rate” is based upon four percent, not the actual interest rate, which is five percent.