Interest-Only Mortgage Calculator

An interest-only mortgage is a home loan where you pay only the interest portion of the balance for a set period, typically 5 to 15 years. During this time your monthly payment is lower because you make no principal reduction. After the IO period ends, the loan converts to a fully amortizing mortgage and your payment increases, sometimes substantially.

The IO payment is simply your loan balance multiplied by the monthly interest rate: IO Payment = P times (annual rate divided by 12 divided by 100). For a $300,000 loan at 6%, the monthly rate is 0.5%, so IO Payment = $300,000 times 0.005 = $1,500 per month. No principal is included.

After the IO period, the loan converts to a standard amortizing mortgage. You now owe the same original balance but must repay it over the remaining, shorter term. The formula becomes the standard mortgage payment formula applied to the original balance over the remaining months. This produces a higher payment than a conventional mortgage originated on the same day, because the term is shorter.

A conventional 30-year mortgage spreads principal repayment over all 360 months. An IO loan with a 10-year IO period only begins repaying principal at month 121, leaving just 240 months to cover the same original balance. The shorter repayment window means each payment must be larger to retire the debt on time.

Yes, significantly. Because the balance does not decrease at all during the IO period, the base on which interest accrues remains at its maximum the entire time. A $350,000 loan at 6.5% on a 30-year term with a 10-year IO period will cost approximately $50,000 to $70,000 more in total interest than a conventional 30-year mortgage at the same rate.

IO mortgages are used primarily by real estate investors who want maximum cash flow during the holding period and plan to sell before the IO phase ends, high-income borrowers expecting large future raises or bonuses who want low payments now, buyers of jumbo properties in high-cost markets, and some homebuyers who invest the payment difference aggressively. They are less common for owner-occupied primary residences.

Most IO mortgages allow voluntary principal payments during the IO phase. Making extra principal payments reduces your balance, which lowers both your IO payment and your post-IO amortizing payment. This is one of the best ways to manage the risk of IO loans: voluntarily pay principal when cash is available so the transition shock is smaller.

IO mortgages are considered higher-risk products. Most lenders require a minimum credit score of 700 to 720, with better rates available above 740. They also typically require a loan-to-value ratio of 80% or lower (20%+ down payment), significant cash reserves (6 to 12 months of payments), and strong documented income to qualify for the higher post-IO payment.

IO mortgages make sense in specific situations: for investors planning to hold a property for less than the IO period, for buyers with irregular income who expect large future cash events, or for buyers in high-cost markets where the IO payment makes ownership possible at all. They are risky for buyers who may not be able to handle the payment jump or who plan to stay in the home long-term, because they build no equity during the IO phase and cost more in total interest.

Both have low initial payments, but they differ in what happens at the end. An IO mortgage converts to a fully amortizing loan after the IO period. A balloon mortgage requires a single large lump-sum payoff of the remaining balance at a fixed future date, typically 5 to 7 years from origination. If you cannot refinance or sell in time, a balloon mortgage forces default or emergency refinancing. IO mortgages are generally less risky because they continue as ongoing loans.

Yes. Many IO borrowers plan to refinance before the IO period ends, especially if they expect rates to fall or if they want to lock in a conventional fixed-rate loan once they have more equity. However, refinancing requires qualifying for the new loan at prevailing rates, which may be higher. If your home value has not increased, you may also lack the equity needed to qualify for favorable refinancing terms.

IO mortgages can be either fixed-rate or adjustable-rate. Fixed-rate IO mortgages are less common but exist, especially in the jumbo market. More frequently, IO loans are structured as adjustable-rate mortgages (ARMs) where the IO period aligns with the fixed rate period (for example, a 7/1 ARM with a 7-year IO period). After both the IO and the fixed periods end, the loan converts to an amortizing adjustable-rate loan, exposing the borrower to both payment and rate risk simultaneously.