Interest-Only Mortgage Calculator

An interest-only mortgage separates the cost of borrowing from the repayment of principal. For a limited period, the required payment covers interest and leaves the loan balance unchanged. The interest-only mortgage calculator shows that low opening payment, then estimates the larger amortizing payment that begins after the IO period. It also reports the payment jump, interest during the IO period, total interest over both periods, balance entering amortization, and total paid.

This calculator is not a standard mortgage-payment page. Use the mortgage calculator for a fully amortizing loan from month one, the mortgage interest calculator to isolate lifetime interest, and the amortization calculator when you want a detailed principal schedule. If the IO loan also has adjustable-rate features, compare the reset risk with the ARM mortgage calculator.

How interest-only math works

During the IO period, the required payment is simply the monthly interest charge. If the loan amount is $400,000 and the annual rate is 6.50%, the monthly rate is the annual rate divided by 12. The required IO payment is the balance multiplied by that monthly rate. The balance does not decline unless you enter voluntary principal paid during the IO period.

After the IO period, the calculator subtracts any voluntary principal reduction and amortizes the remaining balance over the later term. This is where many borrowers discover payment shock. The later payment must pay interest and repay principal, and it must do so over fewer years than a normal loan with the same total timeline.

Formula

The interest-only payment is:

$\text{IO payment} = \text{principal} \times \frac{\text{annual rate}}{12}$

The balance entering amortization is:

$\text{balance after IO} = \text{principal} - \text{principal paid during IO}$

The later amortizing payment is:

$\text{later payment} = \text{balance after IO} \times \frac{\text{monthly rate}}{1 - \left(1 + \text{monthly rate}\right)^{-\text{amortizing months}}}$

The calculator then adds IO-period interest to later amortizing interest. Total paid includes voluntary principal paid during the IO period, all IO interest, and all later amortizing payments.

Worked example using the default form values

Assume a $400,000 loan, a 6.50% annual interest rate, 10 years of interest-only payments, 20 years of later amortization, and $0 in voluntary principal reduction. The monthly rate is 6.50% divided by 12. The interest-only payment is therefore $2,166.67 per month.

Because no principal is voluntarily paid, the balance entering amortization remains $400,000. The later amortizing term is 240 months. Using the standard payment formula, the later principal-and-interest payment is $2,982.29. The payment jump after the IO period is $815.63 per month. Interest during the IO period is $260,000.00. Total interest over both periods is $575,750.21, and total paid is $975,750.21.

That example demonstrates the trade-off. The first payment is lower than a fully amortizing 30-year payment would be at the same rate, but delaying principal repayment increases total interest and creates a larger payment later. If you enter voluntary principal payments during the IO period, the required IO payment does not change in the model, but the later balance and later payment fall.

Pros, cons, and risk controls

The main advantage is flexibility. A borrower with seasonal income may prefer a low required payment and then make larger principal payments during high-income months. A buyer planning to sell before amortization begins may value the lower required cash outflow. An investor may compare the IO payment with rent or business cash flow.

The disadvantages are serious. Equity builds more slowly, total interest can be higher, and the later payment may be unaffordable if income does not rise. If home prices fall, refinancing out of the loan can be harder. If the loan is also adjustable-rate, the borrower may face both amortization shock and rate shock at the same time. Use the budget calculator to test the later payment, the debt-to-income calculator to check qualification pressure, and the refinance calculator only as a scenario, not a guarantee.

Tips for evaluating an IO offer

Ask whether the quoted rate is fixed or adjustable, when amortization begins, whether there is a balloon payment, and whether extra principal can be paid without penalty. Compare the IO loan with a fully amortizing mortgage over the same total horizon. Do not treat the IO payment as the housing budget; treat the later amortizing payment as the minimum affordability test. If the IO structure is part of a jumbo or portfolio loan, also review reserve requirements and documentation standards.

Sources

Source version: issuer pages current when accessed July 9, 2026; no unstated effective year is assumed.

• CFPB, What is an interest-only mortgage? — consumer explanation of IO payments and principal risk.
• CFPB, Consumer Handbook on Adjustable Rate Mortgages — payment-change and ARM-disclosure concepts relevant to IO ARMs.
• Freddie Mac, Primary Mortgage Market Survey — market-rate context for comparing IO, fixed, and adjustable loan scenarios.