What you pay each month on an interest-only loan
On an interest-only loan, your monthly payment covers only the interest that accrues—not the principal balance itself. To calculate it, multiply your loan balance by the annual interest rate, then divide by 12. That number is what you owe each month.
For example: if you borrowed $200,000 at 5% annual interest, the calculation is ($200,000 × 0.05) ÷ 12 = $833.33 per month. That payment stays the same as long as the interest rate and loan balance do not change.
The catch is that you are not reducing what you owe. Every dollar you pay goes to interest; none of it shrinks the principal. After five years of $833.33 payments, you still owe the full $200,000.
Key Takeaways
- Monthly interest-only payment = (Loan balance × Annual interest rate) ÷ 12.
- Your payment covers only interest, so the principal balance stays the same until you make a lump-sum payment or the loan converts to principal-and-interest.
- Interest-only periods typically last 5 to 10 years, after which your payment jumps to include both principal and interest.
- If your interest rate is variable, your monthly payment will change when the rate adjusts, even during the interest-only period.
- The longer you stay interest-only, the larger your principal-and-interest payment will be when the period ends.
When the interest rate changes during the interest-only period
Many interest-only loans have a variable rate, meaning the interest rate can move up or down based on market conditions. When that happens, your monthly payment changes immediately, even though you are still in the interest-only phase.
If your rate rises from 5% to 6%, that $200,000 loan now costs ($200,000 × 0.06) ÷ 12 = $1,000 per month instead of $833.33. The lender will notify you of the rate change and your new payment amount before it takes effect.
Some loans have a fixed rate during the interest-only period, which means your payment stays exactly the same month to month. Check your loan documents or call your lender to confirm whether your rate is fixed or variable.
What happens when the interest-only period ends
Interest-only periods are temporary. Most last 5, 7, or 10 years, depending on the loan terms. When that period ends, the loan converts to principal-and-interest (also called amortizing), and your payment jumps significantly.
At conversion, the lender recalculates your payment to cover both the remaining principal and the interest over the remaining loan term. If you still owe $200,000 after a 5-year interest-only period on a 30-year loan, you now have 25 years left to pay it off. Your new monthly payment will be much higher than the $833.33 you were paying before.
You can calculate the new payment using an amortization formula or an online calculator, but the key point is this: the longer you stay interest-only, the steeper the jump. Some borrowers refinance before conversion to avoid the shock, while others plan to sell the property or pay down principal before the period ends.
How to handle a variable-rate interest-only loan
If your rate can change, your lender is required to tell you when an adjustment is coming and what your new payment will be. Read those notices carefully—they show the new rate, the new monthly payment, and the date it takes effect.
Track your rate changes over time. If rates have risen significantly since you took out the loan, you may want to refinance to a fixed rate or a shorter interest-only period before conversion happens. If rates have fallen, refinancing might also save you money.
Set aside money during the interest-only period if you can. Even though you are not required to pay principal, putting extra money toward the balance now means a smaller jump when the loan converts. Every dollar of principal you pay down reduces the amount you will owe when the interest-only period ends.
Interest-only loans on mortgages versus other debts
Interest-only terms are most common on mortgages and home equity lines of credit (HELOCs), but they also appear on some business loans and construction loans. The calculation method is the same regardless of the loan type: balance × rate ÷ 12.
Mortgages with interest-only periods often have lower starting payments than traditional 30-year fixed mortgages, which is why some borrowers choose them. However, the trade-off is that you build no equity during the interest-only phase, and your payment will rise sharply later.
HELOCs often stay interest-only indefinitely if you want them to—you can choose to pay only interest or to start paying principal at any time. Other loans, like construction financing, may require you to convert to principal-and-interest or pay off the balance entirely when the project is complete.
Common mistakes when calculating or managing interest-only payments
The biggest mistake is assuming your payment will stay low forever. Interest-only is a temporary structure. If you have not planned for the conversion, the payment shock can be severe—sometimes doubling or tripling your monthly obligation.
Another mistake is ignoring rate changes on variable-rate loans. If you receive a notice that your rate has adjusted, recalculate your payment right away so you know what to expect. Do not assume it will stay the same.
Some borrowers also fail to account for property taxes, insurance, and HOA fees on mortgages. Your interest-only payment covers only the interest portion of your mortgage payment, not these other costs. Your total monthly housing expense is higher than the interest-only number alone.
Frequently Asked Questions
Can I pay principal during the interest-only period?
Yes. Most lenders allow you to pay extra toward principal at any time without penalty. Doing so reduces the balance and lowers your interest-only payment going forward, and it also means a smaller principal-and-interest payment when the period ends.
What if I cannot afford the payment after the interest-only period ends?
Contact your lender before conversion happens. You may be able to refinance to a longer loan term, extend the interest-only period, or modify the loan. Waiting until after the payment jumps makes your options much more limited.
Is the interest-only payment tax-deductible?
On a mortgage, the interest portion of your payment may be deductible if you itemize deductions on your tax return. Consult a tax professional about your specific situation, as rules vary by loan type and your income level.
How do I know if my interest-only loan has a fixed or variable rate?
Check your loan documents or the disclosure statement you received at closing. It will state the rate type and whether it can adjust. You can also call your lender and ask directly.
What is the difference between interest-only and a balloon payment loan?
Interest-only means you pay only interest each month, but the full principal is due at the end. A balloon loan typically has principal-and-interest payments, but a large lump sum is due at maturity. The structures are different, though both require planning for a large payment later.