What you pay each month on an interest-only loan
On an interest-only loan, your monthly payment covers only the interest that accrues that month. You do not pay down the principal—the original amount you borrowed. The calculation is straightforward: multiply your loan balance by the annual interest rate, then divide by 12 to get the monthly interest charge.
For example, if you borrowed $200,000 at 5% annual interest, the yearly interest is $10,000. Divided by 12 months, your monthly payment is $833.33. That payment stays the same each month as long as the interest rate and loan balance do not change. After the interest-only period ends—typically 5 to 10 years—the loan converts to a standard amortizing loan, and your payment jumps because you now pay both interest and principal.
Key Takeaways
- Monthly payment on an interest-only loan equals (loan balance × annual interest rate) ÷ 12.
- Your payment covers only interest, so the principal you owe stays the same throughout the interest-only period.
- When the interest-only period ends, your payment increases significantly because you begin paying down principal.
- If the interest rate is variable, your monthly payment will change when the rate adjusts.
- Making extra payments toward principal during the interest-only period reduces what you owe when the amortization period begins.
The formula and how to use it
The basic formula is: Monthly Payment = (Loan Balance × Annual Interest Rate) ÷ 12
Start with your current loan balance—not the original amount, but what you actually owe right now. Multiply that by the annual interest rate expressed as a decimal. For a 5% rate, use 0.05. Then divide the result by 12 to convert the annual interest to a monthly figure.
If your loan balance is $150,000 and your rate is 4.5%, the math looks like this: ($150,000 × 0.045) ÷ 12 = $562.50 per month. That $562.50 is pure interest. None of it reduces what you owe.
How interest-only periods work in real loans
Most interest-only loans are structured with two distinct phases. The first phase, usually 5 to 10 years, is the interest-only period. During this time, you pay only interest using the formula above. Your loan balance does not shrink.
When the interest-only period ends, the loan converts to a fully amortizing loan. Now your payment includes both interest and principal, and it is calculated to pay off the remaining balance over the remaining loan term. This new payment is almost always much higher than your interest-only payment was, sometimes 50% to 100% higher depending on how much principal remains and how long you have left to repay.
For instance, if you borrowed $200,000 at 5% for a 30-year loan with a 10-year interest-only period, your first 10 years cost $833.33 per month. When year 11 arrives, the remaining $200,000 must be paid off over the final 20 years. Your new payment jumps to approximately $1,325 per month.
Variable-rate interest-only loans and payment changes
Some interest-only loans have a fixed rate for the entire interest-only period. Others have a variable rate that adjusts periodically—often every 6 months or annually—based on a market index plus a margin set by your lender.
When the rate changes, your monthly payment changes when ready. If your rate rises from 4% to 4.5%, your payment increases. If it falls to 3.5%, your payment decreases. You recalculate using the new rate and the same formula: (balance × new rate) ÷ 12.
Variable-rate loans are riskier because you cannot predict your future payments. A rate that starts at 3% could climb to 6% or higher if market conditions shift. Before taking a variable-rate interest-only loan, understand the rate cap—the maximum rate you could be charged—and calculate what your payment would be at that cap.
What happens when you make extra principal payments
During the interest-only period, you can pay extra toward principal if you choose to. This extra payment does not change your required monthly interest payment, but it does reduce your loan balance.
If you owe $200,000 and send an extra $500 toward principal one month, your balance drops to $199,500. The next month, your interest-only payment is still $833.33 (assuming the rate has not changed), but you have reduced the amount you will owe when the amortization period begins. When the loan converts, your new payment will be lower because the principal is smaller.
This strategy makes sense if you have cash available and want to reduce the shock of the payment increase later. It also reduces the total interest you pay over the life of the loan.
Common mistakes in calculating interest-only payments
The most frequent error is using the original loan amount instead of the current balance. If you borrowed $250,000 but have paid down $50,000 in principal (through extra payments or a previous amortization period), your balance is $200,000. Use $200,000 in the formula, not $250,000.
Another mistake is forgetting to divide the annual rate by 12. The interest rate is always quoted as an annual figure. If you multiply the balance by the annual rate without dividing by 12, you will calculate a yearly interest charge, not a monthly one.
A third error is confusing the interest-only payment with the fully amortizing payment that comes later. These are two different numbers. The interest-only payment is lower and stays constant (unless the rate changes). The amortizing payment is higher and includes both interest and principal.
When to use interest-only loans and what to watch for
Interest-only loans are common in real estate investing, where the borrower expects property value to rise or rental income to cover the payment. They are also used by some homebuyers who expect their income to increase significantly before the amortization period begins.
The risk is clear: if your income does not rise as expected, or if property values fall, you will face a much larger payment in a few years while still owing the full original amount. Before committing to an interest-only loan, calculate what your payment will be when the interest-only period ends, and confirm you can afford it.
Also check whether your loan has a prepayment penalty. Some lenders charge a fee if you pay off the loan early or pay extra principal during the interest-only period. If a penalty applies, factor that cost into your decision.
Frequently Asked Questions
Can I pay principal during the interest-only period?
Yes. Extra principal payments reduce your balance and lower the payment you will owe when the amortization period begins. Check your loan documents to confirm there is no prepayment penalty, as some lenders charge a fee for paying extra principal.
What happens if I cannot afford the payment when the interest-only period ends?
Contact your lender before the conversion date. Some lenders offer loan modifications, refinancing, or extended amortization periods. Waiting until you miss a payment makes your options much narrower and damages your credit.
Does the interest-only payment include property taxes or insurance?
No. The interest-only payment is interest only. If your loan has an escrow account, your lender collects property taxes and insurance separately each month. Your total monthly housing cost is the interest-only payment plus the escrow amount.
If my interest rate is variable, how often does my payment change?
The rate adjustment schedule depends on your loan terms. Some rates adjust every 6 months, others annually, and some have a fixed period before adjustments begin. Check your loan documents for the adjustment frequency and any rate caps that limit how much the rate can change.
Is an interest-only loan the same as a balloon loan?
No. An interest-only loan converts to amortization at a set date. A balloon loan requires a large lump-sum payment at the end. Some loans combine both features—interest-only payments for several years, then a balloon payment due. Read your promissory note to know which type you have.