Monthly Interest-Only Payment Calculator and Formula

Calculate a monthly interest-only payment by multiplying the loan balance by the annual interest rate and dividing by 12. Enter the balance and APR below to estimate the monthly interest due; this calculator also shows the balance and interest across a chosen interest-only period.

Formula

The monthly interest-only payment is the loan balance multiplied by the monthly interest rate:

Monthly payment = P × (APR ÷ 100 ÷ 12)

• P is the outstanding loan balance, in dollars.
• APR is the annual percentage rate, entered as a percentage per year.
• APR ÷ 100 ÷ 12 converts the annual percentage to a monthly decimal rate. In the calculator expression this is APR ÷ 1200.

The other results are based on a constant balance and rate: total interest = monthly payment × number of months, and the ending balance equals the starting balance. The interest-only period is converted from years to months by multiplying by 12. These calculations do not include principal repayments.

Worked Example

Using the calculator’s defaults—$20,000 balance, 8% APR, and a five-year interest-only period:

1. Convert the annual rate to a monthly decimal: 8 ÷ 1200 = 0.0066667.
2. Multiply by the balance: $20,000 × 0.0066667 = $133.333…, which rounds to $133.33 per month.
3. Convert the period to months: 5 × 12 = 60 months.
4. Multiply the displayed monthly payment by 60: $133.33 × 60 = $7,999.80 total interest.
5. With no principal paid, the balance remains $20,000 at the end of the period.

The monthly result is rounded to cents before the total is calculated. As a result, the displayed total uses the rounded payment rather than unrounded precision.

Assumptions

• The balance and APR stay unchanged throughout the entered period.
• Payments are made monthly, with interest calculated on the stated balance for each month.
• Interest is treated as monthly at APR ÷ 12; this is a simplified estimate and may not match a lender’s daily accrual or payment-date conventions.
• No principal is repaid during the interest-only period.
• Fees, taxes, insurance, PMI, escrow, and any other charges are not included.

How Interest-Only Payments Work

An interest-only payment covers interest that accrues for the period, not the amount borrowed. If the balance is $20,000 and the monthly rate is about 0.6667%, that month’s estimated interest is about $133.33. Paying only that amount leaves the $20,000 principal intact. If the payment is less than accrued interest, unpaid interest may be added to the balance under the loan agreement; this calculator does not model that possibility.

Interest-only terms may apply for a limited phase of a loan, after which payments can change. The next phase might require principal repayment, or the remaining balance might be due under the contract. The exact structure is specific to the loan documents, so the calculator’s selected period should not be mistaken for a complete repayment schedule.

How to Use and Read the Result

Use the current outstanding balance, not necessarily the original amount borrowed, and enter the APR as a percentage (for example, enter 8 for 8%, not 0.08). Set the period to the number of years during which you want to estimate interest-only payments. The first result is the estimated monthly interest payment; the total is the sum of those rounded monthly payments over the selected period.

• If the balance changes because of repayments, additional borrowing, or capitalized interest, recalculate using the updated balance.
• If the rate is variable, a single APR cannot describe future payments; calculate separate estimates for possible rates rather than treating one result as a forecast.
• Use the lender’s stated rate and payment schedule. APR and the rate used to accrue interest can be presented differently in loan disclosures.

Common Mistakes and Limits

A frequent error is dividing a percentage by 12 without first converting it to a decimal. For example, 8% per year means 0.08 ÷ 12 per month, not 8 ÷ 12. The calculator’s division by 1200 performs both conversions at once.

Another common misunderstanding is assuming an interest-only payment reduces the debt. Under the assumptions here, it does not: the payment covers interest alone, so the balance at the end remains unchanged. A payment that includes principal would lower the balance and would need a different repayment calculation.

The estimate also simplifies how interest accrues. Loan agreements can use daily balances, different day-count methods, payment dates, variable rates, rounding rules, or fees. This calculator does not reproduce those contract details or calculate a later principal-and-interest payment. Ask the lender for the payment schedule and figures applicable to the actual loan.

Frequently Asked Questions

What is the formula for a monthly interest-only payment?
Multiply the loan balance by the annual percentage rate expressed as a decimal, then divide by 12: balance × APR ÷ 1200 when APR is entered as a percentage.

Does an interest-only payment reduce the principal?
No. A payment equal to the interest leaves principal unchanged. Principal falls only if a payment also covers some of the amount borrowed, subject to how the loan applies payments.

How do I calculate interest-only payments for a different balance?
Replace the balance in the formula with the current amount outstanding and use the applicable annual rate. Recalculate if either changes.

Can an interest-only payment change?
Yes. The amount can change if the balance or interest rate changes, or if the interest-only phase ends. The contract determines how and when payments are recalculated.

Is the total interest shown the full cost of the loan?
No. It estimates interest for the selected period under the stated assumptions. It excludes fees and other charges and does not include principal repayment or interest after that period.

This result is an estimate, and the lender’s disclosure gives the exact figures.

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in bal | Loan balance | $ | 20000 | 0.01 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Interest-only period | years | 5 | 0.0833 | 50
out pmt | Monthly interest-only payment | $ | bal * (apr / 1200) | 2
out tot | Interest paid during period | $ | pmt * yrs * 12 | 2
out end | Balance at end of period | $ | bal | 2