Interest-Only Payment Formula and Calculator

The interest-only payment formula is monthly payment = loan principal × annual interest rate ÷ 12, with the rate written as a decimal. This calculator estimates the monthly interest-only payment from a loan amount and APR, and shows the interest paid over the selected term if the principal stays unchanged.

Formula

For a fixed annual percentage rate and monthly payments, the interest-only formula is:

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

Because the calculator accepts APR as a percentage, its equivalent expression is round(P × APR ÷ 1200, 2). It rounds the monthly amount to the nearest cent. Total interest over the term is the rounded monthly payment multiplied by the number of monthly payments.

• P = loan principal, in dollars.
• APR = annual percentage rate, entered as a percentage (for example, 8 means 8%).
• 12 = monthly periods per year.
• Term = the number of years entered; the payment count is term × 12 months.

This calculation assumes the principal remains constant throughout the interest-only period. It does not calculate a later payment that repays principal.

Worked Example

Using the calculator’s default inputs—$20,000 principal, 8% APR, and a five-year term:

1. Convert the annual rate to a monthly decimal rate: 8 ÷ 1200 = 0.0066667.
2. Multiply the principal by that monthly rate: $20,000 × 0.0066667 = $133.333…, which rounds to $133.33 per month.
3. Find the number of monthly payments: 5 × 12 = 60.
4. Multiply the displayed, rounded payment by the number of months: $133.33 × 60 = $7,999.80 total interest.

The $7,999.80 figure is interest only. The $20,000 principal is still outstanding at the end of the assumed five-year period, so it is not included in that total.

Assumptions

• The rate remains fixed for the full term.
• Payments are monthly and made at the end of each period.
• Interest is calculated monthly using APR ÷ 12.
• The principal does not decrease during the interest-only period.
• Fees, taxes, insurance, mortgage insurance, and escrow are not included.

How Interest-Only Payments Work

A regular fully amortizing payment includes both interest and some principal. An interest-only payment covers the interest charged for that period, leaving the balance unchanged if the borrower pays exactly the amount due and no other charges or adjustments apply. For a constant balance and rate, the basic monthly interest amount is consequently the same from month to month.

For example, at 8% APR, a $20,000 balance generates about $133.33 in monthly interest under the calculator’s convention. Paying that amount does not reduce the $20,000 balance. If the amount due is not paid in full, the unpaid interest may be handled according to the loan agreement; this simple calculator does not model that possibility.

Interest-only structures can apply for a defined period or under particular loan terms. After that period, payments may change because principal repayment begins, the rate changes, or both. The payment formula here does not forecast those later payments. The loan documents or lender disclosure are needed to identify the actual schedule and any adjustments.

How to Read the Results

The first result is the estimated monthly interest-only payment for the inputs shown. The second is an arithmetic estimate of interest across the stated number of months, calculated using the rounded monthly payment. Neither result represents a payoff amount: the principal remains due unless it is paid separately or the loan terms provide otherwise.

Use the APR and principal that apply to the specific loan when available. The example’s 8% rate is only a calculator default, not a claim about an available or typical rate. Rates, fees, and terms vary; obtain the applicable figures from the lender’s disclosure.

Common Mistakes and Limits

• Entering 8 instead of 0.08 as a percent: enter 8 for 8% APR. The calculator converts the percentage internally.
• Confusing annual and monthly rates: this formula divides APR by 12 and by 100. Do not enter a monthly rate in the APR field.
• Assuming interest-only means the debt is paid off: the formula estimates interest, not principal repayment. The original balance can remain due.
• Ignoring rounding: the calculator rounds the payment to cents and multiplies that displayed amount by the number of months. A lender’s accounting may use more precise daily or periodic calculations, so its totals can differ.
• Treating APR as every cost: the calculation does not add fees or other charges, nor does it model variable rates, irregular payment dates, missed payments, or balance changes.

APR ÷ 12 is the stated monthly-rate convention for this calculator. A loan may accrue interest using a different method or schedule, so the simplified result is not a substitute for the payment terms disclosed for that loan.

Frequently Asked Questions

How do I calculate an interest-only payment?
Multiply the loan principal by the annual rate as a decimal, then divide by 12. For an APR entered as a percentage, the calculation is principal × APR ÷ 1200.

Does an interest-only payment reduce the loan balance?
Not when the payment covers only the interest and there are no other balance adjustments. Principal repayment requires an additional amount or a different payment structure.

What happens when the interest-only period ends?
The payment may change as principal repayment begins or if the rate changes under the contract. The exact amount depends on the remaining balance, rate, repayment period, and loan terms; this calculator does not determine it.

Is total interest the same as the amount needed to repay the loan?
No. The calculator’s total is interest across the selected period. It excludes the outstanding principal, which remains to be repaid under the assumed interest-only arrangement.

Why might a lender’s figure differ from this estimate?
Actual calculations can depend on the contract’s rate method, day-count convention, payment dates, fees, rate changes, and rounding. Review the lender’s disclosure for the applicable calculation and schedule.

This result is an estimate for educational arithmetic; the lender’s disclosure gives the exact figures.

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in p | Loan amount | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Loan term | years | 5 | 0.0833 | 50
out pmt | Monthly interest-only payment | $ | round(p * apr / 1200, 2) | 2
out int | Total interest over term | $ | pmt * yrs * 12 | 2