Interest-Only Payment Calculator and Formula

An interest-only payment is the amount needed to cover the interest accruing during a payment period, without reducing the loan principal. Enter the loan amount, annual interest rate, and term to estimate the monthly interest-only payment and the total interest paid if that payment stays unchanged for the full term.

Formula

For a fixed principal balance and a nominal annual interest rate divided into monthly periods, the monthly interest-only payment is:

Monthly payment = P × (APR ÷ 100) ÷ 12, or equivalently P × APR ÷ 1,200.

• P is the loan principal, in dollars.
• APR is the annual percentage rate entered as a percentage per year, such as 8 for 8%.
• 12 converts the annual rate to a monthly rate, assuming monthly periods.

The calculator rounds the monthly payment to the nearest cent. Its second result is that rounded payment multiplied by the number of months in the term: Total interest = monthly payment × years × 12. It assumes every payment covers interest only, so the principal balance does not decline. The rate is treated as fixed for the entire term.

Worked Example

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

1. Convert the annual rate to a monthly rate: 8 ÷ 1,200 = 0.0066667, or about 0.66667% per month.
2. Multiply the principal by that monthly rate: $20,000 × 0.0066667 = $133.3333…
3. Round the monthly interest payment to cents: $133.33 per month.
4. Convert the term to months: 5 × 12 = 60 months.
5. Multiply the rounded payment by the months: $133.33 × 60 = $7,999.80 total interest.

That total is based on the calculator’s rounded monthly payment. The principal remains $20,000 throughout this simplified example, so it is not included in the $7,999.80 interest figure and would still be due separately under the loan’s terms.

Assumptions

• The interest rate remains fixed for the full term.
• Payments are monthly and made at the end of each monthly period.
• Interest is calculated using a monthly rate equal to the annual rate divided by 12.
• The estimate excludes fees, taxes, insurance, PMI, escrow, and any charges not entered as inputs.
• The loan is interest-only for the entire entered term; no principal payments, missed payments, or rate changes are modelled.

When This Calculation Applies

This calculation is useful when a loan’s scheduled payment covers accrued interest but does not pay down principal during a specified period. It can also help explain the interest component of a loan payment. It is not the regular payment formula for a fully amortizing loan: an amortizing payment includes both interest and principal, so the balance generally falls over time.

Interest-only periods may be part of a loan’s schedule rather than its entire life. If the interest-only phase ends and principal must then be repaid, the later payment can differ substantially from this estimate. To work out that later payment, you would need the balance remaining at the transition, the rate then applicable, and the number of repayment periods remaining. This calculator does not infer or model those terms.

How to Read the Result

The monthly figure is an estimated interest payment for one month at the stated balance and rate. The total-interest figure adds those monthly amounts across the full term, assuming the payment remains exactly the rounded amount shown. Neither number is the amount required to repay the loan in full. If no principal is paid during the term, the original principal remains outstanding.

For instance, a $133.33 monthly payment in the example does not mean the $20,000 debt is gradually being paid off. It represents interest only under the stated assumptions. A separate principal payment, balloon payment, or later repayment schedule may be specified in the loan documents; those arrangements are not included here.

Common Calculation Mistakes and Limits

• Using a whole-number rate as a decimal incorrectly: enter 8 for 8%, not 0.08. The formula divides the percent input by 1,200.
• Confusing interest-only with amortizing payments: this formula does not divide principal across the term. It therefore does not calculate a payoff payment.
• Ignoring the outstanding balance: interest changes if the balance changes. The calculator assumes the full original principal remains outstanding.
• Mixing periods: the formula assumes monthly payments and an annual rate divided by 12. A daily accrual method, irregular payment schedule, or different compounding convention can produce a different amount.
• Rounding differences: lenders may calculate and round interest period by period using their contract’s method. Small differences can accumulate, so the calculator’s total is an estimate.
• Assuming APR captures every cost: this arithmetic uses the entered annual rate as a monthly rate. It does not add fees or other charges, even if a particular rate disclosure accounts for costs differently.

The formula is most straightforward for a fixed balance, fixed annual rate, and regular monthly periods. For a variable rate or changing balance, calculate each period using the balance and rate applicable to that period, then add the period amounts; the simple total shown here will not represent that schedule.

Frequently Asked Questions

How do I calculate an interest-only payment?
Multiply the principal by the annual rate expressed as a decimal, then divide by 12 for monthly periods. For example, at 8%, use principal × 0.08 ÷ 12.

Does an interest-only payment reduce the loan balance?
Not if the payment covers only interest. Unless principal is paid separately, the balance stays the same and remains to be repaid under the loan terms.

Is an interest-only payment lower than an amortizing payment?
For the same principal, rate, and payment schedule, the interest-only amount excludes principal repayment, while an amortizing payment includes principal as well as interest. Actual payment terms depend on the loan agreement.

Why might my lender’s number differ from this calculator?
Possible reasons include daily rather than monthly interest accrual, payment dates, rounding rules, fees, a changing balance or rate, or a different contractual calculation method. Check the loan disclosure for the applicable figures and method.

What happens when the interest-only period ends?
The remaining principal may become payable or be repaid over a later schedule, depending on the contract. This calculator does not project that later payment; the schedule and remaining term are needed.

This result is an estimate, and 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