Interest-Only Loan Payoff Calculator and Balance Guide

An interest-only loan payoff calculator estimates the remaining balance and the amount needed to pay off the loan after a chosen number of payments. Enter the original loan amount, annual interest rate, interest-only period, and any principal paid; the results show the balance and estimated payoff amount, before lender-specific charges.

Formula

The calculator estimates the balance by applying monthly interest and subtracting the interest-only payments and separately paid principal. The balance formula is B = P(1 + r)ⁿ − I × [((1 + r)ⁿ − 1) / r] − E. Here, B is estimated remaining principal in dollars, P is the starting balance in dollars, r is the monthly rate as a decimal (APR ÷ 1,200), n is the number of monthly periods (years × 12), I is the monthly interest-only payment in dollars (P × r), and E is additional principal paid in dollars.

The other output, monthly interest-only payment, is I = P × r. This model assumes the interest-only payment is calculated from the original balance and that no principal is included in each scheduled payment. With no extra principal paid, the balance stays at the original amount under the stated monthly-payment assumptions. The balance expression accounts for interest accruing and being paid each month; it is not a model of unpaid, capitalized interest.

Worked Example

Using the defaults in the calculator: starting balance P = $200,000, APR = 6%, elapsed period = 5 years, and extra principal E = $0.

1. Convert APR to a monthly decimal rate: r = 6 ÷ 1,200 = 0.005.
2. Convert elapsed years to months: n = 5 × 12 = 60.
3. Find the scheduled interest-only payment: I = $200,000 × 0.005 = $1,000 per month.
4. Apply the balance formula. Each month, interest of $1,000 is paid and no principal is reduced; with E = $0, the remaining principal is $200,000.

Estimated remaining balance: $200,000.00. Monthly interest-only payment: $1,000.00. These are the displayed results rounded to cents.

Assumptions

• The interest rate is fixed throughout the elapsed period.
• Payments are made monthly at the end of each period, with no missed or late payments.
• Interest compounds monthly at APR divided by 12; the APR is treated as a nominal annual rate.
• The model excludes fees, taxes, insurance, PMI, escrow, payment caps, rate changes, and lender-specific payoff charges.

When to Use the Calculator

Use it for a rough principal-balance estimate when considering an interest-only loan or checking how scheduled interest payments differ from principal repayment. It can also help illustrate what a lump-sum principal payment might do to an assumed balance. The extra-principal field subtracts the amount entered directly; it does not model the date or timing of that payment, or any lender rules about applying it.

For an actual payoff, the figure from this calculator is not a payoff quote. A lender’s payoff calculation may depend on the payment date, interest accrued since the last payment, the contract’s rate and day-count method, pending transactions, and charges specified by the loan documents. Ask the lender or servicer for a dated payoff figure and instructions for sending payment.

How to Interpret the Results

An interest-only payment generally covers interest for the period, not principal. Consequently, making every scheduled interest-only payment does not by itself pay down the amount borrowed. In the calculator’s example, $1,000 per month services the assumed interest on $200,000 at 6% APR, while the principal remains $200,000 after five years.

That does not mean every real interest-only loan keeps the same balance. A contract may have a changing rate, a limited interest-only phase, scheduled principal payments, or other payment terms. After an interest-only phase, required payments can change because principal must then be repaid over the remaining term. This calculator estimates the balance during the interest-only phase; it does not calculate the later amortizing payment or the loan’s final payoff schedule.

Common Errors and Model Limits

• Confusing interest paid with principal repaid: Interest payments compensate for borrowing and ordinarily do not reduce principal in an interest-only structure.
• Entering the wrong rate: Enter the annual percentage rate as a percentage, such as 6 for 6%, not 0.06. The formula converts it to a monthly decimal.
• Assuming the calculator knows the contract: The rate, payment structure, timing, and extra-principal treatment may differ from the assumptions here. Check loan documents and current account details.
• Using an undated estimate as a payment instruction: Daily interest or other amounts can change the sum due. Obtain a payoff quote for the date you intend to pay.
• Rounding or timing differences: The displayed values are rounded to cents. Actual payment allocation and accrued interest can differ according to the lender’s calculation method.

The calculation is intentionally limited to a fixed-rate, monthly interest-only model. It does not determine whether a payment is due, verify the account balance, model variable-rate changes, or include fees and other amounts. Enter the loan’s actual terms where known and treat any unmodelled amounts separately.

Frequently Asked Questions

Does an interest-only payment reduce the loan balance?
Usually, a payment that covers only interest does not reduce principal. The balance may fall if the borrower makes principal payments or if the contract applies part of a payment to principal.

How do I calculate the monthly interest-only payment?
For a fixed annual APR with monthly periods, multiply the balance by APR ÷ 1,200. For example, $200,000 at 6% gives $1,000 in monthly interest under this calculator’s assumptions.

What happens when the interest-only period ends?
The payment terms may change, often requiring principal repayment as well as interest. The amount depends on the contract, remaining balance, remaining term, and applicable rate; this calculator does not estimate that later payment.

Is the estimated balance the amount needed to pay off my loan?
No. It estimates principal under simplified assumptions, not a dated payoff amount. Ask the lender or servicer for the exact figure and payment instructions.

Can extra payments reduce an interest-only balance?
They can reduce principal if the lender applies them that way and the loan terms permit it. The calculator subtracts the entered extra amount as a simple adjustment and does not model timing or contractual restrictions.

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

Related

• Biweekly Payment Calculator: Payoff Time and Interest Saved
• Refinance Calculator: New Payment, Monthly Saving and Break-Even
• Extra Payments on a Loan Calculator: Months and Interest Saved

in p | Starting loan balance | $ | 200000 | 0 | 100000000
in apr | Interest rate (APR) | % | 6 | 0.01 | 100
in yrs | Interest-only period elapsed | years | 5 | 0.0833 | 50
in extra | Principal paid separately | $ | 0 | 0 | 100000000
out bal | Estimated remaining balance | $ | p * (1 + apr / 1200) ^ (yrs * 12) - (p * (apr / 1200) * ( ((1 + apr / 1200) ^ (yrs * 12) - 1) / (apr / 1200) )) - extra | 2
out pay | Monthly interest-only payment | $ | p * apr / 1200 | 2