Interest-Only Loan Amortization Calculator With Balloon

This interest-only loan amortization calculator estimates the monthly interest payment, the final balloon balance, and total interest for a loan that makes interest-only payments before a lump-sum payoff. Enter the principal, annual interest rate, and term; the results assume the principal remains unpaid until the balloon is due.

Formula

For a fixed-rate, interest-only loan with monthly payments, the monthly interest payment is the principal multiplied by the annual percentage rate divided by 1,200. The principal is not reduced by those payments.

• pmt = round[p × (APR ÷ 1,200), 2], the monthly interest payment in dollars.
• bal = p, the balloon principal due at the end in dollars.
• int = pmt × 12 × years, the sum of scheduled interest payments in dollars.
• tot = int + p, the scheduled interest plus balloon principal in dollars.
• p is the original loan principal in dollars; APR is the annual rate entered as a percentage; years is the term in years.

Rounding the monthly payment to cents before multiplying is important: the displayed total uses the payment the calculator shows, rather than an unrounded value.

Worked Example

Using the default inputs—$20,000 principal, 8% APR, and five years:

1. Convert the annual percentage rate to a monthly decimal rate: 8 ÷ 1,200 = 0.0066667.
2. Calculate monthly interest: $20,000 × 0.0066667 = $133.333…, rounded to $133.33.
3. There are 5 × 12 = 60 monthly payments. Total scheduled interest is $133.33 × 60 = $7,999.80.
4. The principal has not been amortized, so the balloon is $20,000.00.
5. Total scheduled payments including the balloon are $7,999.80 + $20,000 = $27,999.80.

Assumptions

• The interest rate is fixed for the full term.
• Payments are monthly, with interest calculated on the stated principal; the estimate treats each scheduled payment as one month of interest.
• Compounding is monthly at APR ÷ 12. The principal does not decline before maturity.
• Fees, taxes, insurance, PMI, escrow, late charges, prepayments, and payment-date variations are not included.

How an Interest-Only Balloon Loan Works

An amortizing loan ordinarily includes principal and interest in each scheduled payment. As principal is paid down, the remaining balance falls. An interest-only structure works differently during its interest-only period: scheduled payments cover interest, while the original principal remains due. If the loan requires the outstanding amount in one payment at maturity, that final amount is the balloon.

“Interest-only” describes the scheduled payment structure, not a promise that the rate or payment will never change. Some contracts have an adjustable rate, an interest-only period shorter than the full loan term, or later payments that repay principal. This calculator models only a fixed-rate structure with interest-only payments for the entire term and a balloon of the full principal at the end.

How to Read the Results

The monthly payment is an estimate of interest only; it is not the amount that pays off the loan. The balloon result is the principal still outstanding at maturity. Total interest adds the rounded monthly payment across the entered number of months, and total paid adds the balloon principal.

For the example, paying $133.33 each month does not turn a $20,000 balance into a smaller balance under the model. The borrower would still owe $20,000 at the end, in addition to having paid $7,999.80 in scheduled interest. A contract may define the final payment differently if there are unpaid charges, rate changes, missed payments, or other adjustments.

When This Calculator Is Useful

Use the estimate to understand the payment pattern and identify the size of the final principal obligation in a simple interest-only balloon arrangement. It can help with general cash-flow comparisons, but it does not determine whether a loan is suitable, affordable, or available. Use the actual principal, rate, payment schedule, and maturity date disclosed by the lender when checking a specific offer.

• Confirm whether the quoted rate is fixed or variable and how it may change.
• Check whether payments are truly interest-only for the entire term or only an initial period.
• Find out how the contract calculates accrued interest, payment dates, and the maturity balance.
• Account separately for any fees or other amounts that are due, since they are not included here.

Common Misconceptions and Limits

A frequent misconception is that making every monthly payment automatically pays off the loan. In this model, it does not: the payments cover interest alone. Another is that the balloon is the sum of all remaining monthly payments; here it means the unpaid principal due at maturity, separate from the scheduled interest payments.

Actual loan documents may use daily interest accrual, different payment intervals, variable rates, or an interest-only period followed by amortizing payments. Rounding and payment timing can also change exact totals. This calculator does not model those contract-specific details, delinquency, extra principal payments, or refinancing. Ask the lender for a payment schedule and maturity balance based on the loan terms.

Frequently Asked Questions

How do you calculate an interest-only loan payment?
For monthly payments at a fixed annual percentage rate, multiply the principal by APR ÷ 1,200. For example, $20,000 at 8% produces $133.33 per month after rounding to cents.

Does an interest-only payment reduce the loan balance?
Not in the structure modelled here. The payment covers interest, leaving the principal unchanged. A contract with extra principal payments or a later amortizing phase would have a different balance.

How is the balloon payment calculated?
In this calculator, the balloon equals the original principal because no principal is repaid before maturity. Additional unpaid charges or contract provisions could change the amount actually due.

Is the balloon included in total interest?
No. The balloon is principal, not interest. Total interest is the sum of the monthly interest payments; total paid combines that interest with the balloon principal.

Can the payment change during the loan?
It can if the agreement has a variable rate, a changing payment schedule, or other terms not represented here. The calculator assumes a fixed rate and consistent monthly payments.

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

Related

• APR Calculator: The True Rate of a Loan With Fees
• Simple Interest Loan Calculator: I = P × r × t
• Interest-Only Loan Calculator: Payment During and After the Interest-Only Period

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 bal | Balloon payment at maturity | $ | p | 2
out int | Total interest over term | $ | pmt * yrs * 12 | 2
out tot | Total paid including balloon | $ | pmt * yrs * 12 + p | 2