Partially Amortized Loan Calculator

A partially amortized loan separates the payment schedule from the maturity date. The monthly payment is calculated as if the debt will be paid over a long amortization period, but the actual payment period ends sooner. Whatever balance remains at that earlier date becomes the balloon payment. This calculator estimates both pieces: the monthly principal-and-interest payment and the lump sum due at the end of the shorter payment period.

The structure is common in some commercial real estate loans, business loans, private notes, and seller-financed purchases. It can make monthly payments more manageable, but it shifts a large financing question into the future. For a loan that fully amortizes by the last scheduled payment, use the loan calculator. For home-loan context, compare the payment with the mortgage calculator. To see the same balance decline through time, use the amortization calculator.

How to use this calculator

Enter the full loan amount borrowed, the annual interest rate, the amortization time, and the shorter payment period. The amortization time sets the monthly payment. The payment period sets how many of those monthly payments are made before the balloon is due. For example, a 30-year amortization with a 10-year payment period means the payment is sized like a 30-year loan, but the balance is tested after only 120 monthly payments.

The calculation rounds years into whole months. It also caps the payment period at the amortization period. If you enter a 10-year amortization and a 15-year payment period, the calculator uses 120 payment months, not 180, because a fully amortizing 10-year schedule has no balance left after month 120.

Formula used by the calculator

The monthly interest rate is the annual rate divided by 12 and by 100. The amortization months are the amortization years multiplied by 12 and rounded to the nearest whole month. The monthly payment is:

$\text{monthly payment} = \text{loan amount} \times \frac{\text{monthly rate}}{1 - (1 + \text{monthly rate})^{-\text{amortization months}}}$

After the shorter payment period, the remaining balance is:

$\text{remaining balance} = \text{loan amount} \times (1 + \text{monthly rate})^{\text{payments made}} - \text{monthly payment} \times \frac{(1 + \text{monthly rate})^{\text{payments made}} - 1}{\text{monthly rate}}$

The balloon payment is the remaining balance floored at zero:

$\text{balloon payment} = \max(0, \text{remaining balance})$

When the interest rate is zero, the monthly payment is the loan amount divided by the amortization months, and the remaining balance falls by that same principal amount each month.

Example: estimating a balloon balance

Use a $1,000,000 loan, 10% annual interest, a 30-year amortization, and a 10-year payment period. The amortization schedule has 360 months. The payment period is 120 months. The monthly rate is 10% divided by 12, or about 0.8333% per month.

The calculator’s payment formula gives a monthly payment of $8,775.72. Making that payment for 120 months produces total regular payments of $1,053,085.88 during the payment period. Because the payment was sized for 360 months, the loan is not close to paid off at month 120. The remaining balance is $909,380.19, so the primary result is a balloon due after the payment period of $909,380.19.

Total cash paid through the balloon date is $1,962,466.08: the regular payments plus the balloon. Subtracting the original $1,000,000 loan amount leaves $962,466.08 of interest over the modeled loan period. That number is not the interest for a full 30-year loan; it is the cost through the 10-year payment period plus the balloon payoff.

How it helps borrowers and lenders

The calculator makes the tradeoff visible. A partially amortized structure can produce a lower required monthly payment than a 10-year fully amortizing loan, which may fit a business plan or commercial lease income. But the lower payment does not make the debt disappear. It leaves a specific refinancing or payoff target. Borrowers can use the balloon amount to plan a sale price, refinance application, reserve account, or investor capital need.

For lenders and sellers, the balloon estimate shows how much principal remains exposed at maturity. That can affect collateral requirements, personal guarantees, renewal terms, and interest-rate risk. If the loan is tied to real estate, compare the balloon with expected property value and with debt service coverage. If the borrower plans to accumulate cash for the balloon, the savings-goal calculator can help convert that future obligation into a monthly savings target.

Caveats before relying on the result

Partial amortization is sensitive to the assumed rate, term, and maturity. A small rate increase can raise the payment, and a shorter payment period can leave a much larger balloon. The calculator assumes fixed monthly payments, monthly interest, no fees, and no extra principal payments. It does not model adjustable rates, interest-only periods, lender renewal options, default interest, late fees, or tax and insurance escrow.

The biggest practical risk is refinance risk. A borrower may expect to refinance the balloon, but future credit standards, income, property value, and market rates are unknown. Build a backup plan before signing a loan that depends on a large maturity payment.

Sources

• Consumer Financial Protection Bureau, What is a balloon payment and when is one allowed? — consumer explanation of balloon payments and regulatory context.
• Consumer Financial Protection Bureau, Mortgage resources — borrower education on mortgage terms and repayment obligations.
• Electronic Code of Federal Regulations, 12 CFR 1026.43 — federal ability-to-repay and qualified mortgage rule text involving balloon-payment rules.

Formula references

• Claim: r=annualRate/1200; PMT=Pr/(1-(1+r)^(-12amortizationYears)); balance_k=P(1+r)^k-PMT*((1+r)^k-1)/r; use straight-line branches when r=0. Source: Regulation Z Appendix J — Annual Percentage Rate Computations, Electronic Code of Federal Regulations / CFPB. Version: 2025 annual edition of 12 CFR Part 1026, Appendix J; accessed 2026-07-09. Jurisdiction: United States federal consumer credit. Accessed 2026-07-09.
• Claim: r=annualRate/1200; PMT=Pr/(1-(1+r)^(-12amortizationYears)); balance_k=P(1+r)^k-PMT*((1+r)^k-1)/r; use straight-line branches when r=0. Source: Principles of Finance, OpenStax, Rice University (peer-reviewed open textbook). Version: 2022 first edition, ISBN 978-1-951693-54-1. Jurisdiction: Jurisdiction-neutral finance definitions. Accessed 2026-07-09.