Seller financing formula: payment, balloon, and amortization

The short answer

A seller-financed note has two clocks:

• The amortization term sets the monthly principal-and-interest payment.
• The balloon term sets when the remaining balance must be paid.

The note amount is the contract price minus the down payment. If the buyer puts down $57,000 on a $285,000 purchase, the seller note is $228,000. The seller is financing the balance, not the entire price.

The basic payment formula is the same one used for any amortizing loan:

Payment = P × r × (1 + r)^n / ((1 + r)^n − 1)

Where:

• P is the note amount.
• r is the monthly interest rate (annual rate divided by 12).
• n is the number of payments in the amortization term.

For a $228,000 note at 7% with a 30-year amortization, the payment is about $1,516.89 per month. The 5-year balloon does not change that payment. It changes the balance that is still owed after five years.

The seller financing calculator builds the full schedule, shows the balloon, and stress-tests the refinance.

Why the balloon is a separate risk

A 30-year amortization with a 5-year balloon is not a 30-year loan. It is a 5-year loan with a 30-year payment.

After five years of payments on the example note, the balance is roughly $214,620. The buyer must pay that balance at the balloon. Most buyers do not have that cash sitting in a checking account. The realistic exit is a refinance or a sale.

That creates three questions:

1. Will the property be worth enough? A refinance lender will apply an LTV limit, not the buyer’s purchase price.
2. Will the rent support the new payment? A new loan at a higher rate can fail a DSCR test even if the old seller note passed.
3. Will the buyer qualify? Credit, income, entity structure, and lender guidelines can change over five years.

A balloon is not automatically bad. It can be a useful bridge when the buyer needs time to improve the property, season the rental, or wait out a higher-rate market. It is dangerous when the deal only works if everything goes right.

Amortization, balloon, and interest-only

A seller note can combine three features:

• Amortization: how the payment is calculated.
• Balloon: when the unpaid balance is due.
• Interest-only period: a period when the payment covers interest but not principal.

An interest-only period lowers the initial payment and increases the balance at the balloon. If the note is interest-only for three years and then amortizes, the payment steps up after year three. The buyer needs to be able to carry the higher payment, not just the teaser payment.

The calculator shows both the initial payment and the payment after the interest-only period. It also warns when an interest-only period is present, because a low initial payment can hide a later affordability problem.

Work the example backward

Start with the exit, then decide whether the note terms are realistic.

Step 1: Project value at the balloon

If the property is worth $285,000 today and appreciates at 3% per year, the value in five years is about $330,393. That is a projection, not a guarantee. Run a flat-value case as well.

Step 2: Apply a refinance LTV

At a 75% LTV, the maximum new loan is about $247,795. Subtract the balloon balance and refinance closing costs from that loan amount to estimate the cash out or cash needed.

In the example, the refinance produces a positive cash-out of roughly $26,980 before considering any lender DSCR constraint. A positive cash-out does not mean the refinance is approved.

Step 3: Test the new payment and DSCR

The new loan at 6.75% over 30 years has a principal-and-interest payment of about $1,607 per month. Add taxes, insurance, and HOA to get PITIA. Then compare the rent with the new PITIA.

A refinance can clear the balloon and still fail a lender’s rent-coverage test. The calculator shows both the cash-out and the coverage ratios so the two questions stay separate.

What changes the balloon balance

The balloon balance moves with:

• Note amount: a larger note leaves a larger balance.
• Note rate: a higher rate means less principal is paid down early.
• Amortization term: a longer term means slower principal repayment.
• Interest-only period: no principal is paid during the IO period.
• Balloon term: a longer balloon gives the buyer more time to pay down principal.

A common mistake is to compare two offers by payment alone. A 30-year amortization with a 5-year balloon can have a lower payment and a larger balloon than a 20-year amortization with the same balloon. The buyer needs to plan for the balance, not just the monthly check.

The seller’s side

The seller should also look at the note as an investment:

• Yield: the note rate compared with what the seller could earn elsewhere.
• Credit risk: the buyer’s down payment, reserves, and rental experience.
• Collateral risk: the property value and the seller’s position if the buyer defaults.
• Liquidity: the seller cannot spend the balloon balance until it is paid or refinanced.
• Tax: the installment sale can defer gain, but recapture and interest are taxed differently.

A seller who needs all the cash at closing should not offer a long balloon with a thin down payment. A seller who wants a steady income stream may prefer a shorter amortization or a larger down payment to reduce the balloon risk.

A practical checklist

Before signing a seller-financed note:

• Confirm the note amount equals the price minus the down payment.
• Confirm the amortization and balloon terms are written separately.
• Build the amortization schedule through the balloon month.
• Run the refinance at a flat value, a lower value, and a higher rate.
• Compare the new PITIA with the market rent.
• Check the buyer’s reserves for the refinance closing costs.
• Confirm the note rate against the applicable federal rate.
• Get the note, mortgage or deed of trust, and disclosure documents reviewed by counsel.

The math is not complicated. The risk is in treating a balloon payment as a promise instead of a deadline.