Interest-Only Loan Math: The Payment Jump After the Interest-Only Period
October 29, 2026
With an interest-only loan, you pay only the interest for an initial period, typically 5 to 10 years, and the principal balance does not decrease. After that, the loan becomes fully amortizing over the remaining term, and payments rise sharply. The monthly payment during the interest-only period is simply balance × annual rate ÷ 12. For a $300,000 loan at 7%, that is $1,750 a month, compared with $1,995.91 for a standard 30-year amortizing loan.
The lower initial payment of $246 a month looks like a bargain, but the principal remains $300,000 after five years. When the interest-only period ends, you must repay $300,000 over the remaining 25 years, and the payment becomes $2,120.34, which is $125 more than the standard 30-year payment and $370 more than the interest-only payment.
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The interest-only borrower pays $1,750 for 60 months, a total of $105,000, and still owes $300,000. The standard borrower pays $1,995.91 for 60 months, a total of $119,755, and owes about $282,400 afterward. Over the full 30 years, the interest-only loan costs 60 × $1,750 + 300 × $2,120.34 = $105,000 + $636,102 = $741,102, compared with $718,527 for the standard loan, an extra $22,575 in total interest, because the principal was not reduced early.
The risk of payment shock is significant: the payment rises 21% at the reset ($1,750 to $2,120), and if the loan is adjustable, the rate may rise at the same time.
| Interest-only (5 years) | Standard 30-year | |
|---|---|---|
| Payment years 1-5 | $1,750 | $1,995.91 |
| Balance after 5 years | $300,000 | about $282,400 |
| Payment years 6-30 | $2,120.34 | $1,995.91 |
| Total paid over 30 years | $741,102 | $718,527 |
Who Uses Interest-Only Loans?
Interest-only loans are used by borrowers whose income will rise (such as professionals early in their careers), by investors who plan to sell or refinance before the amortization starts, and by those with irregular income who prefer flexibility. Some use the savings in monthly payments to invest, hoping for a higher return than the loan rate, though that adds risk. Real estate investors sometimes use them to maximize cash flow. The strategy depends on the home price rising or income growing, neither of which is guaranteed. If prices fall, you may owe more than the property is worth, with no equity buildup from paydown.
Risks and Safeguards
Payment shock; no equity growth from payments; possible negative equity; refinance risk if rates rise or credit weakens; and in some contracts, a balloon payment. To manage the risk, make voluntary principal payments during the interest-only period, which reduce the balance and the payment after reset. Check whether the loan allows extra payments without penalty. Plan the budget for the higher payment from day one: if you could not afford $2,120 a month, the loan is too risky. Compare with a standard loan or a 15-year mortgage, which offers the lowest total cost.
Regulations and Alternatives
In many countries, regulators restrict interest-only mortgages for owner-occupiers after the 2008 financial crisis, requiring ability-to-repay checks. Home equity lines of credit commonly have interest-only draw periods too. For those seeking lower payments, alternatives include a longer term, an adjustable-rate loan, a larger down payment, or a less expensive home.
Frequently Asked Questions
How is an interest-only payment calculated?
Balance times the annual rate divided by 12.
What happens when the interest-only period ends?
The loan starts to amortize over the remaining term, and the payment rises.
Does an interest-only loan build equity?
Not from payments. Equity grows only if the home's value rises or you pay extra principal.
Is interest-only cheaper overall?
No. It typically costs more in total interest than a standard loan.
Can I make extra principal payments?
Often yes, but check for prepayment penalties.