How an interest-only mortgage works
An interest-only mortgage lets a borrower pay only the interest that has accrued on the loan balance during a defined initial period, with none of that monthly payment reducing the principal. The Consumer Financial Protection Bureau (CFPB) describes this interest-only feature as commonly applying to an initial period -- rather than the full loan -- after which the loan converts to a standard payment that repays both principal and interest.
Because the interest-only payment depends only on the outstanding balance and the interest rate, it does not depend on the loan's overall term the way a fully amortizing payment does. The formula is simply principal multiplied by the monthly interest rate: on a $300,000 loan at a 5.5% annual rate, the interest-only payment is $300,000 x (0.055 / 12) = $1,375.00 per month, unchanged for as long as the IO period lasts and the rate stays fixed.
The payment jump when amortization begins
Because no principal is repaid during the interest-only period, the balance owed at the end of that period is identical to the amount originally borrowed. When the IO period ends, the lender recalculates the payment needed to fully repay that same balance, but now over a shorter remaining term than the loan's original length, which produces a materially higher monthly payment -- an effect often called payment shock.
In a worked example, a $300,000 loan at a 5.5% annual rate with a 5-year interest-only period on a 30-year total term has an interest-only payment of $1,375.00. Once the IO period ends, the same $300,000 balance is amortized over the remaining 25 years (300 months) at 5.5%, producing a payment of $1,842.26 -- an increase of $467.26 per month, or approximately 34%, with no change in the interest rate itself.
| Phase | Monthly payment | What it covers |
|---|---|---|
| Interest-only period (years 1-5) | $1,375.00 | Interest only; principal unchanged at $300,000 |
| After amortization begins (years 6-30) | $1,842.26 | Principal and interest, repaying $300,000 over the remaining 25 years |
The no-equity risk during the IO period
Because zero principal is repaid during the interest-only period, a borrower who has made only IO payments builds no home equity from those payments -- the outstanding balance is exactly what it was at origination for as long as the IO period runs. Any equity gained during that period comes only from the property's market value rising or from a down payment made at purchase, neither of which the interest-only payment itself contributes to.
This matters most in a market downturn: a borrower with little or no equity who needs to sell or refinance during or shortly after the IO period has less of a cushion against a decline in the home's value than a borrower who has been steadily paying down principal on a standard amortizing loan over the same years. The combination of no principal reduction and the eventual payment jump is the structural trade-off an interest-only loan carries in exchange for a lower payment early on.
Who interest-only loans suit -- and who they don't
Interest-only structures appear more often in jumbo loans, certain adjustable-rate products, and investment-property financing than in standard 30-year fixed conforming mortgages. They can suit borrowers with variable or back-loaded income -- such as commission-based earners or those expecting a future income increase -- who plan to make voluntary principal payments, refinance, or sell before or shortly after the payment jump occurs.
An interest-only loan is a weaker fit for a borrower who intends to hold the property long-term without a clear plan for the payment increase, since the jump is a certainty built into the loan structure, not a possibility. Borrowers considering an IO loan should model both the interest-only payment and the fully amortizing payment before committing, using the same rate and remaining term the loan will actually apply.
Fixed-rate versus adjustable-rate interest-only loans
An interest-only feature describes how the payment is calculated during the IO period and is a separate question from whether the loan's rate is fixed or adjustable. Some interest-only loans carry a fixed rate for the full term, in which case the only change at the end of the IO period is the shift from interest-only to amortizing; other interest-only loans are also adjustable-rate mortgages, in which case the rate can change independently of, or at the same time as, the shift to amortization.
A borrower evaluating an adjustable-rate interest-only loan is exposed to two separate sources of payment change -- the amortization shift and any rate adjustment -- which can compound if both occur close together. Reviewing the loan's Truth in Lending disclosures for the specific structure, including whether and when the rate can adjust, is a necessary step before relying on any single projected post-IO payment.
Часто задаваемые вопросы
What is an interest-only mortgage?
An interest-only mortgage is a loan in which the required monthly payment, for a defined initial period, covers only the interest accrued on the balance and none of the principal. The Consumer Financial Protection Bureau describes this as an interest-only feature that typically applies to an initial period before the loan converts to a standard payment that repays both principal and interest.
How much can the payment jump after an interest-only period ends?
The size of the jump depends on the loan's balance, rate and remaining term, but it can be substantial because the full original principal must be repaid over a shorter remaining period. In a worked example on a $300,000 loan at 5.5% with a 5-year interest-only period on a 30-year term, the payment rises from $1,375.00 to $1,842.26 once amortization begins over the remaining 25 years, an increase of $467.26, or about 34%.
Does an interest-only mortgage build home equity?
Not from the payments themselves during the interest-only period, since none of that payment reduces the principal balance. Any equity gained during the IO period comes only from the property's market value rising or from a down payment made at purchase, which the interest-only payment does not affect.
Are interest-only mortgages fixed-rate or adjustable-rate?
Both exist. The interest-only feature describes how the payment is calculated -- interest only for a period, then amortizing -- and is separate from whether the underlying rate is fixed or adjustable for the loan's life. An adjustable-rate interest-only loan can see both the amortization shift and a rate change affect the payment, sometimes at the same time.
Who typically takes out an interest-only mortgage?
Interest-only structures are more common in jumbo loans, certain adjustable-rate products, and investment-property financing than in standard 30-year fixed conforming mortgages. Borrowers with variable or expected future income sometimes use them to lower payments early on, provided they have a concrete plan for the payment increase when the IO period ends.
Источники
- Consumer Financial Protection Bureau (CFPB). What is an interest-only mortgage and how does an interest-only feature work? consumerfinance.gov.
- Consumer Financial Protection Bureau (CFPB). Your Home Loan Toolkit — a step-by-step guide to shopping for a mortgage. consumerfinance.gov.
- Federal Reserve Board. A consumer's guide to mortgage refinancing. federalreserve.gov.
- Freddie Mac. Understanding mortgage options and loan types. freddiemac.com.
- Brueggeman WB, Fisher JD. Real Estate Finance and Investments. 15th ed. McGraw-Hill Education, 2019.