Interest-Only Mortgage Calculator
Enter your loan amount, interest rate, total loan term, and the length of your interest-only period. This calculator shows your monthly interest-only payment, what your payment jumps to once the full-amortization phase begins, total interest paid over both phases, and a side-by-side comparison with a standard mortgage. A year-by-year schedule lets you see exactly how your balance and payments evolve.
What is an interest-only mortgage?
An interest-only mortgage lets you pay just the interest portion of your loan for a set period, typically 3 to 10 years. During this phase, your monthly payment is lower because you are not reducing the principal balance at all. Once the interest-only period ends, the remaining principal must be repaid over the rest of the loan term, so your monthly payment rises - sometimes sharply. Some lenders then convert to a standard amortizing schedule; others require a balloon payment. The structure is common among investors, borrowers expecting income growth, or buyers who plan to sell or refinance before the IO period expires.
How to read the results of this calculator
The primary result is your monthly interest-only payment, which equals your loan principal multiplied by the monthly interest rate (annual rate divided by 12). Once the interest-only phase ends, the full principal still needs to be repaid over the remaining term, so the calculator applies the standard PMT formula to compute the new, higher amortizing payment. The payment increase and extra lifetime interest figures show you the true cost of deferring principal repayment. The comparison column shows what you would pay on a plain fixed-rate mortgage at the same rate, so you can judge whether the lower initial payments are worth the higher total cost.
When an interest-only mortgage can make sense
Interest-only loans suit a narrow set of situations. Real estate investors sometimes use them to maximize cash flow on a rental property while they add value before selling. Professionals expecting a significant income increase (a doctor completing residency, for example) may use the lower payments to manage cash flow in the short term. Borrowers who plan to sell within the IO window get the benefit of lower payments without facing the payment shock. Outside these cases, the higher lifetime interest cost and the payment jump at the end of the IO phase make standard mortgages the safer choice for most long-term homeowners.
The payment shock: what happens when the interest-only period ends
The most important risk in an interest-only loan is the payment shock when the IO phase closes. Because you made no principal payments, the full original loan amount still needs to be repaid - but now over a shorter remaining term. For example, on a 30-year loan with a 10-year IO period, the entire principal must be amortized over just 20 years. That compresses a 30-year payoff into 20 years of payments, producing a meaningfully higher monthly bill. This calculator shows you both figures side by side, and the year-by-year schedule lets you see exactly when the transition happens and what the new balance looks like.
Interest-Only vs. Standard Mortgage at a Glance
| Feature | Interest-Only Loan | Standard Mortgage |
|---|---|---|
| Initial monthly payment | Lower (interest only) | Higher (principal + interest) |
| Principal repayment during IO period | None | Yes, from payment 1 |
| Equity built during IO phase | Only from appreciation | From payments and appreciation |
| Payment after IO period ends | Jumps significantly | Stays the same |
| Total lifetime interest | Higher | Lower |
| Typical IO period length | 3-10 years | N/A |
| Balloon payment risk | Possible if short term | None on fixed loans |
| Best suited for | Short-term owners, investors | Long-term homeowners |
Key differences between interest-only and conventional fully-amortizing mortgages.
Frequently asked questions
How is the interest-only monthly payment calculated?
The interest-only payment is simply the loan principal multiplied by the monthly interest rate. Monthly rate = annual rate / 12. So for a $400,000 loan at 6.5%, the monthly rate is 0.065 / 12 = 0.005417, and the payment is $400,000 x 0.005417 = $2,166.67. No principal is repaid during this phase.
What happens to my payment when the interest-only period ends?
When the interest-only phase ends, the full original balance is still outstanding. Your lender recalculates the payment using the standard amortization formula over the remaining loan term. Because you are now paying both interest and principal over a shorter window, the monthly payment rises noticeably - sometimes by 30% to 60% or more depending on rates and the remaining term.
Do I build any equity during the interest-only period?
You build no equity through payments during the IO phase, because every dollar goes to interest. Any equity you gain comes only from your property appreciating in value. If property values fall during the IO period, you can end up owing more than the home is worth, a situation called being underwater.
Are interest-only mortgages riskier than standard mortgages?
Yes, generally. The three main risks are: no equity build-up through repayment, a significant payment shock at the end of the IO period, and higher lifetime interest costs because you are paying interest on the full principal for longer. These risks are manageable if you have a clear plan for the IO period - selling, refinancing, or absorbing the higher payment - but they make interest-only loans unsuitable for most long-term homeowners.
Can I pay principal during the interest-only period?
Most interest-only loans allow you to make extra principal payments at any time, and doing so reduces your balance before the amortizing phase begins, which lowers the eventual higher payment. Check your loan agreement for any prepayment penalties before making extra payments.
How much more interest does an interest-only loan cost compared to a standard mortgage?
The extra interest depends on the length of the IO period, the loan size, and the rate. The key driver is that your full principal continues accruing interest throughout the IO phase instead of shrinking month by month. For a $400,000 loan at 6.5% with a 10-year IO period on a 30-year term, the extra interest versus a standard 30-year fixed can be $50,000 or more. This calculator shows the exact figure for your inputs.