Interest-Only Guide

Interest-Only Mortgage Calculator: The Real Cost

George Smith
George Smith — Founder, Klickify Agency

You hear about interest-only mortgages. The payment is much lower. On a $400,000 loan, a standard 30-year payment at 6.8% is $2,609. An interest-only payment is only $2,267 — $342 less per month. That sounds great. But there is a catch.

With an interest-only mortgage, you do not pay down any principal during the interest-only period (typically 5 or 10 years). After that, the loan recasts to a fully amortizing payment over the remaining term. Your payment jumps significantly. And you have built zero equity from payments. If home prices drop, you could owe more than the home is worth.

Most online calculators do not show the full cost of interest-only loans. They only show the initial low payment. Lenders love interest-only loans because they keep you paying interest forever. Bankrate's interest-only calculator is buried under pages of ads. NerdWallet's calculator is limited to the first 5 years. Truly Free Mortgage Calculator shows the entire 30-year timeline, including the payment shock at the end of the interest-only period.

The Real Math of an Interest-Only Mortgage

Let me compare a standard 30-year fixed mortgage to a 5-year interest-only mortgage (then 25-year amortizing) on a $400,000 loan at 6.8%.

Standard 30-year fixed: Monthly payment $2,609. After 5 years, you have paid down about $19,000 in principal. Balance remains $381,000.

Interest-only (first 5 years): Monthly payment $2,267 (interest only). You pay $2,267 × 60 = $136,020 over 5 years. All interest. Your loan balance remains $400,000. Zero principal paid.

Then, after 5 years, the loan recasts. The remaining term is 25 years (300 months). The new payment on a $400,000 balance at 6.8% over 25 years is $2,777. That is $168 higher than the standard 30-year payment ($2,609). So your payment jumps from $2,267 to $2,777 — an increase of $510 per month.

Over the full 30-year period, the interest-only loan total paid is: (5 years × $2,267 × 12) + (25 years × $2,777 × 12) = $136,020 + $833,100 = $969,120. The standard 30-year total paid is $2,609 × 360 = $939,240. The interest-only loan costs you $29,880 more, and you end with zero equity (unless home prices appreciated).

The only scenario where an interest-only loan makes sense is if you have a very high income but are temporarily cash-constrained, or if you are an investor who plans to sell before the interest-only period ends. For most homeowners, it is a bad deal.

Step-by-Step: Use the Interest-Only Calculator

1. Go to trulyfreemortgage.com and select Interest-Only Calculator
Found under "Mortgage Types."
2. Enter your loan amount
Example: $400,000.
3. Enter the interest rate
Interest-only loans typically have slightly higher rates than standard loans. Use 6.8% for comparison, but actual may be 7.0%.
4. Enter the interest-only period in years
Common terms: 5 years or 10 years.
5. Enter the total loan term (usually 30 years)
Interest-only period plus amortizing period equals total term.
6. Click "Calculate"
The calculator will show the interest-only payment, the recast payment after the IO period, the standard payment (for comparison), total interest paid, and the payment shock amount.
7. Look at the "break-even" analysis
The calculator will show how many years you need to stay in the home for the interest-only loan to be worse than the standard loan. Usually, it is worse from day one.
8. View the amortization table
You will see that for the first 5 years, your balance stays flat. Then it starts dropping.

The Lead-Gen Problem With Free Mortgage Calculators

Bankrate's interest-only calculator is designed to make the product look attractive. The page has minimal warnings about the risks. Instead, it has a "Get Quotes from Lenders" button. Lenders love interest-only loans because they generate higher total interest. Bankrate gets paid per lead.

NerdWallet's interest-only content is actually more responsible. They warn about the risks. But their calculator is basic — it does not show the payment shock clearly. And of course, they want your email to "help you find the right loan."

LendingTree does not have an interest-only calculator. They want you to apply for a loan, and then they will try to sell you an interest-only product after they have your information.

Truly Free Mortgage Calculator gives you the full picture. I do not benefit from you choosing any particular loan type. My revenue is from ads. So I can tell you the truth: interest-only is usually a bad idea for homeowners.

See the Full Interest-Only Timeline Free

No account. No email. Runs in your browser.

Frequently Asked Questions

Is an interest-only mortgage ever a good idea?
For a small subset of borrowers: high-income professionals who expect large bonuses in the future, real estate investors who plan to flip within a few years, or retirees with illiquid assets. For 95% of home buyers, a standard amortizing loan is better.
Can I make principal payments during the interest-only period?
Yes, most interest-only loans allow extra principal payments without penalty. If you do that, the recast payment will be lower. But few people actually do this.
How does an interest-only ARM compare to a fixed-rate interest-only?
Worse. An interest-only ARM has a low fixed rate for the first 5-7 years, then adjusts up. The combination of payment shock from the interest-only period ending plus a rate increase can double your payment. Avoid.
Are interest-only loans still available in 2026?
Yes, primarily for jumbo loans (over $766,000) and for investors. They are less common since the 2008 financial crisis. Lenders require larger down payments (20-30%) and excellent credit.
What happens if home prices drop during the interest-only period?
You will have no equity. You cannot sell without bringing cash to closing. This is the biggest risk. In 2008, millions of homeowners with interest-only loans went underwater.
Can I refinance out of an interest-only loan before the recast?
Yes. Many people take an interest-only loan with the plan to refinance later. But refinancing costs money, and rates might be higher. That plan can backfire.

Before you sign an interest-only loan, run the full numbers. See the payment shock. Calculate the total interest. Most people decide it is not worth it.

Figures on this page are for educational purposes only. Rates and program availability vary by lender, location, and borrower profile. Consult a licensed lender for loan-specific figures. Truly Free Mortgage Calculator does not collect personal data and does not connect users with lenders.

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George Smith
WRITTEN BY
George Smith
Founder, Klickify Agency
info@klickifyagency.comLinkedIn
George builds free web tools that respect user privacy. Founder of Klickify Agency.