Interest Only Loan Calculator

How Does an Interest-Only Loan Work?

An interest-only (IO) loan allows you to pay only the interest for a specified period (typically 5-10 years) before switching to full amortization for the remaining term. Enter the loan amount, rate, interest-only period, and total term in the calculator above. It shows the IO payment, the fully amortized payment after the IO period ends, the payment increase, and total interest over the life of the loan. IO loans provide lower initial payments but cost significantly more in total interest because principal repayment is deferred.

Interest-Only Payment vs Fully Amortized Payment

A $400,000 mortgage at 7% interest-only for 10 years then amortized over the remaining 20 years: IO payment = $2,333/month (years 1-10). Amortized payment = $3,101/month (years 11-30). Payment jump: +$768 (33% increase). Total interest: $741,960. Compare to a standard 30-year amortized loan at 7%: payment $2,661/month throughout. Total interest: $558,036. The IO loan saves $328/month for 10 years ($39,360 total) but costs $183,924 more in total interest. The "savings" during the IO period are borrowed, not saved - the deferred principal still accrues interest for two additional decades.

Who Benefits from Interest-Only Mortgages?

Real estate investors: IO loans maximize cash flow on rental properties where the goal is income, not equity building. If the rent covers the IO payment with profit, the investor earns positive cash flow while the property appreciates. High-income professionals with variable compensation (bonuses, commissions): the lower IO payment provides breathing room during lean months, with the ability to make principal payments during high-income months. Short-term owners who plan to sell before the IO period ends: lower payments without concern about the amortization reset. IO loans are not appropriate for buyers stretching their budget because the payment increase after the IO period often creates financial stress.

The Payment Shock at IO Period End

The transition from IO to fully amortized payments creates "payment shock." On a $300,000 loan at 6.5%: IO payment $1,625. After a 7-year IO period with 23 years remaining: amortized payment $2,270 (+$645, a 40% increase). If rates are variable and have risen during the IO period, the shock is even greater. A 7% IO payment of $1,750 becoming a 9% amortized payment over 23 years: $2,805 (60% increase). Borrowers must plan for this transition years in advance. If your income will not support the higher payment, you must sell, refinance, or face potential default when the IO period expires.

Interest-Only HELOCs

Most HELOCs have a built-in IO structure: 5-10 year draw period (interest-only on drawn balance) followed by a 10-20 year repayment period (amortized). A $100,000 HELOC at 8% during the draw period: IO payment $667/month. After draw period ends: amortized over 15 years at $956/month (+$289). Many HELOC borrowers are unaware of this transition because they opened the line years ago and have only ever made IO payments. The shift to amortized payments on mature HELOCs has caught numerous homeowners off guard, particularly those who drew the maximum and assumed the low payment would continue indefinitely.

Interest-Only Construction Loans

Construction loans are inherently interest-only during the build phase because the full loan amount is not yet disbursed. The lender releases funds in draws as construction milestones are reached. Interest accrues only on the drawn balance. A $350,000 construction loan at 8%: after the first draw of $70,000, IO payment is $467/month. After 50% drawn ($175,000): $1,167/month. At full draw: $2,333/month. Upon construction completion, the loan converts to a permanent mortgage (construction-to-perm) that is fully amortized. The IO phase serves a practical purpose here because paying principal on a construction loan before the house is finished makes no economic sense.

Risks of Interest-Only Borrowing

Zero equity building: after 10 years of IO payments, you owe the same amount as day one. If the property has not appreciated, you have no equity buffer. Negative amortization risk: some IO loans allow payments below the interest amount, adding unpaid interest to the principal balance (the loan grows). Rate risk on variable IO loans: rates increasing during the IO period raise both the IO payment and the future amortized payment. Refinance risk: if property values decline or credit deteriorates during the IO period, refinancing to avoid payment shock may be impossible. The 2008 financial crisis was significantly driven by borrowers in IO and negative-amortization loans who could not afford the reset payments and could not refinance because their homes were underwater.

Comparing IO Loans to Standard Amortization

IO makes sense when: the rate is lower than your investment return (deploying the payment difference profitably), the holding period is shorter than the IO period (selling before amortization begins), or the income profile is genuinely variable and the lower floor payment prevents default during low-income periods. Standard amortization is better when: you plan long-term ownership, prefer predictable payments, want to build equity automatically, and value the forced savings that principal payments represent. For the majority of homebuyers, standard amortization provides a simpler, less risky path to ownership. IO loans are tools for specific situations, not general-purpose mortgages for everyday buyers.

Frequently asked questions