Interest-Only Loan Payment Calculator

Lower Payments Now, But the Balance Stays Put

You borrow $350,000 and pay $1,750/month for five years. Then your statement arrives: $2,900/month. Same loan, same rate, but the interest-only period ended. That's the reality of interest-only loans, and this calculator shows you both numbers upfront so you're not caught off guard.

How hard does an interest-only loan hit when principal kicks in? The payment holds flat through the IO period, then jumps when the full balance amortizes over a shorter remaining term. On a $315,000 loan at 7.25% with a 7-year IO period inside 30 years, the payment climbs from $1,903 to about $2,452 in year eight, and total interest runs near $411,000. That total-interest penalty, plus a rate that's usually fixed on the whole balance, is what sets an interest-only loan apart from a HELOC.

During the interest-only (IO) period, you pay only the interest accruing on your balance. Your principal doesn't shrink—you owe the same amount you borrowed on day one. When IO ends, the loan converts to full amortization: you now pay principal AND interest, squeezed into a shorter remaining term. The payment jump can be 40-70%.

Enter your loan amount, rate, total term, and IO period. The calculator displays both payment phases, the exact percentage increase, and total interest over the loan's life. Use it to decide if an IO structure fits your financial plan—or if it's a trap waiting to spring.

The Variables That Drive Your IO Payment

Loan amount: IO payments scale directly. $200,000 at 6% costs $1,000/month interest-only. $400,000 at 6% costs $2,000/month. Double the balance, double the payment.

Interest rate: Every 1% rate change moves your IO payment by $83 per $100,000 borrowed monthly. An IO ARM (adjustable-rate mortgage) adds uncertainty—your IO payment can rise mid-period if rates increase.

IO period length vs. total term: A 30-year loan with a 10-year IO period leaves only 20 years for amortization. The same loan with a 5-year IO period has 25 years to amortize. Longer IO = lower payment now, higher payment later, more total interest paid.

What Payment Shock Really Looks Like, One Loan in Full

Marcus buys a $420,000 rental with 25% down and takes a $315,000 interest-only loan at 7.25%: a 7-year IO period inside a 30-year term. For those first seven years he pays $1,903 a month, all of it interest, and the $315,000 principal doesn't move an inch.

Then year eight arrives. The loan now has to amortize the full $315,000 over the 23 years that remain, so the payment climbs to $2,452, up $549 (29%) with no change in rate or balance. And because he carried the entire principal at interest for seven extra years, the loan runs $411,480 in total interest, well above what a standard amortizing loan of the same size would cost. The mortgage calculator shows that fully amortizing payment, with taxes, insurance, and PMI, if you want the side-by-side.

For Marcus the structure still works, but only because he has a way out. Rent covers the IO payment with room to spare, and the plan is to refinance or sell before year seven if the property appreciates. If values stay flat, he either absorbs the $549 jump or refinances into whatever rates exist then.

That exit is the whole line between a smart interest-only loan and a dangerous one. A resident doctor three years from an attending salary can use a short IO the same way, banking the lower payment through the lean years with a raise already in sight. Use IO because you can't afford the real payment, though, and you've just bought a bill you'll eventually have to run from.

Where IO Loans Go Wrong

Using IO to afford more house: If you can only afford the IO payment, you can't afford the house. When payments increase, you'll either struggle or be forced to sell. IO should be a strategy, not a stretch.

No equity building: During IO, your principal balance doesn't decrease. If home values drop, you could owe more than the house is worth with no equity cushion.

Refinancing isn't guaranteed: Many IO borrowers plan to refinance before the payment jump. But rates might be higher, your credit might change, or lending standards might tighten. Don't count on refinancing as your only exit.

Combining IO with variable rates: An IO ARM doubles your risk. If rates rise AND your IO period ends simultaneously, your payment could jump 60%+ overnight.

Balloon payment structures: Some IO loans require the full balance due at the end instead of converting to amortization. Miss this detail and you face a six-figure payment with no warning.

How the Calculator Computes Payments

IO phase: Monthly payment = (Principal × Annual Rate) ÷ 12. The balance remains constant since no principal is paid.

Post-IO phase: The full original principal is amortized over the remaining term using standard loan formulas. A 30-year loan with 7-year IO becomes a 23-year amortization.

Total interest: IO phase interest + post-IO phase interest. IO loans typically cost more total interest than equivalent amortizing loans because you pay interest on the full balance longer.

Assumptions: Fixed interest rate (if you have an ARM, actual results will vary), no prepayments during IO, and full amortization after IO ends (not balloon).

Sources

• Consumer Financial Protection Bureau — What is an interest-only loan?
• FDIC Consumer Resources — Mortgage protections and disclosures