Interest-Only Mortgage Calculator
Estimate your monthly interest-only payment, total interest paid during the interest-only period, and possible payment shock once principal repayment begins.
Tip: This calculator assumes a fixed interest rate during the selected period.
How an interest-only mortgage works
An interest-only mortgage lets you pay only the interest portion of your loan for a set period, commonly 5 to 10 years. During that window, your monthly payment can be much lower than a standard principal-and-interest payment. The tradeoff is simple: your loan balance usually does not go down.
For example, if you borrow $400,000 at 6.75%, your monthly interest-only payment is based on interest alone: principal × monthly interest rate. You get payment flexibility now, but you should plan for a potentially higher payment later when principal repayment begins.
Mortgage interest-only calculator formula
1) Monthly interest-only payment
Monthly IO Payment = Loan Amount × (Annual Rate ÷ 12)
If your annual rate is expressed as a percentage, divide by 100 first. In formula form: Payment = P × (r / 100) / 12
2) Total interest paid during interest-only period
Total IO Interest = Monthly IO Payment × 12 × IO Years
Because principal is typically unchanged, this value is easy to estimate and useful for budget planning.
3) Payment after interest-only period
Once the interest-only period ends, many loans “recast,” which means you begin paying principal over a shorter remaining timeline. This can create payment shock. Our calculator estimates that next payment using a standard amortization formula over the remaining years.
Why people choose an interest-only mortgage
- Lower monthly payment in the early years.
- More cash flow for investing, business growth, or reserves.
- Potential flexibility for borrowers with variable income (commission, bonus, self-employment).
- Useful for short ownership horizons when you expect to sell or refinance before IO ends.
Risks you should understand first
- No automatic principal reduction: your balance may stay exactly the same.
- Payment shock: payment can rise sharply when principal repayment starts.
- Refinance risk: if rates rise or home values fall, refinancing may be harder.
- Budget illusion: lower payment can tempt over-borrowing.
Quick planning checklist before choosing interest-only
Cash flow safety
Keep a reserve fund that covers several months of expenses, including the higher payment you may face later.
Exit strategy
Decide now how you’ll handle the end of the IO period: refinance, sell, or start paying principal aggressively.
Rate assumptions
If your loan is adjustable, run scenarios with higher future rates. A small rate increase can materially affect monthly cost.
Compare alternatives
Evaluate a fixed-rate mortgage, a 15-year term, and an IO option side by side. The “best” choice depends on risk tolerance, time horizon, and overall financial plan.
Example scenario
Suppose you take a $500,000 interest-only mortgage at 7.0% with a 10-year IO period and a 30-year total term. The monthly interest-only payment is about $2,916.67 (before taxes/insurance). Over 10 years, you would pay roughly $350,000 in interest and still owe about $500,000 in principal.
Then, with 20 years left, principal-and-interest payments can jump significantly. This is why a calculator is essential: it helps you stress-test your plan before signing loan documents.
Final thoughts
A mortgage interest calculator for interest-only loans is most useful when it does more than output one payment. You need to see both today’s affordability and tomorrow’s repayment reality. Use the calculator above to model realistic taxes, insurance, and holding period assumptions so your decision is informed, not emotional.