If you want to quickly estimate your interest-only mortgage repayments, this calculator gives you a fast answer. It also shows total interest paid during the interest-only period and an estimate of what repayments could look like once the loan switches to principal-and-interest.
Interest-Only Loan Calculator
Estimate only. Assumes a constant interest rate and excludes fees, redraw costs, offset effects, and lender-specific rules.
What is an interest-only loan?
An interest-only home loan lets you pay only the interest portion for a set period (for example, 1 to 5 years). During this time, your loan balance usually does not reduce unless you make extra repayments.
That means your minimum repayments are lower at the start than with a principal-and-interest mortgage, but the trade-off is that you build less equity early and can face a repayment jump later.
How this calculator works
Core formula
- Interest-only repayment per period = Loan Amount × (Annual Rate ÷ Payments Per Year)
- Total interest during IO period = Repayment Per Period × Number of IO Periods
When principal-and-interest starts
If your loan reverts to principal-and-interest after the interest-only phase, repayments are recalculated over the remaining loan term. Because there is less time left to repay principal, this amount is often much higher.
Why people use interest-only mortgages
- Lower short-term repayments and improved cash flow.
- Flexibility for investors managing tax strategy and rental timing.
- Temporary breathing room while income is expected to increase.
Key risks to understand
- Repayment shock: payments can rise sharply when IO ends.
- Slower equity growth: principal stays mostly unchanged.
- Higher total interest cost: you generally pay more interest over the life of the loan than with early principal reduction.
Tips for smarter use
1) Model both phases now
Use this interest-only repayment calculator to estimate today’s minimum payment, then plan for the principal-and-interest phase before you commit.
2) Stress test your budget
Try higher rates (for example +1% to +2%) to see whether your budget still works if interest rates rise.
3) Make extra repayments when possible
Even in an interest-only period, extra contributions can reduce balance and future repayment pressure, depending on your loan terms.
Frequently searched comparisons
People often compare this tool with a standard mortgage repayment calculator, amortization calculator, home loan interest calculator, and principal-and-interest loan estimator. Using both gives a clearer side-by-side view of cash flow and long-term cost.
Final thoughts
An interest-only loan can be useful in the right scenario, but it is not automatically cheaper in total. The best approach is to calculate both short-term affordability and long-term repayment capacity. Use the calculator above as a planning tool, then confirm final figures with your lender or broker.