What is an interest-only mortgage repayment?
An interest-only mortgage repayment means you pay only the interest charged on the loan balance for a set period. During that period, your principal (the amount you borrowed) usually does not reduce unless you make extra payments.
This calculator helps you estimate how much those repayments are likely to be, based on your loan amount, interest rate, and payment frequency.
How this calculator works
In an interest-only period, the repayment formula is straightforward:
Interest-only payment per period = (Loan amount × annual interest rate) ÷ number of payments per year
For example, if your loan is $600,000 and your rate is 6.00%:
- Annual interest = $600,000 × 0.06 = $36,000
- Monthly interest-only repayment = $36,000 ÷ 12 = $3,000
That amount does not reduce the loan principal. You are effectively servicing the cost of borrowing only.
Why compare with principal-and-interest (P&I)?
Interest-only repayments are usually lower than principal-and-interest repayments in the short term. This can improve cash flow today, but it can also increase total interest over the life of the loan if the principal remains high for longer.
This calculator includes an optional comparison using a standard amortization formula so you can see the gap between:
- Interest-only repayment (paying interest only), and
- Principal-and-interest repayment (paying interest plus reducing debt).
When interest-only loans can make sense
1) Property investors focused on cash flow
Some investors use interest-only loans to keep repayments lower while rental income is building or while they manage other debt and investment priorities.
2) Temporary income disruption
If a borrower expects income to rise later (for example, after parental leave or a business transition), an interest-only period may offer breathing room in the short term.
3) Strategic financial planning
Some borrowers choose interest-only for a defined period while directing surplus cash into offset accounts, emergency savings, or higher-return opportunities. This approach requires discipline and a clear plan.
Key risks to understand
- No automatic debt reduction: Your balance stays the same unless you make extra repayments.
- Repayment shock: Once interest-only ends, required repayments can jump significantly when the loan reverts to principal-and-interest.
- Higher total interest: Keeping a larger principal for longer can increase lifetime borrowing costs.
- Rate sensitivity: If rates rise, your payments rise immediately because interest cost is directly linked to the rate.
Tips for using an interest-only period wisely
Build an exit strategy from day one
Know what happens after the interest-only period. Model the future principal-and-interest payment and make sure it fits your budget.
Use the cash flow advantage intentionally
If interest-only lowers your repayment by several hundred dollars per month, allocate that difference on purpose (savings, offset, debt reduction, investments, or liquidity).
Review rates and structure regularly
Even small interest rate changes can materially affect repayment amounts. Reassess your loan features, rates, and repayment strategy at least annually.
Quick FAQ
Does interest-only mean I pay less forever?
No. You usually pay less during the interest-only period, but later repayments can be higher and total interest can be greater over time.
Can I still pay extra during interest-only?
Often yes, depending on loan terms. Extra repayments can reduce principal and lower future interest, but always check your lender’s conditions.
Is this calculator financial advice?
No. It is an educational estimate tool. Loan products, fees, compounding methods, and lender policies vary. For personal advice, consult a licensed mortgage broker or financial adviser.