Interest-only vs principal-and-interest

The promise and the catch

An interest-only mortgage promises a smaller monthly payment for the first few years of the loan. For some borrowers — typically property investors, or households with temporary cash-flow constraints — that lower payment unlocks options they couldn’t otherwise pursue. For others, it’s the start of a much more expensive path that catches them by surprise when the interest-only period ends.

Understanding interest-only is mostly understanding the trade-off: smaller payments today in exchange for either bigger payments later, more interest paid overall, or both. Whether the trade is worth it depends entirely on what the borrower plans to do with the cash flow saved during the interest-only window.

How interest-only actually works

During the interest-only period — usually 3 to 10 years, sometimes longer — the borrower pays only the interest accruing on the loan balance each month. The principal remains untouched. On a $500,000 loan at 6%, the monthly interest-only payment is $500,000 × 6% / 12 = $2,500 per month. The balance at the end of the interest-only period is still $500,000.

When the interest-only period ends, the loan converts to standard principal-and-interest. The full $500,000 balance must now be amortised over the remaining term — not the original term. If the original loan was 30 years and 5 of those were interest-only, the post-IO P&I phase has only 25 years to clear the principal.

The payment shock

This shorter remaining term is what creates the “payment shock”. The same $500,000 amortised over 30 years from day one would require a P&I payment of around $3,000 per month at 6%. The same balance over 25 years requires roughly $3,200 per month. So the borrower who enjoyed five years of $2,500/month payments suddenly faces a $3,200 payment — a 28% increase, every month, for the next 25 years.

For longer interest-only periods the shock is more pronounced. A 10-year interest-only window leaves only 20 years to amortise the full balance, pushing the post-IO payment higher. The shorter the remaining term, the bigger the jump.

The total interest cost

Beyond the payment shock, interest-only also costs more in total interest. Two factors drive this:

1. Interest accrues on a non-shrinking balance during the IO period. Every month of interest-only is a month where the principal isn’t reducing. That same period under P&I would have chipped away at the balance, reducing future interest.
2. Post-IO amortisation is over a shorter term. Slightly higher payments compress more interest into a shorter period, but the cumulative amount is still significantly higher than a standard 30-year P&I.

A typical $500,000 / 30-year / 6% loan paid as P&I from day one accrues around $580,000 in total interest. The same loan with a 5-year interest-only period costs around $640,000–$650,000 — an extra $60,000–$70,000 over the life of the loan, paid simply for the privilege of smaller payments early on.

When interest-only can make sense

Interest-only is a tool, and like any tool it has applications where it works well:

• Investment property structuring. In jurisdictions where interest is tax-deductible against rental income but principal repayments aren’t, investors sometimes use interest-only to maximise deductible expenses while channelling principal repayments to non-deductible debt elsewhere.
• Bridging temporary cash-flow constraints. A new business, a parental leave period, a renovation project — short-term scenarios where cash flow is genuinely tight, with a clear plan to switch to P&I when the constraint passes.
• Imminent sale or refinance. If the borrower expects to sell the property within the IO window — for example, a developer or a deliberately short-hold investor — interest-only avoids paying down a balance that will be cleared by the sale anyway.

In all of these cases, the borrower has a specific structural reason for choosing interest-only and a clear plan for when the period ends.

When interest-only doesn’t make sense

The dangerous use of interest-only is borrowing more property than you can actually afford by relying on the smaller IO payments to fit the budget. This is the path that produces financial distress when the IO period ends and the budget doesn’t have room for the new, higher P&I payment.

Signs that interest-only is being used as a borrowing-capacity hack rather than a structuring tool:

• The borrower can’t afford the post-IO P&I payment under their current budget.
• There’s no specific plan for what changes during the IO period (rising income, sold asset, completed renovation).
• The expectation is that “rates will be lower” or “I’ll earn more by then” — both fundamentally hopes, not plans.

If any of these apply, the right move is usually a smaller property at standard P&I — not the larger property at interest-only.

How to decide

Before choosing interest-only, run both scenarios side by side:

1. The interest-only path you’re considering — including the post-IO P&I payment, not just the IO payment.
2. A standard P&I loan from day one, ideally at the same loan amount.

Compare three numbers: monthly payment now, monthly payment after the IO period, and total interest over the life of the loan. If the post-IO payment is uncomfortable or the lifetime interest premium feels disproportionate to the structural benefit, P&I from day one is the safer choice.

Use the calculator

Run your own numbers through our interest-only vs principal-and-interest calculator to see both paths side by side — payment shock, total interest premium, and the month-by-month balance trajectory. Pair it with our mortgage repayment calculator for a deeper look at the standard P&I baseline.