Calculator
Interest Only Mortgage Calculator
Interest Only Mortgage Calculator
Most Australian lenders grant interest-only periods of up to 5 years on investment loans, often at a 0.25–1% rate loading. When the IO period ends, the payment rises and the remaining term is shorter — lenders re-test the new repayment under APRA's buffer.
Interest-Only Payment
$3,412.5/mo
Interest-Only Analysis
During the 5-year interest-only period you pay $3,412.5/month — $204,750 total, none of it reducing the $650,000 balance. When the IO period ends, payments jump $895.46/month to $4,307.96 to amortise the full balance over the remaining 25 years. If the $650,000 loan grows at 4% a year, the debt is serviced by asset growth rather than repayments during the IO phase. Across the full 30-year term, the IO structure costs $847,138.73 in interest — $48,742.41 more than straight P&I payments of $4,023.32/month from day one. For investors the interest is generally deductible when the property earns rent; for owner-occupiers it is not — which changes the real cost arithmetic entirely.
*Models an interest-only period followed by principal-and-interest repayments at the rate you enter. Australian lenders typically cap IO periods at 5 years for investment loans and usually charge an IO rate loading. Actual serviceability is tested at the lender's assessment rate including APRA's buffer. Educational only, not lending or financial advice.
The Investor's Lever and the Owner-Occupier's Trap
An interest-only mortgage is exactly what it sounds like: for a fixed period — usually up to five years in Australia, sometimes renewable for investors — you pay only the interest on the loan. The balance does not shrink by a dollar. When the period ends, repayments jump to full principal-and-interest amounts over the remaining term, often a 40–70 per cent payment rise overnight. For Australian property investors the structure is central to strategy: while interest-only dominates, the debt stays at full balance, the rental deduction applies to the whole amount, and every dollar of cash flow stays available for the next deposit. For owner-occupiers the calculus is inverted. Without the tax deduction, interest-only means you are paying maximum interest for minimum progress — and you still carry the full debt when rates rise or the IO period expires. APRA requires lenders to assess borrowers at higher test rates, and banks have periodically tightened IO lending after the 2017–18 clampdown. Yet the structure persists because it serves a real purpose: bridging cash flow while income is temporarily low, holding a property for a defined horizon, or deliberately deferring principal while deploying cash elsewhere. The question this tool forces is the only one that matters: what does the extra interest cost, and is what you do with the freed cash actually worth more than that cost?
Two Phases: Interest First, Then a Compressed Repayment Runway
During the interest-only phase the payment is the simplest in lending: IO Payment = Loan × (Rate ÷ 12). On $650,000 at 6.6 per cent, that is $3,575 a month — every dollar going to interest, nothing to principal. When the IO period ends, the same balance must be amortised over however many years remain, and because those years are now fewer, the payment jumps sharply: Post-IO Payment = Balance × r ÷ (1 − (1 + r)^−n), with n = remaining months. The same loan after a five-year IO period on a 30-year term must repay in 25 years, pushing the payment to roughly $4,443 — an $868 monthly leap. The total cost comparison is the arithmetic of the trade-off. Interest paid over the life of an IO-structured loan always exceeds a straight P&I loan of the same term, because for the IO years the balance never declines and interest is charged on the full amount, while the P&I loan is steadily reducing both. This tool reports the difference as 'Extra Interest vs Straight P&I' — often tens of thousands on an Australian-sized loan. The expected-appreciation input shows the asset side: at 4 per cent growth, a $650,000 loan position is worth roughly $790,000 at the five-year mark, and whether the strategy worked depends on that asset growth outearning the interest bill — the investor's real scorecard, not the repayment comfort during the easy years.
Expert Insights
The IO Rate Loading Is the First Cost Most Borrowers Ignore
Australian lenders almost universally price interest-only periods at a premium to the P&I equivalent — commonly 0.25 to 1.00 percentage points higher. On a $650,000 loan that loading alone can cost over $400 a month during the IO phase, before the structural extra interest is counted. Any honest comparison must add the loading to the IO rate input; using the advertised P&I rate understates the true cost. It is also why the calculator should be run at the IO rate the lender actually quotes, not the comparison rate from the brochure. The loading alone can erase much of the cash-flow benefit for shorter IO periods.
The Deduction Changes the Arithmetic Entirely
For a genuine investment property, interest is generally tax-deductible against rental income — so a 6.6 per cent IO loan for someone on the 37 per cent marginal rate effectively costs closer to 4.2 per cent after tax. That after-tax figure is what should compete against alternative uses of the cash flow, not the headline rate. For an owner-occupier, none of the interest is deductible, and the IO structure is a pure extra cost with no offset. This asymmetry explains why investors dominate the IO market and why owner-occupiers using the structure are paying the most expensive form of housing finance available. Never compare IO and P&I for an investor without running the marginal-rate tax adjustment.
Plan the Repayment Jump Before It Arrives
The single most common IO failure in Australia is the unprepared reversion: the IO period ends, the lender converts the loan to P&I, and the borrower — who budgeted the lower payment for years — suddenly faces a 50 per cent-plus rise. Lenders re-test serviceability at the higher repayment when the period ends, and borrowers whose circumstances have deteriorated (rates up, income flat) can face forced restructuring at the worst time. The fix is structural and free: from month one, pay the full post-IO amount and park the difference in an offset account. That converts the IO product into a P&I outcome you control, keeps the flexibility, and makes the reversion a non-event.
Actionable Tips
- 1
Run the Full-Term Cost Before Choosing IO
Enter your loan amount and term here and read the 'Extra Interest vs Straight P&I' figure first — that is the price of the IO strategy in dollars. If the answer is $60,000 of extra interest, the question is whether the freed cash flow earns or saves more than that over the same period. For investors the comparison should be against the after-tax rate. For owner-occupiers it is against accelerated equity, which can never lose. Most IO decisions that fail simply never ran this number.
- 2
Match the IO Period to a Defined Exit or Transition
Interest-only works best when it funds a defined, time-bound strategy: completing a building project, bridging while saving a larger deposit for the next purchase, or holding a property for a planned sale horizon. If you cannot name what happens when the IO period ends, the structure is not strategy — it is delay with a cost. Write the transition plan before signing: the sale date, the refinance goal, or the cash-flow threshold that triggers switching to P&I. The reversion should be a scheduled event in the plan, not a surprise from the lender.
- 3
Park the Savings in an Offset, Not in Consumption
The cash-flow benefit of IO evaporates if the saved repayment is absorbed into lifestyle. The disciplined version: pay the P&I-equivalent amount from day one, with the difference flowing into an offset account linked to the loan. That buffer reduces interest charged, builds the reserve for the IO expiry, and converts a 5 per cent buffer into a real option when the period ends. It also builds a track record that helps renegotiate terms. Borrowers who do this have the IO product's flexibility at close to a P&I outcome's cost — the best of both, without giving up either.
Real-World Examples
The Chens Built Two Rentals on IO Cash Flow
The Chens bought their first investment property at $580,000 on a five-year IO structure at 6.75 per cent including the loading. The payment was $3,263 a month; the P&I equivalent would have been $3,753. The $490 monthly difference, plus the full interest deduction at their 37 per cent marginal rate, funded deposits on two more properties within four years. At the five-year mark the revalued portfolio supported a switch to P&I on the first property with no serviceability stress. The strategy worked because the freed cash had a named job, and the IO period had a planned ending — not because IO itself was cheap.
Gary Paid for the Breather for Years After
Gary, an owner-occupier, switched his $520,000 home loan to IO for two years when his income dipped — a sensible bridge. But the IO rate loading and the unshrinking balance meant he paid roughly $63,000 of interest and reduced principal by $0 over the period. When he returned to P&I the repayment was noticeably higher than it had been, because two years of amortisation had been used up. The IO period got him through the income gap, but the cost of it followed him for the rest of the term. His hard-won rule: IO for owner-occupiers is emergency equipment, not a lifestyle setting.
Priya Made the Reversion a Non-Event
Priya took a five-year IO on her $700,000 investment loan and immediately began paying the post-IO P&I amount, parking the difference in an offset. Over five years she accumulated over $30,000 in the offset while the lender's records showed an IO loan — the best of both worlds. When the IO period ended, nothing changed in her household budget: she had been paying the higher figure for years, and the offset buffer meant the new P&I balance calculation started from a position of strength. The lesson she passes on: the right way to use IO is to behave as if you are not.
Glossary of Terms
- Interest-Only Period
- The fixed phase of a loan during which repayments cover only interest and the balance does not reduce. Australian lenders typically grant up to 5 years for investment loans, at a higher rate, and may require requalification to extend.
- Reversion (IO Expiry)
- The automatic conversion of an interest-only loan to principal-and-interest repayments when the IO period ends. The payment rises to amortise the full balance over the remaining term, often a 40–70 per cent jump that must be planned before it arrives.
- IO Rate Loading
- The extra interest rate a lender charges on the interest-only portion of a loan compared with the P&I equivalent, commonly 0.25–1.00 percentage points in Australia. It is a real cost that must be included in any honest comparison.
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Ivy Sinclair-Wren
Financial Chaos Analyst
Ivy Sinclair-Wren is a Financial Chaos Analyst covering investing, AI, wealth psychology, and the emotional consequences of opening finance apps during market crashes. Based in Melbourne, she specializes in demystifying the Australian tax code and helping users navigate the intersection of spreadsheet logic and human irrationality.