Interest Only Mortgage Calculator
Model an interest-only mortgage through both phases: the low initial payment, and the sharp reset when principal repayment begins on an undiminished balance.
Interest Only Mortgage Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Results are a model, not a quotation — an institution's own figures govern.
- Every input is an assumption; change one and the answer changes with it.
- Rounding is applied only at the display layer, so totals may differ by a cent from a statement that rounds each line.
In short: An interest-only mortgage defers all principal, so the balance never falls during the initial phase. A $400,000 loan at 6.75% costs $2,250.00 monthly for ten years, then jumps to $3,041.46 — a 35.2% increase — when the full balance must amortise over the remaining twenty years.
Formula
Because no principal is repaid, the reset amortises the original balance over a shortened remaining term, which is what makes the increase so steep.
Worked Example
- Interest-only payment. $400,000 × 6.75% ÷ 12 = $2,250.00.
- Ten years pass. $270,000.00 of interest paid, balance still $400,000.
- Reset. $400,000 must now amortise over 240 months: $3,041.46.
- Measure the shock. A $791.46 rise, or 35.2%.
- Compare the alternative. A normal 30-year loan costs $2,594.39 throughout and $65,968.19 less overall.
Analyst note. The early saving is $344.39 a month against a conventional loan, but it is borrowed, not earned: the full $400,000 is still owed a decade later. Interest-only is defensible for borrowers with genuinely lumpy income who will invest the difference, and hazardous for anyone relying on appreciation or a future refinance.
Strengths & Limits Of This Model
Where this engine is strong
- Runs entirely in your browser — no figure you type is transmitted or stored.
- Shows the full working, so every number can be traced and challenged.
- Free, unmetered and free of affiliate incentives.
Where it stops
- Generalised assumptions cannot capture every individual circumstance.
- Jurisdiction-specific rules and mid-year changes may not be reflected.
- A model output is not a substitute for a professional review of your position.
Practical Use Cases
Managing irregular or bonus-weighted income
A low mandatory payment with voluntary principal reduction suits borrowers paid largely in bonuses. Model the voluntary payments with the Extra Payment Mortgage Calculator.
Bridging a known short holding period
If you will certainly sell within the interest-only phase, deferring principal maximises liquidity. The risk is the certainty being wrong — test the alternative with the Rent Vs Buy Calculator.
Stress-testing the reset before committing
The decisive question is whether you can afford $3,041.46 in ten years, not $2,250.00 today. Check it against income with the Debt To Income Calculator.
Methodology & Editorial Standards
During the interest-only phase the payment is the balance multiplied by the monthly rate, so no amortisation occurs and the balance is unchanged at reset. The reset payment amortises the full original balance over the remaining term, which is the standard structure and the source of the steep increase. The comparison baseline is the same loan fully amortising from the outset over the total term. Rates are held constant, though many interest-only products are adjustable and would reset on rate as well as structure, compounding the shock. No appreciation is assumed, since relying on it to refinance out of the reset is the central risk of the product rather than a mitigant. The engine implements the standard published formula for this calculation. Inputs are validated for domain and sign before evaluation, and any undefined case returns an em-dash rather than a spurious value.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Interest Only Mortgage Calculator — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How does an interest-only mortgage work?
You pay only the interest for an initial period, typically five to ten years, so the balance never falls. Here $2,250.00 a month for ten years leaves the full $400,000 still owed.
What happens when the interest-only period ends?
The loan reprices to amortise the entire balance over the remaining term. The payment jumps from $2,250.00 to $3,041.46, a 35.2% increase, because twenty years must now do thirty years of work.
Is an interest-only mortgage cheaper?
Only monthly, and only at first. It costs $65,968.19 more over the full term than a conventional loan, because interest accrues on an undiminished balance for a decade.
Who should consider interest-only?
Borrowers with lumpy or bonus-weighted income who will make voluntary principal payments, and those with a genuinely certain short holding period. It is unsuitable for anyone relying on appreciation.
Can I pay principal during the interest-only period?
Yes, on most products, and doing so reduces both the balance and the eventual reset payment. That is the disciplined way to use the structure — the low payment becomes a floor, not a target.
Are interest-only mortgages still available?
Yes, though far more tightly underwritten than before 2008. They are now largely a jumbo and portfolio-lender product requiring substantial deposits, reserves and strong credit.
Do I build any equity?
None from payments during the interest-only phase. Any equity gain comes purely from appreciation, which is precisely the dependency that makes the product risky in flat or falling markets.
What is the difference from a balloon loan?
An interest-only loan resets to a higher amortising payment; a balloon loan demands the entire remaining balance as a single lump sum. Both defer principal, but the balloon defers all of it.
Can I refinance before the reset?
That is the common plan, but it depends on qualifying at future rates with an undiminished balance and no payment-based equity. If rates rise or values fall, the exit may not be available.
Are interest-only rates higher?
Usually slightly, since the lender carries the balance longer without amortisation. Many are also adjustable rather than fixed, which layers rate risk on top of the structural reset.
How much interest do I pay in the IO phase?
$270,000.00 over ten years in this example, with the balance unchanged at $400,000. That is the true cost of the deferral, and it is substantial.
Is the reset payment always this steep?
The shorter the remaining term, the steeper. A ten-year interest-only phase on a thirty-year loan compresses repayment into twenty years. A five-year phase resets more gently because twenty-five years remain.
Is this interest only mortgage calculator free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.
Can I use it offline?
Largely, yes. Because computation is client-side, the page continues to calculate without a network connection once it has loaded.
Which currency does it use?
Amounts display in US$ accounting format, but the underlying mathematics is currency-agnostic. The result is identical in any currency, so simply read the figures in your own.
Why does a result show an em-dash?
An em-dash indicates the calculation is not defined for the inputs given — typically a division by zero or a value outside the valid domain. We show a dash rather than a misleading number.
How do I report an error?
Email apexconverter.praxiscalc@gmail.com with the tool URL, your exact inputs, the output received and the output you expected. Verified mathematical errors are patched within 72 hours.