Interest-Only Mortgage Calculator
Interest-only monthly cost versus repayment, the deferred principal cliff and investor use-cases.
Formula
Disclaimer: Assumes a constant rate across both phases; many IO products are ARMs whose reset compounds the payment jump. Not financial advice.
Disclaimer: This tool is for general informational and estimation purposes only and is not professional financial, tax, accounting or legal advice. All figures are estimates — verify with a qualified professional before making decisions. Read the full disclaimer.
Need interest-only mortgage calculator results fast? Skip the spreadsheet and get a clear, defensible answer in one step — free, private and instant, recalculating live as you change any input.
About Interest-Only Mortgage Calculator
Interest-only looks like relief and is actually deferral: at the defaults ($400,000 at 6.8%, 10 years IO inside a 30-year term) you pay $2,267 monthly while the principal sleeps untouched — then the SAME balance must amortize over just 20 remaining years, jacking the payment ~19% precisely when the structure assumed you'd be richer. The four outputs above keep the whole story visible at once. Legitimate uses are real but specific: property investors maximizing deductible interest while deploying capital elsewhere (the dominant Australian/UK buy-to-let pattern), lumpy-income professionals smoothing cash flow, and bridge situations with a defined exit (sale, bonus, vesting). The common thread is a credible plan for the principal that isn't 'the house will appreciate'. The math that decides it: IO costs you the amortization you didn't do — at the defaults, ~$272,000 of interest in the IO decade alone, with the balance still intact. If the freed-up monthly difference (vs a repayment loan) is genuinely invested at returns above the mortgage rate, IO can win on paper; if it leaks into lifestyle, you've rented your own house from the bank for ten years. Be honest about which household you run.
How to use Interest-Only Mortgage Calculator
- 1Enter Loan amount, Interest rate (%), Interest-only period (years), Total term (years) into the Interest-Only Mortgage Calculator.
- 2The result is computed automatically using IO payment = balance × rate ÷ 12 ; afterwards the UNTOUCHED balance amortizes over the remaining (term − IO) years — there is no button to press; it updates live as you type.
- 3Change any input to model a different scenario, then use “Copy result link” to share the exact numbers.
Why use Interest-Only Mortgage Calculator?
- ✓Computes interest-only mortgage calculator instantly with the correct formula — no spreadsheet needed
- ✓100% free and unlimited, with no sign-up, login or paywall
- ✓Runs entirely in your browser, so the figures you enter are never uploaded or stored
- ✓Shows the formula, a live worked example and references so you can defend the number
Frequently asked questions
Why is the post-IO payment so much higher?+
Compression: a 30-year loan that ignored principal for 10 years must clear 100% of the balance in 20 — fewer years, same debt, so the amortization formula demands more monthly. Longer IO windows make the cliff worse. The 'jump' output is the number to budget-test, not the teaser.
Is interest-only cheaper overall?+
Never in total interest — you pay full-rate interest on the full balance for the entire IO period, which a repayment loan steadily shrinks. IO 'wins' only if the cash-flow difference earns MORE than the mortgage rate elsewhere, after tax and after your own discipline. For owner-occupiers without an investment engine, repayment wins almost mechanically.
Can I make principal payments during the IO period?+
Usually yes, voluntarily — and it's the best of both: a low REQUIRED payment for safety, with optional principal attacks in good months that immediately reduce the next month's interest. Confirm no prepayment penalties and that extra amounts recast the IO payment (most IO products recompute monthly interest on the new balance automatically).
Who actually qualifies for interest-only now?+
Post-2008 rules pushed IO upmarket: lenders want strong equity (often ≤75–80% LTV), high credit quality and documented repayment strategies — or investment-property purposes where rent and exit logic stand in. Mass-market owner-occupier IO is largely gone in the US; investors and high-net-worth borrowers remain the audience.
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