Interest Only Mortgage Calculator — IO vs Standard

How to use

1. Enter the loan amount and interest rate.
2. Set the interest-only period and the total term.
3. Calculate to see the IO payment and the post-IO payment.
4. Compare total interest against the standard-loan column.

General information only — not medical, financial or legal advice. Figures cross-checked against public official sources (IRS / HMRC / NHS / WHO guidance); content last reviewed September 2026. For decisions that matter, confirm with a licensed professional in your country.

About this tool

Interest-only mortgages keep payments small by paying no principal for a set period — the balance never falls. This calculator shows the IO payment (loan times monthly rate), then reprices the same balance over the remaining term once the IO window ends, which is where payment shock lives: on a $400,000 loan at 6%, a 10-year IO starts at $2,000 a month and re-amortizes to roughly $2,866 because thirty years of payoff compress into twenty. Alongside, it runs the standard fully amortizing payment for the whole term and totals interest both ways, so you see the exact dollar price of flexibility — commonly five or six figures more over the life of the note. Useful for borrowers with lumpy income, an expected liquidity event or a short hold, and sobering for everyone else. Educational estimate computed entirely on this page; nothing you type is uploaded anywhere.

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