Interest-Only Mortgage Calculator
See the interest-only payment next to the conventional principal-and-interest payment — and exactly how far the payment jumps the month the interest-only period ends.
Loan & Rate Details
Most lenders offer 5 or 10. Principal is untouched until it ends.
Interest-only loans are usually quoted above a comparable fixed rate — raise this to price that in.
The fully amortizing loan you're comparing against.
Interest-only payment
$2,300
per month for 10 years — none of it touches principal
Conventional P&I payment
$2,634
per month for 30 years — pays the loan off in full
Interest-only frees up $334 a month today. In year 11 the payment jumps to $3,077 — a $777 (34%) increase — because the full $400,000 still has to be repaid, now over just 20 years.
Side-by-Side Over 30 Years
Interest-Only (10/30)
Conventional (30-year fixed)
Held to term at these rates, the interest-only loan costs $66,151 more in total interest, and leaves you $57,562 behind on principal at the moment the interest-only period ends.
Principal Paid Down by Year
Equity you build from payments alone, ignoring appreciation. The interest-only bar is flat at zero until the period ends — every dollar goes to the lender.
The interest-only risk in one line.
You owe the same $400,000on the last day of the interest-only period as you did on the first. If the home hasn't appreciated, selling may not cover the balance plus closing costs, and refinancing out of the payment jump depends on rates, your credit, and the appraisal — none of which you control.
What Is an Interest-Only Mortgage?
An interest-only mortgage splits the loan into two phases. In the first phase — typically 5 or 10 years — your required payment covers only the interest that accrued that month. Nothing reduces the balance. In the second phase, the loan becomes fully amortizing: the entire original balance must be repaid over the years that remain.
That second phase is where the product gets its reputation. A 30-year loan with a 10-year interest-only period doesn't give you 30 years to repay principal — it gives you 20. The payment after the reset is therefore higher than the payment on a plain 30-year loan of the same size at the same rate, not merely higher than the interest-only payment you had been making. The calculator above shows both numbers so the gap is impossible to miss.
Interest-only is a payment structure, not a rate type. Many interest-only loans are also adjustable-rate, which means the rate can reset around the same time the payment recasts. If that describes the loan you're considering, run the ARM vs fixed-rate comparison as well — the two risks stack.
Who Uses Interest-Only Loans?
After the 2008 crisis, interest-only lending narrowed to borrowers who have a specific reason for it rather than borrowers who simply need a smaller payment. The patterns that hold up:
- Lumpy income. Commissioned salespeople, bonus-heavy employees, and business owners whose cash arrives in a few large chunks. They pay the low minimum in lean months and put lump sums straight to principal when the money lands — the same total, on their own schedule.
- Short holding periods.A buyer confident they'll sell or relocate before the period ends never faces the recast. The principal they skipped was never going to build much equity in the early years anyway.
- Deliberate cash-flow arbitrage. A high earner who genuinely invests the payment difference, rather than spending it, and accepts the risk that the investment underperforms the mortgage rate.
- Bridging to a known liquidity event. Vesting equity, a business sale, or an inheritance that will pay the balance down before the reset.
What each of these shares is an exit. The borrowers who get hurt are the ones who chose interest-only because it was the only way to afford the house.
Risks and When to Avoid Them
- Payment shock. The recast is scheduled, disclosed, and still catches people out. Look at the post-reset payment in the calculator above and ask whether you could afford it today. If not, the loan is betting on a raise.
- No forced savings.A conventional payment quietly converts cash into equity every month whether or not you have the discipline to do it yourself. Interest-only removes that mechanism. “Invest the difference” only works if you actually invest the difference.
- Negative-equity exposure. Your balance is flat, so a price decline eats straight into equity you never built. Sell into a soft market and the proceeds may not cover the balance plus closing costs.
- Refinance dependence.The common plan — “I'll refinance before the reset” — requires favorable rates, intact credit, and a supportive appraisal at one specific future moment. None of those are promised.
- Stacked rate risk. When the loan is an interest-only ARM, the rate reset and the payment recast can arrive together, compounding the increase.
Avoid interest-onlyif the amortizing payment doesn't fit your budget, if your timeline is uncertain, if you need equity for a future move, or if the plan depends on refinancing. If the goal is simply a lower payment on a house you intend to keep, a longer term or a larger down payment gets you there without the reset — compare the options with the 15 vs 30-year calculator or the down payment calculator.
Interest-Only vs Conventional Comparison
| Interest-Only | Conventional (fully amortizing) | |
|---|---|---|
| Monthly payment during the IO period | Lower — you pay only the interest that accrued | Higher — interest plus a slice of principal |
| Principal paid during the IO period | Zero | Rises every month, slowly at first |
| Equity growth | Only from appreciation (or voluntary principal payments) | Appreciation plus every scheduled payment |
| Payment after the IO period | Jumps — the full balance re-amortizes over a shorter term | Unchanged for the life of a fixed-rate loan |
| Rate structure | Frequently an ARM, so the rate can reset too | Commonly fixed for the entire term |
| Total interest over the full term | Higher — the balance stays at its maximum for years | Lower — the balance shrinks from month one |
| Qualified Mortgage status | Non-QM — interest-only features are excluded from the CFPB's QM definition | Typically meets the QM standard |
| Underwriting | Stricter — lenders generally want more equity, reserves, and stronger credit | Standard agency guidelines |
| Prepayment | Extra principal is allowed and lowers later payments, but nothing is required | Principal reduction is built into every payment |
The trade is liquidity now for cost and equity later. Interest-only buys you a smaller required payment during the introductory period; you pay for it with a larger payment afterward, more total interest, and a balance that hasn't moved. Nothing about the structure is dishonest — it's just only worth it when the liquidity is genuinely more valuable to you than the equity.
To see where the principal actually goes on a conventional loan — how little of an early payment reduces the balance, which is the strongest argument for interest-only in a short hold — open the amortization schedule. If you already hold an interest-only loan and the reset is coming, the refinance calculator will tell you whether refinancing out of it pays for itself.
Frequently Asked Questions
What is an interest-only mortgage?
An interest-only mortgage lets you pay only the interest that accrues each month for an introductory period — commonly 5 or 10 years — before the loan converts to a fully amortizing payment for the remaining term. During the interest-only period your balance does not move. A $400,000 loan is still a $400,000 loan on the last day of the interest-only period, no matter how many payments you have made.
How much does the payment go up when the interest-only period ends?
The jump is driven by two things at once: you start repaying principal, and you have less time to do it. A 30-year loan with a 10-year interest-only period has to repay the entire original balance across the final 20 years, so the amortizing payment is larger than it would have been on a plain 30-year loan of the same size. Enter your numbers in the calculator above to see the exact dollar and percentage increase for your loan.
Do you build any equity with an interest-only mortgage?
Not from your payments. During the interest-only period, every dollar goes to the lender as interest and none reduces the balance, so the only equity you gain comes from the home appreciating. If prices are flat or falling, your equity stays flat or shrinks. That is very different from a conventional loan, where each payment converts a little cash into ownership regardless of what the market does.
Can I pay extra principal on an interest-only loan?
Usually yes, and it is the single best way to defuse the payment shock. Any principal you pay voluntarily during the interest-only period lowers the balance that has to be re-amortized later, which lowers the payment after the period ends. This is why interest-only loans suit borrowers with lumpy income — a commissioned salesperson or a bonus-heavy earner can pay the minimum in lean months and dump a bonus into principal when it arrives. Confirm the terms with your lender before relying on it.
Are interest-only mortgages still available after 2008?
Yes, but they are a different product than the pre-crisis version. Under the CFPB's ability-to-repay rules, a loan with interest-only payments cannot be a Qualified Mortgage, so these are non-QM loans made mostly by portfolio lenders, private banks, and credit unions that keep the loan on their own books. Expect stricter underwriting than a conventional loan: more down payment, documented reserves, and stronger credit. Lenders must also qualify you on the fully amortizing payment, not the low interest-only one.
Is an interest-only mortgage ever cheaper overall?
Held to term, no. Because the balance stays at its maximum through the interest-only period, more interest accrues in total than on an otherwise identical amortizing loan — and interest-only loans are typically quoted at a higher rate on top of that. The only ways an interest-only loan comes out ahead are selling or refinancing before the period ends, or reliably earning more on the payment difference than the mortgage rate costs you. Both depend on things you can't guarantee.
What happens if I can't afford the payment after the IO period?
You are left with the same options any borrower has under payment stress — refinance, sell, or fall behind — but with less equity to work with than a conventional borrower who has been paying down principal for a decade. Refinancing requires qualifying rates, credit, and an appraisal at the time you need it, and selling requires the home to be worth more than the balance plus closing costs. Plan the exit before you sign, not in year eleven.