Interest-Only Mortgage Calculator

See your monthly interest-only payment, the total interest paid during the IO period, and the balloon warning for what comes next. An interest-only mortgage can slash your early payments — but the principal never shrinks while you pay interest alone. This free calculator shows exactly what you will pay each month, how much interest you will hand the lender in total, and what happens the day the interest-only period ends.

How to Use the Interest-Only Mortgage Calculator

Get your full interest-only picture in under a minute with these five steps:

1. Enter the loan amount. Type the total you are borrowing — for example, $300,000. This is the balance on which every interest-only payment is calculated.
2. Enter the interest rate. Use the annual rate as a percentage, such as 6%. If you have an adjustable-rate mortgage, run the numbers at both the starting rate and a realistically higher rate.
3. Enter the interest-only years. Common IO periods are 5, 7, or 10 years. This is how long your payments cover interest alone before the loan recasts.
4. Read your monthly payment. The calculator instantly shows the interest-only payment — on $300,000 at 6%, that is $1,500 per month, every month, for the whole IO period.
5. Study the balloon warning. Check the total interest paid and the principal still owed afterward. On that same $300,000 loan with a 10-year IO period, you will pay $180,000 in interest and still owe the full $300,000 — the loan must then be refinanced, repaid, or converted to much higher principal-and-interest payments.

The calculator’s warning is the point: interest-only payments do not reduce the principal, so you must plan for what comes next before you sign.

About Interest-Only Mortgages

A standard amortizing mortgage payment has two parts: interest, which is the lender’s fee for the loan, and principal, which slowly pays down what you borrowed. An interest-only (IO) mortgage strips out the second part for a fixed interest-only period — typically 5 to 10 years — so your monthly payment covers only the interest charge. The principal balance stays exactly where it started, down to the dollar, for the entire IO period.

That single fact drives every pro and con of the product. The advantage is cash flow: because none of your payment goes toward the loan balance, the monthly bill is 20% to 30% lower than a fully amortizing payment on the same loan. Investors love this — lower payments mean better monthly cash flow on rental property, and they would rather deploy capital elsewhere than trap it in home equity. Borrowers with lumpy incomes, such as commission earners or business owners, use IO periods to keep obligatory payments low while they build reserves or wait for a liquidity event.

The danger is the recast, also called the reset. When the IO period ends, the loan converts to principal-and-interest payments amortized over the remaining term — and because you have made zero progress on the balance, the new payment is dramatically higher. This payment shock is the number-one reason IO borrowers get into trouble. On a $350,000 loan at 7% with a 5-year IO period, the payment jumps from $2,042 to about $2,474 a month — a 21% overnight increase — and it only gets worse if rates have risen.

There is also the balloon risk. Some IO loans do not recast into amortizing payments at all; instead the entire principal comes due as a lump sum — a balloon payment — at the end of the IO period. Borrowers then must refinance, sell, or pay cash. If home values have fallen or credit has tightened, refinancing may be impossible, which is how IO borrowers end up forced sellers.

And crucially, you build equity only through price appreciation, never through your payments. With a standard mortgage, each payment buys a little more of your home; with interest-only, your equity after 10 years of $1,500 payments on a $300,000 loan is exactly what appreciation alone gave you — minus nothing, plus nothing. If prices are flat, a decade of payments leaves you owning precisely as much of the home as the day you bought it. That is why lenders treat IO loans as riskier, often reserve them for jumbo loans and well-qualified borrowers, and charge slightly higher rates for the privilege.

Interest-Only Mortgage Formula

The math behind an interest-only payment is refreshingly simple — there is no amortization schedule during the IO period:

Monthly IO payment = loan amount × (annual interest rate ÷ 12)

Total interest during IO period = monthly IO payment × (IO years × 12)

Principal owed after IO period = original loan amount (unchanged)

Worked example 1: $300,000 at 6% with a 10-year interest-only period.
Step 1 — monthly rate: 6% ÷ 12 = 0.5% per month, or 0.005.
Step 2 — monthly payment: $300,000 × 0.005 = $1,500 per month.
Step 3 — total interest: $1,500 × 120 months = $180,000 paid to the lender over 10 years.
Step 4 — balance remaining: $300,000 — not one dollar of principal repaid.
A standard 30-year amortizing payment on the same loan would be about $1,799 a month, so the IO borrower “saves” $299 a month — but after 10 years the amortizing borrower owes roughly $255,000 while the IO borrower still owes the full $300,000.

Worked example 2: $500,000 at 7% with a 5-year interest-only period, then amortized over the remaining 25 years.
Step 1 — monthly IO payment: $500,000 × (0.07 ÷ 12) = $500,000 × 0.0058333 = $2,916.67 per month.
Step 2 — total interest in 5 years: $2,916.67 × 60 = $175,000.
Step 3 — balance after 5 years: $500,000, untouched.
Step 4 — recast payment: the $500,000 must now amortize over 25 years (300 months) at 7%. Monthly payment = $500,000 × 0.0058333 ÷ (1 − 1.0058333⁻³⁰⁰) = $2,916.67 ÷ 0.8253 = $3,534 per month.
The payment jumps from $2,916.67 to $3,534 — an extra $617 every month, or $7,400 a year, for 25 years. This is the payment shock every IO borrower must be able to survive.

Practical Example 1: An Investor’s Cash-Flow Play

Daniel buys a $500,000 rental property with a $400,000 loan at 6.5% and chooses a 7-year interest-only period because his priority is monthly cash flow, not equity buildup.

His interest-only payment is $400,000 × (0.065 ÷ 12) = $2,166.67 per month. A fully amortizing 30-year payment at the same rate would be about $2,528 per month — so the IO structure frees up $361.60 every month, or $30,374 over the 7-year IO period. That cash covers vacancies, repairs, and property tax increases while the rent checks keep coming.

Total interest paid over the 7 years: $2,166.67 × 84 = $182,000. And the loan balance after 7 years? Still exactly $400,000.

Daniel’s plan works only because he has an exit strategy: he expects the property to appreciate about 3% per year. After 7 years the $500,000 property should be worth roughly $500,000 × 1.03⁷ = $614,937, giving him about $214,937 in equity against the unchanged $400,000 loan. He can then sell, refinance into a conventional loan, or keep the property with strong cash flow. But notice the fragility: if prices stay flat, he has paid $182,000 in interest and owns no more of the building than on day one. Interest-only is a cash-flow tool for disciplined investors with a plan — never a way to “afford” a property whose amortizing payment you cannot handle.

Practical Example 2: The Payment Shock, Up Close

Sofia takes a $350,000 interest-only mortgage at 7% with a 5-year IO period, planning to refinance before it ends. Her IO payment is $350,000 × (0.07 ÷ 12) = $2,041.67 per month — comfortable on her salary.

Five years pass. She has paid $2,041.67 × 60 = $122,500 in interest and — as the calculator’s balloon warning stresses — still owes the full $350,000. Then life intervenes: rates rise and a job change dents her credit, so refinancing is no longer available. The loan recasts.

Her new payment must amortize $350,000 over the remaining 25 years at 7%: $350,000 × 0.0058333 ÷ (1 − 1.0058333⁻³⁰⁰) = $2,474 per month. That is $432 more every month — a 21% overnight increase — on a balance that never shrank despite $122,500 of payments.

Sofia’s story is the cautionary tale behind every IO loan: the product is safe only for borrowers who can afford the post-recast payment from day one, or who have a concrete, realistic exit — a scheduled sale, a vested bonus, a certain refinance. Anyone stretching to afford the IO payment itself is one recast away from distress. Before choosing interest-only, run this calculator, then ask yourself the hard question: “Can I afford $2,474, not just $2,042?”

5 Common Mistakes with Interest-Only Mortgages

1. Having no exit plan for the recast. The IO period always ends. Borrowers who assume they will “figure it out later” face payment shock or a balloon they cannot pay. Every IO loan needs a written plan on day one: refinance by a date, sell by a date, or verified ability to afford the amortizing payment.
2. Thinking interest-only is cheaper. The monthly payment is lower, but the lifetime cost is higher — you pay interest on the full balance for years while an amortizing borrower steadily shrinks theirs. On the $300,000, 6% example, 10 years of IO costs $180,000 in interest with zero principal reduction; the amortizing borrower pays about $170,800 in interest over the same decade and cuts the balance by $45,000.
3. Counting on appreciation for equity. IO builds equity only if prices rise. Buyers in 2006 who chose IO loans learned this brutally: when prices fell 20%, they owed the full balance on an underwater home with no amortization cushion, making refinancing impossible exactly when they needed it most.
4. Ignoring rate adjustments on IO ARMs. Many IO loans are adjustable-rate mortgages, so the recast can coincide with a rate increase — a double shock. Always model the post-IO payment at a rate 2 to 3 points higher than today’s, not just at the teaser rate.
5. Skipping the comparison with a standard mortgage. Run both. If the IO payment is $1,500 and the 30-year fixed is $1,799, ask what the $299 monthly “saving” actually buys you versus $45,000 less debt after 10 years. Sometimes the answer justifies IO; often it does not — but you should see both numbers before deciding.

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Cite this page as: “Interest-Only Mortgage Calculator” at https://sigmacalculator.net/financial-calculators/interest-only-mortgage-calculator/ from SigmaCalculator, https://sigmacalculator.net — Online Calculators. Last updated: October 7, 2026.