property calculator

Interest-Only Mortgage Calculator

Both payment phases, the payment shock when the interest-only period ends, and the equity forgone.

During the interest-only period the payment is Loan × Rate ÷ 12 and the balance never falls, so afterwards the full balance must amortise over a shorter remaining term. Example: $500,000 at 6.75% with a 10-year interest-only period on a 30-year term pays $2,812.50, then jumps 35.18% to $3,801.82 — and leaves $73,495 less equity than a conventional loan at the same date.

Quick actions: Save inputs, view saved runs, print, share, or export results

Saved Runs

Inputs

Loan Amount$

Interest Rate%

Interest-Only Periodyrs

Total Loan Termyrs

Results

Interest-Only Payment

$2,812.50

Payment After the IO Period$3,801.82

Payment Shock$989.32 (35.2%)

Interest-Only Months120 months

Amortising Months240 months

Interest Paid During IO Phase$337,500

Interest Paid During Amortising Phase$412,437

Total Interest$749,937

Conventional Payment (same term)$3,242.99

Conventional Total Interest$667,477

Extra Interest vs Conventional$82,460

Monthly Saving During IO Phase$430.49

Balance When the IO Period Ends$500,000

Conventional Balance at That Point$426,505

Equity Forgone$73,495

Tip

The payment that matters is the second one. Keep this to hand before the interest-only period ends, not after.

Formula

Interest-only mortgage formulas

Interest-Only Payment = Loan × (Annual Rate ÷ 12)

Amortising Months = Total Months − Interest-Only Months
Amortising Payment = Loan × [ r(1+r)ᵐ ] ÷ [ (1+r)ᵐ − 1 ]

Payment Shock = Amortising Payment − Interest-Only Payment
Equity Forgone = Loan − Conventional Balance at the same date

The interest-only payment has no principal in it, so the balance is exactly the same on the last day of the interest-only period as on the first. That full balance then has to amortise over whatever term is left, which is why the shock is driven by the length of the interest-only period rather than by the rate: a ten-year interest-only period on a thirty-year term is arithmetically a twenty-year mortgage with a ten-year delay attached.

Step by step

How the calculation works

1The interest-only payment is the simplest calculation in mortgage finance: balance times rate divided by twelve. Nothing is repaid, so nothing changes month to month.
2The second payment is the one to plan around. It is calculated as a fresh amortising loan for the full original balance over the months that remain, which is always shorter than the original term — and the shorter it is, the larger the step.
3Compare the total interest against the conventional figure alongside it. Keeping the balance at its maximum for years costs real money even at the same rate.
4The equity comparison assumes flat property values, which is the honest base case. With no amortisation, an interest-only borrower's equity depends entirely on appreciation, so a flat market leaves no cushion to refinance or sell against. The [amortization calculator](/calculators/amortization-calculator) shows what a conventional borrower would have repaid in the same window.
5Voluntary principal payments are usually permitted and change the outcome substantially. Used as a flexible amortising loan rather than a permanently lower payment, the structure is genuinely useful — the [mortgage calculator](/calculators/mortgage-calculator) gives the conventional baseline to pay against.

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The conventional structure this one defers, with taxes and insurance included.

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Borrowing against equity, rather than deferring its accumulation.

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Bridge Loan Calculator

Interest-only borrowing priced over months instead of years.

Finance Tool

Amortization Calculator

See the principal and interest split an interest-only period postpones.

property calculator

Interest-Only Mortgage Calculator

Both payment phases, the payment shock when the interest-only period ends, and the equity forgone.

Quick actions: Save inputs, view saved runs, print, share, or export results

Saved Runs

Inputs

Loan Amount$

Interest Rate%

Interest-Only Periodyrs

Total Loan Termyrs

Results

Interest-Only Payment

$2,812.50

Payment After the IO Period$3,801.82

Payment Shock$989.32 (35.2%)

Interest-Only Months120 months

Amortising Months240 months

Interest Paid During IO Phase$337,500

Interest Paid During Amortising Phase$412,437

Total Interest$749,937

Conventional Payment (same term)$3,242.99

Conventional Total Interest$667,477

Extra Interest vs Conventional$82,460

Monthly Saving During IO Phase$430.49

Balance When the IO Period Ends$500,000

Conventional Balance at That Point$426,505

Equity Forgone$73,495

Tip

The payment that matters is the second one. Keep this to hand before the interest-only period ends, not after.

Summary

Key output metrics

Interest-Only Payment

$2,812.50

$430.49 less than a conventional loan

Payment After the IO Period

$3,801.82

A 35.2% jump, arriving in month 121

Equity Forgone

$73,495

Conventional balance at that point would be $426,505

D3 visualization

Three Payments, One Loan

The interest-only payment, the payment it becomes, and what a plain amortising loan would have cost throughout.

Guide

How to understand and use this calculator

Quick Answer — How Does an Interest-Only Mortgage Work?

During the interest-only period you pay only the interest, so the balance never falls. When that period ends, the full original balance must amortise over the remaining term, which is shorter — so the payment jumps.

Interest-Only Payment = Loan × (Annual Rate ÷ 12)
Amortising Payment = Loan × [ r(1+r)ᵐ ] ÷ [ (1+r)ᵐ − 1 ], where m = total months − interest-only months
Payment Shock = Amortising Payment − Interest-Only Payment

Worked example: a $500,000 loan at 6.75% with a 10-year interest-only period on a 30-year term. The interest-only payment is $2,812.50 a month. After 120 months the same $500,000 must amortise over the remaining 240 months, at $3,801.82 — a jump of $989.32, or 35.18%.

The cost of the structure is visible in two places. Total interest across both phases is $749,936.81 against $667,476.57 on a conventional 30-year loan — $82,460.24 more. And at the end of the interest-only period the balance is still $500,000, where a conventional borrower would owe $426,504.99: $73,495.01 of equity forgone.

How to Use This Calculator: A $500,000 Loan With a 10-Year IO Period

Enter the loan amount, the rate, the length of the interest-only period, and the total term. The two payments appear immediately, because those are the only two numbers most borrowers are shown — and usually only the first one is emphasised.

The interest-only payment is trivially simple: $500,000 × 6.75% ÷ 12 = $2,812.50. There is no principal in it at all, which is why it is $430.49 a month lower than the $3,242.99 a conventional 30-year loan would cost. That saving is the entire appeal of the structure, and it is real: $430 a month for ten years is $51,659 of cash flow available for something else.

The amortising payment is where the arithmetic turns. The balance is still the full $500,000, but only 240 months remain, so the payment is calculated as a 20-year loan rather than a 30-year one. That produces $3,801.82 — $989.32 more than the interest-only payment and $558.83 more than a conventional borrower would be paying at the same point.

The equity figure is the one worth sitting with. A conventional borrower ten years in owes $426,504.99; an interest-only borrower owes $500,000. If house prices are flat over that decade, the interest-only borrower has $73,495.01 less equity and correspondingly less room to refinance, sell, or borrow against the property. Compare against the plain structure with the mortgage calculator, and see the principal-versus-interest split over time in the amortization calculator.

A Second Example: A Shorter IO Period at a Higher Rate

A shorter interest-only period produces a smaller shock, and the difference is instructive. Take $280,000 at 7.25% with a five-year interest-only period on a 30-year term.

The interest-only payment is $1,691.67 and the amortising payment is $2,023.86 — a shock of $332.19, or 19.64%. That is a far gentler step than the first example's 35.18%, for one reason: after five years there are still 300 months left to amortise over, against 240 in the ten-year case. The length of the interest-only period, not the rate, is what drives the payment shock.

The other numbers scale accordingly. Total interest is $428,657.77 against $407,633.69 conventional, a premium of $21,024.08. Monthly saving during the interest-only phase is $218.43. And equity forgone at the end of the period is $15,739.42 — the amount a conventional borrower would have paid down in those five years.

Read together, the two examples show the trade clearly: a longer interest-only period buys more monthly relief for longer, but costs disproportionately more in both interest and payment shock, because it compresses the same principal into ever fewer remaining months. A ten-year interest-only period on a 30-year term is, arithmetically, a 20-year mortgage with a ten-year delay attached.

When an Interest-Only Mortgage Makes Sense — and When It Does Not

The defensible cases share a structure: income that is genuinely lumpy rather than merely optimistic. A borrower paid substantially in annual bonus or commission can service a low monthly payment and make large principal reductions when the money arrives. An investor may prefer to keep capital deployed elsewhere at a higher return than the mortgage rate. A borrower with a known, dated liquidity event — a vesting schedule, a business sale — may bridge to it deliberately. In each case the plan exists before the loan does.

The dangerous case is using it to afford a house you cannot otherwise afford. The payment shock is not a risk; it is a certainty, scheduled to the month. A borrower who needs the interest-only payment to qualify comfortably today will need a materially higher income in ten years to keep the house, and betting on that is how interest-only lending earned its reputation in the 2000s.

Two structural risks compound this. Many interest-only products are adjustable rather than fixed, so the shock can coincide with a rate reset — the payment recalculates on a shorter term *and* at a higher rate at the same moment. And with no amortisation, equity depends entirely on price appreciation, so a flat or falling market leaves an interest-only borrower with no cushion, no refinance route, and no ability to sell without bringing cash to closing.

Voluntary principal payments change the picture entirely. Nothing prevents paying principal during the interest-only period, and doing so reduces both the eventual payment and the total interest. An interest-only loan used as a flexible amortising loan — paying the conventional amount most months, dropping to interest-only when cash is tight — is a genuinely useful instrument. Used as a permanently lower payment, it is a deferred problem. For other short-term structures, the bridge loan calculator and the second mortgage calculator cover the two most common alternatives.

Limitations

This calculator assumes a single fixed rate across both phases. Many interest-only mortgages are adjustable-rate products where the rate resets at or before the end of the interest-only period, in which case the actual amortising payment can be substantially higher than shown and the total interest figures will not hold. If your loan is adjustable, treat the output as a best case.

It also assumes no voluntary principal payments during the interest-only period and no recast afterwards. Both are common and both improve the outcome, so the payment shock shown is the maximum rather than the expected value for a disciplined borrower. Similarly, it assumes the loan runs its full term rather than being refinanced or the house sold, which is what actually happens in most cases — though relying on a refinance is relying on future credit conditions and future property value.

The payments shown are principal and interest only. Property taxes, homeowners insurance, mortgage insurance, and HOA dues sit on top and are unaffected by the structure, so the proportional shock to your total housing payment is smaller than the shock to the principal-and-interest figure. The equity comparison assumes flat property values and ignores transaction costs. This is a general educational tool, not lending advice — read the note carefully for reset dates and rate caps, and consult a qualified professional before choosing this structure.

Related Calculators

Compare directly against the conventional structure with the Mortgage Calculator, which adds taxes and insurance to the picture, and see how principal and interest split over time in the Amortization Calculator. For other short-term or secondary financing structures, the Bridge Loan Calculator prices interest-only borrowing over months rather than years, and the Second Mortgage Calculator covers borrowing against equity rather than deferring its accumulation. The Mortgage Amortization Schedule guide explains what the interest-only period is deferring.

Interest-only structures are most common on large loans, where the jumbo loan calculator checks the balance against the 2026 conforming limits and prices the reserves a jumbo lender will require.

Related concepts

Payment Shock

The step up when the interest-only period ends. Driven mainly by how many months remain to amortise over, not by the interest rate.

Negative Amortisation

Where a payment is smaller than the interest accruing, so the balance grows. Interest-only is one step better: the balance holds flat rather than rising.

Equity Forgone

Principal a conventional borrower would have repaid over the same period. The hidden price of the lower payment when prices are flat.

Frequently Asked Questions

How is an interest-only mortgage payment calculated?+

Loan balance times the annual rate divided by twelve. A $500,000 loan at 6.75% is $500,000 × 0.0675 ÷ 12 = $2,812.50 a month. No principal is included, so the balance does not fall.

What happens when the interest-only period ends?+

The full original balance amortises over whatever term remains. On a 30-year loan with a 10-year interest-only period, $500,000 must be repaid over 240 months rather than 360, which takes the payment from $2,812.50 to $3,801.82 — a 35.18% jump.

How big is the payment shock on an interest-only mortgage?+

It depends mostly on how long the interest-only period was, not on the rate. A 10-year period on a 30-year term produces a 35.18% jump in the worked example; a 5-year period produces 19.64%, because 300 months remain to amortise over instead of 240.

Does an interest-only mortgage cost more overall?+

Yes. On the $500,000 example, total interest is $749,936.81 against $667,476.57 for a conventional 30-year loan — $82,460.24 more, because the balance stays at its maximum for ten years instead of falling from day one.

Do you build any equity with an interest-only mortgage?+

Only through price appreciation. After ten years, an interest-only borrower still owes the full $500,000 where a conventional borrower would owe $426,504.99 — $73,495.01 less equity, and correspondingly less room to refinance or sell if values have not risen.

Can you pay principal during the interest-only period?+

Almost always, and it substantially improves the outcome. Every dollar of voluntary principal reduces both the eventual amortising payment and the total interest. Used this way — conventional payments most months, interest-only when cash is tight — the structure is a genuinely useful flexibility rather than a deferred problem.

Who should consider an interest-only mortgage?+

Borrowers with genuinely lumpy income, such as substantial bonus or commission pay, who can make large principal reductions when money arrives; investors deliberately keeping capital deployed elsewhere; and borrowers bridging to a known, dated liquidity event. In each case the repayment plan exists before the loan does.

Is the interest-only payment fixed for the whole period?+

Only if the loan is fixed-rate, and many interest-only products are not. On an adjustable product the rate can reset at or before the end of the interest-only period, so the payment recalculates on a shorter term and at a higher rate simultaneously. Check the note for reset dates and rate caps.

Formula

Interest-only mortgage formulas

Interest-Only Payment = Loan × (Annual Rate ÷ 12)

Amortising Months = Total Months − Interest-Only Months
Amortising Payment = Loan × [ r(1+r)ᵐ ] ÷ [ (1+r)ᵐ − 1 ]

Payment Shock = Amortising Payment − Interest-Only Payment
Equity Forgone = Loan − Conventional Balance at the same date

Step by step

How the calculation works

1The interest-only payment is the simplest calculation in mortgage finance: balance times rate divided by twelve. Nothing is repaid, so nothing changes month to month.
2The second payment is the one to plan around. It is calculated as a fresh amortising loan for the full original balance over the months that remain, which is always shorter than the original term — and the shorter it is, the larger the step.
3Compare the total interest against the conventional figure alongside it. Keeping the balance at its maximum for years costs real money even at the same rate.
4The equity comparison assumes flat property values, which is the honest base case. With no amortisation, an interest-only borrower's equity depends entirely on appreciation, so a flat market leaves no cushion to refinance or sell against. The [amortization calculator](/calculators/amortization-calculator) shows what a conventional borrower would have repaid in the same window.
5Voluntary principal payments are usually permitted and change the outcome substantially. Used as a flexible amortising loan rather than a permanently lower payment, the structure is genuinely useful — the [mortgage calculator](/calculators/mortgage-calculator) gives the conventional baseline to pay against.

Related guides

Learn more about this calculation

Guide

Interest-Only Mortgage Calculator

Inputs

Results

Interest-only mortgage formulas

How the calculation works

Learn more about this calculation

Mortgage Amortization Schedule: Formula and Guide

Mortgage Guide: Payment Formula, Costs, and PMI

Mortgage Calculator

Second Mortgage Calculator

Bridge Loan Calculator

Amortization Calculator

Key facts

Interest-Only Mortgage Calculator

Inputs

Results

Key output metrics

Three Payments, One Loan

How to understand and use this calculator

Quick Answer — How Does an Interest-Only Mortgage Work?

How to Use This Calculator: A $500,000 Loan With a 10-Year IO Period

A Second Example: A Shorter IO Period at a Higher Rate

When an Interest-Only Mortgage Makes Sense — and When It Does Not

Limitations

Related Calculators

Related concepts

Frequently Asked Questions

Interest-only mortgage formulas

How the calculation works

Learn more about this calculation

Mortgage Amortization Schedule: Formula and Guide

Mortgage Guide: Payment Formula, Costs, and PMI

Mortgage Calculator

Second Mortgage Calculator

Bridge Loan Calculator

Amortization Calculator

Key facts

Key output metrics

Three Payments, One Loan

How to understand and use this calculator

Quick Answer — How Does an Interest-Only Mortgage Work?

How to Use This Calculator: A $500,000 Loan With a 10-Year IO Period

A Second Example: A Shorter IO Period at a Higher Rate

When an Interest-Only Mortgage Makes Sense — and When It Does Not

Limitations

Related Calculators

Related concepts

Frequently Asked Questions