Interest-Only Mortgage Calculator

What this calculator is, and when to reach for it

An interest-only mortgage does exactly what the name says for a defined opening period: your payment covers the interest and nothing else. The balance you owe at the end of that period is identical to the balance you started with, down to the last unit.

That is not a flaw in the product; it is the product. What you are buying is a lower payment now, and what you are paying for it is that none of those payments build equity. Everything else about interest-only lending follows from that single fact.

The consequence arrives on a known date. When the interest-only period ends, the full original balance must be repaid over whatever term remains — a shorter span than the loan was originally sold against. The payment does not drift upward; it steps, and the step is large.

Reach for this page when an interest-only period is on offer, when yours is approaching its end, or when you want to see plainly what a decade of interest-only payments does and does not achieve.

Who this genuinely suits

Borrowers with irregular income — commission, bonuses, seasonal or self-employed earnings — can benefit from a low compulsory payment while making large voluntary payments when money arrives. The structure gives flexibility to people whose cash flow is lumpy rather than smooth.

It also suits anyone with a credible, separate plan to clear the balance: an investment or endowment intended for the purpose, an expected inheritance, or a property that will be sold within the interest-only window.

It suits least the borrower who chose it because the amortizing payment was unaffordable. There, the lower payment is not a tool but a symptom, and the structure guarantees a much larger payment arrives later, on a schedule that cannot be renegotiated.

Where to go next

To see what the same loan costs on an ordinary repayment basis, use the mortgage calculator, and the amortization calculator to see how much principal a repayment loan would have cleared over the same years.

The HELOC calculator models the same two-phase structure on a credit line, and the ARM calculator covers the other main source of payment shock in mortgage lending.

If you are approaching the end of an interest-only period, the refinance calculator and the loan to value calculator together show whether refinancing is realistically available, and the extra payment calculator shows what voluntary principal payments during the interest-only phase would achieve.

How the two phases are worked out

Two separate calculations, joined by a date. Nothing connects them except the balance, which does not change.

interest-only payment = B × r   |   repayment phase = B × [ r(1 + r)^k ] ÷ [ (1 + r)^k − 1 ]

B

the loan balance, unchanged throughout the interest-only period

r

the periodic rate, being the annual rate divided by twelve

k

the payments remaining once the interest-only period ends

Why the balance cannot move

On a repayment loan the payment exceeds the interest due, and the surplus reduces the balance. On an interest-only loan the payment equals the interest due exactly, so there is no surplus and nothing to apply.

This produces a genuinely unusual property: the payment never changes during the interest-only period, because the balance it is calculated from never changes. Ten years of payments leave you owing precisely what you owed on day one.

The compression that causes the shock

When repayment begins, the full original balance must clear over the remaining term rather than the original one. A thirty-year loan with ten interest-only years becomes a twenty-year repayment loan carrying its entire starting balance.

So the payment rises for two compounding reasons: principal is now included at all, and it is squeezed into two thirds of the time. That is why the increase is typically far larger than borrowers anticipate.

What it costs across the whole loan

Because interest is charged on an undiminished balance for years, the total interest on an interest-only loan substantially exceeds that of an equivalent repayment loan. You are borrowing the same money for longer, in the sense that more of it stays outstanding.

The gap is worth quantifying rather than assuming, and it is the honest price of the lower early payments. Anyone treating interest-only as simply cheaper has looked at the monthly figure and not the total.

Voluntary principal payments change everything

Nothing prevents paying more than the interest during the interest-only period, and doing so reduces the balance exactly as on any loan. Each unit paid also reduces every subsequent interest-only payment, since those are calculated from the balance.

This is the feature that makes interest-only genuinely useful for irregular earners: a low compulsory floor with unlimited voluntary capacity above it. Used that way it behaves like a flexible repayment loan; used passively it behaves like a deferred problem.

What this page assumes

The calculator charges interest only during the interest-only period, so the balance does not move, then works out a level payment over the months that remain.

Because the balance does not fall during the interest-only period, the later payment is worked out over fewer months and is higher than a normal payment on the same loan.

Worked examples, step by step

Take a 350,000 loan at 6.5% over 360 months, with the first 120 payments interest-only and the balance repaid over the remaining 240.

The two phases, and the ordinary loan for comparison

The interest-only payment is 1,895.83, some 316.41 a month below the ordinary repayment figure of 2,212.24. Across ten years that is roughly 38,000 of retained cash flow, which is a genuine benefit if it is being used for something.

Then the payment steps to 2,609.51 — an increase of 713.68, or 37.6%, arriving on a single known date. It is also 397.27 a month above the ordinary loan the borrower declined, and it stays there for twenty years.

What the ten years actually bought

Across the interest-only decade the borrower pays 227,500 in interest and still owes the entire 350,000. Not a single unit of the debt has been repaid, and no equity has been built from payments — only from any change in the property’s value.

Total interest across the full thirty years comes to roughly 503,781, against 446,406 for the ordinary repayment loan. The lower early payments cost about 57,375 more overall, which is the price of the flexibility.

The version where it works

Suppose the same borrower pays the 1,895.83 minimum in lean months but adds substantial amounts in good ones, averaging the 2,212.24 an ordinary loan would have required. The balance falls, every subsequent interest-only payment falls with it, and the transition is far smaller.

That is the intended use. The compulsory payment is a floor for difficult periods, not a target for comfortable ones, and the difference between those two readings is the difference between a useful product and a deferred problem.

The vocabulary, on and around this page

Interest-only mortgage

A loan whose payments cover only accrued interest for a defined opening period, leaving the balance entirely unchanged.

Interest-only period

The opening span during which no principal is repaid. It ends on a known date fixed at the outset.

Repayment phase

The remainder of the term, during which the full original balance must be cleared over a shortened span.

Payment step

The increase at the transition between phases. It arrives at once rather than gradually, on a date known from the start.

Term compression

Repaying the original balance over fewer years than the loan was sold against, which amplifies the payment increase.

Repayment vehicle

A separate plan intended to clear the balance, such as an investment or expected sale. Some lenders require evidence of one.

Voluntary principal payment

Paying above the interest-only minimum. It reduces the balance and every subsequent interest-only payment.

Equity from payments

Ownership built by repaying principal. During an interest-only period it is exactly zero.

Equity from appreciation

Ownership built by a rise in property value. It is the only source of equity available during an interest-only period.

Amortizing loan

A loan whose payments exceed the interest due, so the balance falls. The comparison against which interest-only is judged.

Accrued interest

Interest that has arisen for a period. On an interest-only loan the payment matches it precisely.

Balloon risk

The danger of a large sum falling due when a plan to clear the balance does not materialise.

Loan to value

The balance as a share of property value. It does not improve through payments during an interest-only period.

Refinance dependency

Relying on a new loan to escape the transition, which requires both qualifying and favourable market conditions.

Irregular income

Earnings that arrive unevenly. The main legitimate case for interest-only, since it pairs a low floor with voluntary capacity.

Interest-only stress test

A lender check that the borrower could afford the repayment-phase payment, not merely the interest-only one.

Total interest

All interest across the life of the loan. Substantially higher than an equivalent repayment loan because the balance stays outstanding.

Transition date

The point at which repayment begins. Knowing it years ahead is the difference between preparing and being surprised.

Part and part

An arrangement splitting the loan between interest-only and repayment portions, moderating both the payment and the eventual step.

Underwriting evidence

Documentation a lender may require showing how the balance will be cleared before granting an interest-only loan.

Common mistakes, and what this page will not do

• Expecting the balance to fall. It cannot. After ten years of payments totalling 227,500 in interest, the example borrower still owes the full 350,000.
• Not knowing the repayment-phase payment. It rises from 1,895.83 to 2,609.51 — a 37.6% step arriving on one known date, and 397.27 above the ordinary loan declined.
• Choosing it because the repayment figure was unaffordable. The lower payment is then a symptom rather than a tool, and the structure guarantees a larger one arrives later.
• Having no plan to clear the balance. The full sum falls due regardless. Some lenders require evidence of a repayment vehicle precisely because this fails so often.
• Assuming equity is building. During the interest-only period equity comes only from a rise in property value, which can reverse.
• Treating the minimum as the target. The compulsory payment is a floor for lean months. Paying more when you can is what makes the structure work.
• Relying on refinancing to escape the step. It needs qualifying again and a valuation, and your loan to value has not improved through payments in the meantime.
• Judging the loan on the monthly saving. The example saves about 38,000 over ten years and costs roughly 57,375 more in total interest across thirty.
• Forgetting the transition date. It is fixed at the outset and does not move. Knowing it years ahead is what makes preparation possible.

What this calculator leaves out: This calculator does not include property tax, insurance, or association charges, model variable rates, assess whether a lender would require or accept a repayment vehicle, account for part-and-part arrangements, or model voluntary principal payments during the interest-only period. It applies one fixed rate across both phases.

Frequently asked questions