Interest-Only Mortgage Calculator
Loans & MortgagesLow payments now, full payments later.
Loan & interest-only period
Enter a loan amount to see the interest-only and recast payments.
Advanced — income & affordability
Enter a loan amount to see the interest-only and recast payments.
How this is calculated
- 1During the interest-only phase you pay just the interest on the full balance: $0 × 0.00% ÷ 12 = $0 a month, and the balance stays at $0.
- 2Over 0 yrs that is $0 × 0 months = $0 of interest, with nothing repaid.
- 3When the IO period ends, $0 must amortize over the remaining 0 yrs (1 months), lifting the payment to $0 a month.
- 4That is a jump of $0 a month (0.0%) — the payment shock at year 0.
- 5Total interest is $0 + $0 = $0, about $0 more than a standard mortgage's $0 payment would cost.
Formulas
| Metric | Formula | Your value |
|---|---|---|
| Interest-only payment | loan × rate ÷ 12 | $0 |
| Payment after IO period | P × i ÷ (1 − (1 + i)^−n) | $0 |
| Payment jump | recast − interest-only payment | $0 |
| Total interest | IO-phase interest + repayment interest | $0 |
Your inputs
| Input | What it means | Your value |
|---|---|---|
| Loan amount | The principal you borrow and keep owing through the IO phase. | $0 |
| Interest rate | The annual rate; ÷ 12 it sets both the IO and the recast payment. | 0.00% |
| Total term | The full mortgage length — IO phase plus repayment phase. | 0 yrs |
| Interest-only period | Years of interest-only payments before the loan amortizes. | 0 yrs |
Know what this estimate is based on
- Jurisdiction
- General model; U.S.-specific rules are identified on the relevant tool
- Scope and limitations
- Educational estimate only. A lender may use different compounding, day-count, eligibility, tax, insurance, escrow, fee, or rounding rules.
- Source links checked
- Jul 30, 2026
Built and regression-tested by Smart Tools Lab. It has not been individually reviewed by a licensed financial, tax, or legal professional.
How to use
- 01
Enter the loan amount, the annual interest rate, the total mortgage term and how many of those years are interest-only — and, optionally, your income to gauge affordability.
- 02
Read the interest-only payment you would pay now against the higher principal-and-interest payment that begins once that period ends, plus the payment jump between them.
- 03
Use the standard-mortgage comparison, the affordability read, the two-phase table and the year-by-year schedule to judge whether the recast is one you can absorb.
Formula
An interest-only mortgage has two distinct phases, and this tool prices both. During the interest-only period the lender charges only the interest accruing on the full balance, so the monthly payment is simply the loan amount multiplied by the monthly rate: payment = loan x (rate / 100) / 12. Nothing goes to principal, so the balance never moves — it sits at the loan amount for the entire interest-only period. The interest paid in that phase is just the interest-only payment times the number of interest-only months (loan x rate / 100 x ioPeriod). When the interest-only period ends, the loan does not get smaller or shorter — the same full balance must now amortize over the months left in the total term, which is (term minus ioPeriod) x 12. That amortizing payment uses the ordinary level-payment formula M = P x i x (1 + i)^n / ((1 + i)^n - 1), where P is the unchanged loan amount, i is the monthly rate, and n is the remaining months. The difference between that new payment and the old interest-only payment is the payment jump, and dividing the jump by the interest-only payment gives the percentage rise. The tool also computes a standard repayment payment that would amortize the same loan over the full term from day one, so you can see what you trade for the early low payments. Total interest under the interest-only structure is the interest-only-phase interest plus the repayment-phase interest, and the extra interest figure is how much more that is than the standard mortgage over the same term. Two optional fields drive an affordability read: enter your annual income and any other monthly debts, and the tool measures both the interest-only payment and the recast payment against your monthly income, flagging whether the future payment still fits and grading the overall risk from the size of the payment jump and the affordability of that recast.
Example
Take a 3,000,000 loan at 6% on a 30-year term, with the first 5 years set as interest-only. The monthly rate is 6% / 12 = 0.5%. During the interest-only phase the payment is just that rate on the whole balance: 3,000,000 x 6% / 12 = 15,000 a month. Because none of that 15,000 touches principal, the balance stays at exactly 3,000,000 for all 5 years — that is 15,000 x 60 = 900,000 of interest paid while you owe just as much at the end as at the start. When the interest-only period ends, the same 3,000,000 must be amortized over the 25 years that remain (300 payments) at 6%. The level-payment formula lifts the payment to 19,329 a month — a jump of 19,329 minus 15,000 = 4,329, a rise of 28.9%. The repayment phase carries 2,798,713 of interest. Add the two phases together and the interest-only structure costs 900,000 + 2,798,713 of interest in total. For comparison, a standard 30-year repayment mortgage on the same 3,000,000 at 6% would cost 17,987 a month from day one — more than the interest-only payment now, but less than the 19,329 you face later. Over the full term the interest-only path costs about 223,567 more in interest than that standard mortgage. The two-phase table sums it up: the interest-only phase is 15,000 a month for 5 years (900,000 interest), and the repayment phase is 19,329 a month for 25 years (2,798,713 interest).
Definitions
- Loan amount
- The full principal you borrow and keep owing throughout the interest-only period, since no payments reduce it (0 to 30,000,000).
- Interest rate
- The annual rate on the mortgage; divided by 12 it sets both the interest-only payment and the later amortizing payment (0.1% to 20%).
- Total term
- The full length of the mortgage in years, covering both the interest-only phase and the repayment phase that follows (10 to 35 years).
- Interest-only period
- How many years you pay interest alone before the loan begins amortizing over the years left in the term (1 to 15 years).
- Interest-only payment
- The monthly cost during the opening phase — the balance times the monthly rate, with nothing applied to principal. This is the headline result.
- Payment after the interest-only period
- The principal-and-interest payment once the IO phase ends, when the full balance amortizes over the years that remain. On the defaults it is 19,329 a month.
- Payment jump
- The increase from the interest-only payment to the recast payment, and the percentage rise it represents — the payment shock you must be ready to absorb.
- Remaining principal after the IO period
- What you still owe when the interest-only phase ends. Because no payment touched the principal, it equals the original loan amount.
- Total interest
- Interest paid across both phases — the interest-only phase plus the repayment phase — which is higher than a standard mortgage on the same loan.
- Affordability (payment-to-income)
- The recast payment as a share of your monthly income; the tool checks the future payment, not just the comfortable one you start with, against the usual 28% housing guideline.
Good to know
What an interest-only payment actually buys you
An interest-only payment is the simplest figure in mortgage lending, and that simplicity is exactly what makes it seductive. Each month the lender works out the interest that has accrued on your balance — the loan amount multiplied by the monthly rate, which is the annual rate divided by twelve — and asks for precisely that, no more. On the default loan the arithmetic is 3,000,000 times 6% divided by 12, which is 15,000 a month. Nothing in that payment is set aside to repay what you borrowed; you are renting the money and paying the rent. The consequence runs through everything else about these loans: because not a single dollar reduces the principal, the balance after one payment is identical to the balance before it, and the same is true after the hundredth payment. You are buying a lower monthly outlay, and the price of that lower outlay is that the debt itself does not move. It is worth sitting with how different this is from an ordinary mortgage, where every payment is split between interest and principal and the balance falls a little each month. Here the split does not exist — the payment is pure interest by design. That makes the payment unusually easy to predict and unusually easy to underestimate the cost of, because the number on the statement looks small and stays small for years. Understanding that the interest-only payment is the rent on the full balance, and that the full balance is still waiting for you, is the foundation for every decision that follows about whether this structure fits your situation or quietly sets a trap.
The recast: when the loan finally has to amortize
The defining event of an interest-only mortgage is the moment the interest-only period ends and the loan is recast into a repayment loan. Nothing about the debt has improved in the meantime — the balance is still the full amount you borrowed — but the clock has shrunk. On the default loan, after five interest-only years the same 3,000,000 must now be repaid over the 25 years that remain, not the original 30. Cramming the full principal into fewer years forces the payment up, and the level-payment formula does the work: the monthly figure rises from 15,000 to 19,329. That increase of 4,329, a jump of 28.9%, lands in a single month with no warning beyond the calendar. This is the payment shock, and its size is governed by a simple relationship: the more of the term you spend paying interest only, the fewer years are left to amortize the same balance, so the steeper the eventual rise. A loan that is interest-only for ten of thirty years would jump far more violently than one that is interest-only for three. The shock is not a penalty or a fee; it is the unavoidable arithmetic of having postponed all principal repayment into a shorter window. Many borrowers who took these loans expecting to handle the recast found that the higher payment arrived alongside other pressures and proved harder to absorb than they had imagined. The honest way to use this calculator is to look first not at the comfortable opening payment but at the payment after the interest-only period, and to ask whether that larger number fits a budget you can realistically expect to have when it arrives.
No equity from payments, only from appreciation
On a conventional mortgage, equity grows from two sources at once: the property may rise in value, and every payment chips away at the balance. An interest-only mortgage removes the second engine entirely for the length of the interest-only period. Because the balance never falls, the only way your stake in the home grows during those years is if the property itself appreciates. That turns the loan into a directional bet. If values climb, you enjoy the gain without having had to fund it through principal payments, which is part of the appeal for some buyers. But if values merely hold steady, you finish the interest-only years owning exactly the same sliver of the home you started with, having paid 900,000 in interest on the default loan for the privilege. And if values fall, you can find yourself with less equity than you began with, or none at all. This dependence on appreciation is the quiet risk that distinguishes interest-only borrowing from the ordinary kind. A repayment mortgage builds a cushion automatically; each payment widens the gap between what the home is worth and what you owe, so a moderate dip in prices still leaves you with equity. The interest-only borrower has no such automatic cushion during the interest-only phase and is fully exposed to the market. It is reasonable to accept that exposure if you have a clear reason to expect appreciation or a plan that does not rely on equity at all, but it should be a conscious choice. Treating an interest-only loan as if it built equity the way a normal mortgage does is one of the most common and costly misunderstandings about how these loans work.
Who an interest-only structure can genuinely suit
For all their risks, interest-only mortgages are not a trap for everyone — there are situations where the structure is a sensible tool rather than a hazard. Borrowers with irregular or lumpy income are one example. Someone paid largely in commission, bonuses, or seasonal earnings may value a low mandatory payment that they can comfortably cover in lean months, while voluntarily paying down principal in the months when large sums arrive. The interest-only payment becomes a floor, and the borrower attacks the balance on their own schedule rather than the lender's. A second case is the borrower with a genuinely short expected hold. If you know you will sell within a few years — a relocation is coming, or the property is a stepping stone — paying down principal you are about to hand over anyway can be a poor use of cash, and the lower payment frees that money for other purposes. A third case is the property investor whose plan rests on rent or resale rather than on owning the building outright. An investor counting on rental income to cover costs, or on selling into a rising market, may rationally keep the balance high and the payment low, deploying capital elsewhere for a better return. What unites the sensible cases is that the borrower has a clear, funded plan that does not depend on the balance falling by itself, and the discipline or the certainty to follow it. The structure rewards people who use the freed-up cash deliberately. It punishes those who simply spend the difference between the low payment and what a repayment loan would have cost, arriving at the recast with the same full balance and no plan for the jump.
What happens if property values fall
The single most dangerous scenario for an interest-only borrower is a decline in property values while the full balance is still outstanding. Because nothing has reduced the debt, a falling market presses directly against a stake that was never widened by payments. If prices drop far enough, you can owe more than the home is worth — negative equity — even though you have never missed a payment and have done nothing wrong. This matters far more for an interest-only loan than for a repayment one, because the repayment borrower has been steadily building a buffer of equity that absorbs some of the fall, while the interest-only borrower stands at the full balance with no buffer at all. Negative equity is not just an uncomfortable number on paper; it closes doors precisely when you most need them open. It can make refinancing impossible, because lenders will not lend against a property worth less than the existing balance, which is a problem when your interest-only period is ending and you were counting on a refinance to handle the recast. It can make selling painful, since the sale proceeds would not clear the loan and you would have to find the shortfall in cash. And it leaves you exposed to the higher recast payment with no easy exit. The danger compounds because the conditions that depress property values — a weak economy, rising rates, local shocks — are often the same conditions that strain household budgets and tighten lending. An interest-only loan tends to concentrate its risks into the same bad moment. Anyone using this structure should picture not just the recast but the recast arriving in a soft market, and ask whether they could weather both at once.
The part interest-only loans played in housing busts
Interest-only mortgages carry a reputation earned in real downturns, and the history is worth knowing because the mechanics that caused trouble then are the same ones this calculator lays bare now. In the run-up to past housing crashes, interest-only and similar low-initial-payment loans were marketed heavily to buyers stretching to afford homes whose prices were climbing fast. The low opening payment let people qualify for and buy properties that a full repayment payment would have put out of reach. That worked as long as two things held: prices kept rising, so borrowers could refinance or sell into a gain before any recast bit, and the recast itself stayed comfortably in the future. When prices stopped rising and then fell, both escape routes closed at once. Borrowers who had built no equity through payments now owed more than their homes were worth, so they could not refinance away from the looming jump and could not sell without bringing cash to the table. As interest-only periods ended, payments reset upward into households that had been relying on the low payment to make the numbers work, and defaults followed. The structure did not cause the broader bust on its own, but it amplified it, concentrating risk in exactly the borrowers least able to absorb a shock. The lesson embedded in that history is not that interest-only loans are inherently wicked, but that they are unforgiving when the appreciation they implicitly assume fails to materialize. The protections that have since been added to lending exist precisely because the combination of no equity, a future payment jump, and a dependence on rising prices proved so dangerous when all three turned at once.
The lifetime cost compared with a standard mortgage
The low payment of an interest-only loan is not free, and the full bill shows up in total interest over the life of the loan. Because you carry the entire balance through the interest-only years rather than chipping it down, you pay interest on every dollar of it for longer than a repayment borrower does. On the default loan, the interest-only phase alone costs 900,000 in interest, and you arrive at the end of it owing the same 3,000,000 you started with. The repayment phase that follows then carries another 2,798,713 of interest as the full balance finally amortizes over 25 years. Set that against a standard 30-year repayment mortgage on the same 3,000,000 at 6%, which costs 17,987 a month from day one and steadily reduces the balance throughout. Over the full term the interest-only structure costs roughly 223,567 more in interest than the standard mortgage. That gap is the true price of the early low payments — money handed to the lender in exchange for deferring all principal repayment. It is easy to focus on the comfortable 15,000 figure and lose sight of this, but the comparison is the whole point of the decision. A repayment mortgage front-loads the discomfort and back-loads the savings; an interest-only mortgage does the reverse, front-loading the comfort and back-loading both a higher payment and a larger total cost. Neither is automatically wrong, but the choice should be made with the lifetime number in view, not just the opening one. If the only reason to choose interest-only is that the early payment is lower, the extra lifetime interest is the question you have to answer for it.
Planning your exit before the recast arrives
An interest-only mortgage works best when you treat the end of the interest-only period as a deadline you have prepared for, not a surprise you react to. There are three honest exits, and the time to choose among them is early, while you still have options. The first is to sell the property before the recast. If your plan always rested on a short hold, this is the clean exit — you hand over the balance from the sale proceeds and never face the higher payment at all, provided the property has held or gained value. The second is to refinance into a new loan, which resets the structure and can carry you into a fresh interest-only period or into a conventional repayment loan. Refinancing is powerful but conditional: it depends on your credit still being sound, your income still supporting the loan, and crucially the property still appraising for enough to lend against, none of which is guaranteed when the deadline comes. The third exit is to start repaying principal voluntarily before you are required to, shrinking the balance that will eventually have to amortize and softening the jump when it lands. Even partial early repayment helps, because a smaller balance produces a smaller recast payment over the same remaining years. The mistake to avoid is drifting toward the recast with no plan and no preparation, assuming the future will sort itself out. The borrowers who came to grief in past downturns were largely those whose exit depended entirely on selling or refinancing into a rising market, with no fallback if the market turned. Decide early which exit is yours, keep a second one in reserve, and revisit the plan as the deadline approaches rather than waiting for the payment to jump and forcing a hurried decision.
Frequently asked questions
Why does the balance stay the same during the interest-only period?
Your payment covers only the interest accruing each month, which is the balance times the monthly rate, and not a cent more. With nothing left over to reduce the principal, the amount you owe is identical at the end of the interest-only years as it was on day one. On the default 3,000,000 loan you still owe the full 3,000,000 after five years of paying 15,000 a month.
How big is the payment shock when the interest-only period ends?
When the period ends the same full balance must amortize over only the years left in the term, so the payment jumps sharply. On the defaults it climbs from 15,000 to 19,329, a rise of 4,329 or 28.9%. The shorter the interest-only period relative to the term, the steeper the jump, because the loan has fewer remaining years to spread the principal across.
Do I build any equity during the interest-only years?
Not through payments — because the balance never falls, the only equity you gain comes from the property rising in value. That makes an interest-only loan a bet on appreciation rather than a steady build of ownership. If prices stall or fall while you hold the full balance, you can reach the end of the interest-only period with little or no equity to show for years of payments.
Does an interest-only mortgage cost more over its life?
Yes, because you carry the full balance for longer and pay interest on every bit of it during the interest-only years instead of chipping it down. On the defaults the structure costs about 223,567 more in interest than a standard 30-year repayment mortgage on the same loan. The low early payments are borrowed comfort that the later years pay back with interest attached.
What can I do before the payment jumps?
You have three main exits: sell the property, refinance into a new loan, or begin paying down principal voluntarily before the recast arrives. Each removes or softens the jump — selling ends it, refinancing resets it, and overpaying shrinks the balance that has to amortize. Lining up an exit early matters, since refinancing depends on your credit and the property's value still cooperating when the deadline comes.
Will I be able to afford the payment after the interest-only period?
That is the question that matters most, because the recast payment — not the low one you start with — is the one you live with for most of the loan. Enter your income and the tool measures both payments against it: on a 900,000 income the 15,000 interest-only payment is about 20% of monthly income, while the 19,329 recast is about 26% — still inside the usual 28% housing guideline, but with far less room. If the recast pushes past a comfortable share of your income, the structure is a bet that your earnings will rise to meet it, and the risk grade flags how stretched that bet is.
How does an interest-only mortgage compare with a standard repayment one?
A standard mortgage amortizes from day one, so its payment is higher than your interest-only payment now (17,987 versus 15,000 on the defaults) but lower than your recast payment later (19,329). It also costs less overall: the interest-only path runs about 223,567 more in interest over the full term, because you carry the whole balance through the interest-only years instead of chipping it down. The comparison shows both side by side, so you can see exactly what the early low payments cost you.