Interest-Only Mortgage Calculator

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, which is $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 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.

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.

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.

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.