How Interest-Only Loans Work

An interest-only loan looks like a discount: the same principal at the same rate, a smaller monthly payment, nothing else changed. The payment really is smaller — and the discount has a schedule. Interest-only math is two phases with one formula each, a reset in the middle, and a total-interest bill that only a side-by-side calculation makes honest.

Two phases, two formulas

During the interest-only (IO) period, the monthly payment is the interest accrual and nothing else:

IO payment = principal × r      (r = annual rate ÷ 12)

Nothing touches the principal, so the balance at the end of the IO period is the balance you started with. When the IO period ends, the loan becomes a standard amortizing loan — a fresh payment computed from the same amortization formula used everywhere else:

M = P × r(1 + r)ⁿ / ((1 + r)ⁿ − 1)

with P the original principal and n the number of months in the amortization phase. The Interest-Only Loan Calculator runs both phases and adds their interest, which is the number most IO marketing leaves out.

Why the IO payment never moves

A normal loan's interest shrinks every month because the balance it's charged on shrinks. An IO payment has no such drift: the balance never falls, so the accrual is the same number every month for the entire IO period. On a $300,000 loan at 7.25%, the IO payment is $300,000 × 0.0725/12 = $1,812.50 — in month 1 and in month 60 alike. That flatness is a feature for budgeting and a trap for intuition: sixty payments of $1,812.50 total $108,750 paid, with the balance still exactly $300,000. Zero equity built, by design.

The reset, computed

When the IO period ends, the loan must amortize the full $300,000 over the amortization phase you enter. With a 5-year IO followed by a 20-year amortization, the new payment is a 20-year amortization of the original principal: $2,371.13 per month. That's $558.63 more than the IO payment — a 31% jump arriving on a date you can compute to the month from the loan's start date.

This is the payment shock the mortgage math article warns about, and it's structural, not punitive: the same balance now has fewer years to amortize over, so the principal share of each payment has to grow. The calculator returns both payments together with the total months and the payoff date, so the reset shows up as a line item rather than a surprise.

The total-interest bill

Adding the two phases: $108,750 of interest during IO plus $269,070.71 during amortization gives $377,820.71 of total interest, $677,820.71 paid overall. Now compare a straight 25-year loan on the same $300,000 at 7.25% — payment $2,168.42, total interest $350,526. The IO structure costs about $27,295 more in interest than the straight loan of the same overall length, despite requiring less cash in the early years. The extra cost comes from holding the balance at maximum for five extra years while it accrues at full rate.

That trade — cash-flow relief now, higher lifetime cost later — is exactly why IO structures suit borrowers with lumpy income or a genuine plan to retire principal elsewhere (HELOCs, bridge loans, construction draws). The affordability angle — whether you could carry the reset payment on today's income — belongs to the qualification side of mortgage math, and the underlying per-month interest mechanics to how loan interest works.

What the calculator doesn't model

The tool keeps the model deliberately clean: the rate is fixed for both phases, the IO payment always covers the accrual, and the balance never grows. Real IO products can differ in all three ways — variable rates repriced monthly, payment options that dip below the accrual (producing negative amortization), and balloon terms that require a lump payoff instead of an amortization phase. Treat the output as the baseline case for a fixed-rate IO structure, and read any real note for the repricing and negative-amortization clauses before signing.

Try it yourself

Model your structure in the Interest-Only Loan Calculator — enter the loan amount, rate, interest-only years, and amortization years (plus a start date if you want the payoff date), and read the two payments, two interest totals, and the combined bill. For a standard fully-amortizing loan, the Mortgage Calculator and the Amortization Schedule Generator show where each payment lands.