Interest Only Mortgage Calculator

What this calculator does

An interest-only mortgage lets you pay only the interest for a set stretch of time. The payments are low, which is the appeal. But there is a catch that the low number hides, and it is worth understanding before you are drawn in by it.

This calculator works out two things: your payment during the interest-only period, at whatever frequency you choose, and the total interest you will hand over across that whole period. Read this next part carefully, because it matters. Those two figures describe the interest-only window only. They do not describe what happens after it ends, when the real repayment begins and the payment jumps. This page covers both, so you see the full shape of the loan, not just the comfortable first act.

How to use it

1. Loan amount. The sum you are borrowing.
2. Annual interest rate. The yearly rate, as a percent.
3. Payment frequency. How often you pay during the interest-only period: daily, monthly, quarterly, or yearly.
4. Interest-only period. How long the interest-only phase lasts. Enter a number and pick the unit, days, months, or years.

Press Calculate for the payment and the total interest over the interest-only period, or Reset to clear it.

How interest-only actually works

On a normal mortgage, every payment does two jobs. It covers the interest, and it chips a little off the amount you borrowed, so the balance slowly shrinks and you build ownership in the home. An interest-only mortgage does only the first job.

For the length of the interest-only period, your payment covers the interest and nothing else. That is why it is smaller. But it also means the balance does not move. The US Consumer Financial Protection Bureau puts it plainly: with an interest-only mortgage, the amount you owe does not go down with each payment. At the end of the interest-only years, you still owe every bit of what you first borrowed. You have paid to hold the loan, not to reduce it, and you have built no equity in the meantime.

How the payment is worked out

Because you are paying only interest, the payment is just the interest charged on the full balance for that slice of time. There is no principal in it, so it does not need the usual amortization formula.

Start with the yearly interest, which is the balance times the annual rate. Split that across the year to match how often you pay. A monthly payment is the yearly interest divided by twelve. A quarterly payment is a quarter of the yearly interest, and so on. To get the total across the whole interest-only period, the calculator simply adds up every one of those interest payments from start to finish. Since the balance never falls, each interest payment is the same size, which keeps the arithmetic clean.

A worked example you can check

Say you borrow 300,000 at 6 percent, pay monthly, and the interest-only period runs for 5 years. Let us run it.

• Yearly interest: 300,000 × 6 percent = 18,000
• Monthly payment: 18,000 ÷ 12 = 1,500
• Months in the period: 5 × 12 = 60
• Total interest over the interest-only period: 1,500 × 60 = 90,000

So for five years you pay 1,500 a month, a comfortable figure, and hand over 90,000 in interest along the way. And at the end of those five years, here is the part the low payment quietly leaves out: you still owe the full 300,000. Not a single unit of the principal has been repaid. That is the setup for what comes next.

What happens when the interest-only period ends

This is the section to slow down for. When the interest-only period ends, the loan does not. You still owe the full amount, and now you have to actually repay it, over whatever years are left. So the payment resets, and it jumps.

Take the example above. Suppose that 300,000 loan was a 30 year mortgage with a 5 year interest-only start. When the interest-only phase ends, the full 300,000 now has to be repaid over the remaining 25 years, principal and interest both. The payment climbs from 1,500 a month to about 1,932.90, roughly 29 percent higher, overnight. Lenders and borrowers call this payment shock, and if the loan carries a variable rate, the jump can be steeper still.

This is exactly the feature that got interest-only loans a bad name in the 2008 housing crisis, when borrowers reached the reset and could not absorb the higher payment. Because of that history, these loans now sit outside the safest category of mortgages. Regulators class them as non-qualified, which means they do not carry the same protections as a standard mortgage, and they are not offered through government-backed programs. This calculator deliberately shows you the interest-only payment and the interest cost, so you can see the appeal, but the honest picture is not complete until you have also worked out the payment waiting on the other side.

Who an interest-only loan is really for

None of this makes an interest-only mortgage a trap by default. It makes it a tool for specific situations, and a poor fit for most others. It can work when:

• You expect your income to rise before the reset, and can show a real plan for it, not just a hope.
• Your income is lumpy, from bonuses or commissions, and you intend to pay down the principal in chunks.
• You are an investor or expect to sell before the interest-only period ends.

It fits badly when you are relying on the low payment simply to afford the home, or when your plan for the reset is that rates will probably fall. The single thread running through every borrower who does well with these loans is a concrete plan for the day the payment jumps. If you do not have one, a standard repayment mortgage is almost always the safer road.

The assumptions behind the number

• The interest rate stays the same across the interest-only period.
• The balance is untouched during the period, because you pay only interest.
• Each interest payment is equal, since the balance does not change.
• The figures cover the interest-only period only. The higher payment after the reset is illustrated above but is not part of the calculator's output.

Interest-only loans often carry variable rates and specific terms on extra payments, so your real numbers can differ. Your loan agreement is the binding word. Treat this as a clear view of the interest-only phase, paired with an honest look at what follows.

Questions people ask

Do I build any equity during the interest-only period?

No. Since the balance does not fall, you gain no equity from your payments. Any equity would have to come from the home rising in value, which is never guaranteed.

Why does the payment jump so much afterward?

Because the full amount you borrowed now has to be repaid over fewer years than a normal mortgage would have. Squeezing the whole principal into a shorter remaining term makes each payment larger.

Can I pay down the principal during the interest-only period?

Often yes, but the loan terms control how extra payments are handled, and some charge for it. If your plan depends on paying the principal down early, confirm that in writing before you commit.

If they are so risky, why do they exist?

Because for the right borrower, with rising or irregular income or a short ownership horizon, the flexibility is genuinely useful. The risk comes from using one to stretch into a home you could not otherwise afford.

References

The description of how an interest-only mortgage works, and the fact that the balance does not fall during the interest-only period, follows the US Consumer Financial Protection Bureau's definition. The point that these loans sit outside the qualified mortgage category, and why that matters, also comes from the CFPB. The reset payment used to illustrate payment shock is a standard amortized loan payment over the remaining term, as derived in OpenStax's Principles of Finance.

1. U.S. Consumer Financial Protection Bureau, What is an "interest-only" loan? https://www.consumerfinance.gov/ask-cfpb/what-is-an-interest-only-loan-en-101/
2. U.S. Consumer Financial Protection Bureau, What is a Qualified Mortgage? (interest-only loans as non-qualified mortgages). https://www.consumerfinance.gov/ask-cfpb/what-is-a-qualified-mortgage-en-1789/
3. OpenStax, Principles of Finance 2e, Section 8.3, Loan Amortization. https://openstax.org/books/principles-of-finance-2e/pages/8-3-loan-amortization