Interest-Only Loan Calculator

What the interest-only loan calculator does

The interest-only loan calculator shows the two different payments an interest-only loan produces over its life. During the interest-only period, your instalment covers only the interest, so the balance never moves. Once that period ends, the loan switches to repaying principal as well, and the monthly cost rises sharply. The tool shows both payments side by side with the total interest, so you know what you pay now, what you will pay later, and what the arrangement costs overall.

How an interest-only loan works

A conventional loan blends principal and interest into every payment, so the balance falls from the first month. With an interest-only loan, for an agreed opening period you pay only the interest charged on the balance, and at the end of that period you still owe exactly what you borrowed.

The monthly cost during this phase is the outstanding balance multiplied by the monthly interest rate, which is the annual rate divided by twelve. The balance does not change, so the payment stays the same throughout the interest-only period, and it is noticeably lower than a full principal-and-interest payment on the same loan.

When the interest-only period ends, the loan converts to a repayment loan and the full balance has to be cleared over the remaining term using standard amortisation. Each payment then covers the month's interest and some principal, so the balance begins to fall.

Why the payment jumps later

You spent the opening years without reducing the balance, so the entire original amount still has to be repaid, but over a shorter remaining term. A loan with a five-year interest-only period inside a twenty-five-year term must repay the whole balance over the final twenty years, which pushes the principal-and-interest payment above what it would have been without the interest-only phase.

Seeing both numbers together helps you judge whether you could comfortably afford the higher future payment as well as the lower one at the start.

Lower payments now, more interest later

Because the balance stays at its full size during the interest-only years, you pay interest on the largest possible amount for longer. Total interest over the life of the loan is usually higher than on an equivalent loan that repays principal from day one. Lower early payments improve cash flow, but the interest keeps accruing on a balance that is not shrinking.

For an investor whose priority is minimising monthly outgoings, or a borrower confident their income will rise, the flexibility can justify the extra cost. For someone using it to afford a larger loan than they otherwise could, the later payment shock and higher total interest are serious warnings.

How to use the calculator

1. Enter the loan amount you plan to borrow.
2. Enter the annual interest rate.
3. Enter the length of the interest-only period in years.
4. Enter the total loan term in years.

The tool shows your interest-only payment, the higher payment once repayment starts, and the total interest paid across the whole loan. The bar chart contrasts the two payment levels.

A longer interest-only phase lowers your payments for longer but leaves less time to repay the principal, so the later payment climbs higher and total interest grows. A shorter phase eases the eventual jump but gives up some of the early cash-flow benefit.

Who interest-only loans suit

Property investors often use interest-only borrowing to keep holding costs low while they wait for rental income or capital growth, sometimes intending to sell before repayment begins. Borrowers who expect a reliable rise in income may use the early years to manage tighter budgets and absorb the higher payment later. People with irregular or seasonal income can keep obligatory payments low and overpay when cash allows. In each case there is a clear plan for what happens when the interest-only period ends.

Problems arise when the interest-only period is used only to stretch affordability, with no realistic plan for the higher payment or the untouched balance. If nothing has been repaid and income has not grown, the end of the period can be a shock.

Using the results

The figures assume a fixed rate and a specific structure. Real loans can carry variable rates, fees, and features such as offset accounts, so use the calculator to compare scenarios.

Judge the loan on the later payment. The interest-only payment tells you what is comfortable now; the principal-and-interest payment tells you what you are committing to. If the higher figure looks difficult, the loan is stretching you further than it appears, and the total interest shows what the lower early payments cost.

This article is general information only and is not financial advice.

About This Interest Only Loan Calculator

Use this interest only loan calculator to get an instant estimate based on the figures you enter. Adjust the inputs above to match your situation, or browse all our calculators to plan your next step.