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Interest-Only Mortgage Calculator

Estimate UK interest-only mortgage payments and total interest. See the unchanged capital due at the end and compare a repayment mortgage.

Last reviewed 3 August 2026

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Calculator inputs

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When to use this calculator

  • Before comparing mortgage products, brokers or repayment types.
  • When you want to test how a different deposit, rate or term changes the payment.
  • When you need a quick estimate before using a formal illustration or agreement in principle.
  • When you are stress-testing your budget against a higher rate.

A realistic Ireland planning example

Use these sample inputs as a quick scenario test, then change one variable at a time to compare outcomes.

A realistic Ireland planning example
InputValue
Loan Amount (€)€280,000
Interest Rate (%)5%
Term (Years)25 years

After entering these figures, review monthly payment, total interest and capital due together rather than in isolation. Then rerun the tool with one input adjusted.

How to read your results

Monthly Payment

Estimated monthly mortgage repayment under the rate, term and balance you entered. Compare it with a lender illustration before you apply.

Total Interest

Additional amount paid above the original principal during the term, using the rate you entered. Fees and early-repayment charges are not included unless you added them.

Capital Due

The figure produced by this output for the inputs you entered. Change one variable at a time if you want to see what drives it.

Method & assumptionsAuthoritative sources

An interest-only mortgage means your monthly payment covers only the interest charged on the outstanding loan — the original capital remains unchanged throughout the term and must be repaid in full at the end. This calculator computes the monthly interest charge by applying the annual rate (divided by 12) to the full loan balance, which remains constant because no capital is being repaid. In practice, the interest rate on your mortgage will likely change over time as fixed or tracker periods end and you remortgage, so the real cost will fluctuate. The calculator does not model the performance of any associated repayment vehicle. lenders require a verified, independent repayment strategy before granting an interest-only loan; the financial regulator mandates regular contact between lenders and borrowers to monitor repayment plans throughout the mortgage term.

Common mistakes

  • !Mixing up loan amount and property value, which distorts affordability and LTV.
  • !Using a headline rate but forgetting fees, insurance or repayment type.
  • !Testing only one term length instead of comparing payment and total cost together.
  • !Treating the result as a lender decision rather than a planning estimate.

What to do next

  • Run a second scenario with a cautious assumption so you can see the downside clearly.
  • Compare the result with the related calculators below before making a decision.
  • Check current local rules, eligibility and provider terms before applying or committing money.
  • Keep a record of the assumptions so you can update the estimate when a quote, bill or pay figure changes.

Frequently asked

The monthly payment is the loan balance multiplied by the annual interest rate and divided by 12. Because the payment does not reduce the capital, the full original balance remains due at the end of the term.

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