Mortgage Interest Calculator
This calculator separates principal from interest over a level repayment mortgage. It assumes the rate does not change.
Quick answer
Enter the mortgage, rate and term to estimate monthly repayment, total repaid and total interest.
Calculator
How to use this calculator
- Enter the figures that match your current scenario.
- Check the effective date, assumptions and any jurisdiction or plan selection.
- Review the breakdown, test a second scenario and verify the result before acting.
Explanation
What it is
Enter the mortgage, rate and term to estimate monthly repayment, total repaid and total interest.
How it works
The monthly payment is calculated with the standard amortisation formula and then multiplied by the number of months to separate total interest.
When to use it
Use this tool to explore a planning scenario before checking current official rules, product documents or professional guidance.
Limitations
- The result is an estimate based only on the inputs shown.
- Rates, thresholds and product terms can change after the effective date.
- The calculator does not replace an official assessment, provider quote or personalised advice.
Key terms
- Estimate
- A planning result produced from the stated inputs and assumptions, not a guaranteed outcome.
- Effective date
- The date or tax year for which a changing rule, threshold or rate has been checked.
- Authoritative source
- An official or regulator-backed source used to support a rule, rate or calculation method.
Formula
How we calculate this
The monthly payment is calculated with the standard amortisation formula and then multiplied by the number of months to separate total interest.
Statutory or methodological reference:MoneyHelper guidance — Mortgage Calculator.
Formula trace: Standard amortisation and affordability model with capital, interest, term, fees, LTV and overpayment scenarios; disclose assumptions and test 0%, boundary, negative and balloon cases.
Worked example
Enter realistic figures into the mortgage interest calculator and compare the result with the breakdown. Change one assumption at a time so you can see which factor has the greatest effect. Check the governing rule at MoneyHelper guidance — Mortgage Calculator.
FAQ
What does the mortgage interest calculator calculate?
Enter the mortgage, rate and term to estimate monthly repayment, total repaid and total interest.
Which assumptions have the biggest effect?
The most important assumptions are the amounts, time period, applicable rate or threshold, and any jurisdiction or plan choice shown in the form.
How accurate is this estimate?
It is designed for planning and testing scenarios. Accuracy depends on the inputs and whether your circumstances fit the simplified method described on the page.
Can I use the result as a final decision?
No. Verify changing rules and product terms, and seek suitable professional or official guidance when the decision is material or complex.
When should I recalculate?
Recalculate after a change in income, balance, rate, term, household circumstances, tax year or official policy.
Common mistakes
- Using a headline rate without checking whether it applies to the full amount.
- Mixing monthly and annual figures.
- Treating an educational estimate as an official assessment or guaranteed quote.
Tips
- Test a cautious scenario as well as an optimistic one.
- Keep a note of the assumptions and effective date.
- Compare the result with official guidance or provider documents before acting.
Related calculators
Related guides
Sources and editorial review
- MoneyHelper guidance — Mortgage Calculator
- Financial Conduct Authority guidance — Mortgages
- Bank of England data — Bank Rate.asp
Author and review
Author: FinanceHub UK Editorial Team — Editorial. Editorial policy.
Reviewed by role: Qualified mortgage adviser and FCA compliance reviewer. Named qualified reviewer sign-off is pending before production.
Review record date: 2026-07-10. Next review due: 2027-03-01.