What this calculator does
This free UK mortgage calculator estimates the monthly repayment on a standard capital repayment mortgage. Enter three figures — the loan amount, the annual interest rate and the repayment term in years — and the calculator returns your estimated monthly payment, total amount repaid over the full term and total interest charged.
It is designed for initial planning: comparing the cost of different loan amounts, seeing how much a rate change affects your payment, or working out what term length fits your budget. Results update as you type.
How the calculation works
The calculator uses the standard annuity formula used by lenders and the Bank of England:
Where P is the loan amount, r is the monthly interest rate (annual rate ÷ 12) and n is the total number of monthly payments (years × 12). The formula produces a fixed equal payment for every month of the term.
In the early months, almost all of each payment covers interest, with very little reducing the loan balance. Over time this reverses: as the balance falls, less interest accrues each month and more of each payment reduces the capital. By the final months, payments are almost entirely capital repayment.
Example calculation
A £250,000 mortgage at 4.5% interest over 25 years:
Monthly payment
£1,389
Total repaid
£416,700
Total interest
£166,700
Extending the same mortgage to 35 years reduces the monthly payment to approximately £1,164 — saving £225 per month — but increases total interest paid to around £238,900, an extra £72,200 over the life of the loan.
Reducing the rate from 4.5% to 4.0% on a £250,000 mortgage over 25 years cuts the monthly payment to approximately £1,319 and saves around £21,000 in total interest. Small changes in rate compound significantly over a 25–35 year term.
When to use this calculator
- Before making an offer — quickly check whether the repayments on a likely mortgage are comfortable against your income and outgoings.
- Comparing fixed-rate deals — enter different rates to see the monthly and total cost difference between a two-year and five-year fix.
- Deciding on term length — compare a 25-year and 30-year term to understand the monthly saving versus the long-term interest cost.
- Stress-testing your budget — enter a higher rate (e.g. 6.5% or 7%) to check what happens to your repayments if rates rise when your fixed deal ends.
- Remortgage planning — model your remaining balance at a new rate and term as your current deal approaches expiry.
Common mistakes
Forgetting lender fees
Arrangement fees of £999–£1,999 are common on competitive fixed rates. Adding these to the loan increases the balance and the total interest. A fee-free mortgage at a slightly higher rate can cost less overall — always compare the true cost including fees.
Assuming the rate stays fixed for the full term
Most fixed rates last two or five years. After that, you revert to the lender's standard variable rate (SVR) unless you remortgage. The SVR is typically 2–4 percentage points higher than the initial rate, which can significantly increase repayments.
Ignoring affordability stress tests
Lenders do not just check whether you can afford today's repayment — they stress test your application at a higher rate to check resilience. If the calculator says a mortgage is comfortable but a stress-tested rate makes it unaffordable, the lender may decline or reduce the loan.
Comparing monthly payments without checking term length
A lower monthly payment is only attractive if the term and rate are also comparable. A 35-year mortgage looks cheap monthly, but extending the term on a remortgage can add decades and tens of thousands in interest.
Frequently asked questions
How is a monthly mortgage repayment calculated?
It uses the standard annuity formula, which produces equal monthly payments across the full term. Early payments are mostly interest; later payments are mostly capital. The formula is: PMT = P × [r(1+r)ⁿ] ÷ [(1+r)ⁿ−1].
What is the difference between a repayment and an interest-only mortgage?
A repayment mortgage reduces the loan balance each month — you repay both capital and interest, so the debt is fully cleared at the end of the term. An interest-only mortgage charges interest only, leaving the original loan amount outstanding at the end of the term. This calculator estimates repayment mortgages only.
Does the calculator include lender fees?
No. Results show the repayment based on loan amount, rate and term only. Arrangement fees, valuation fees, broker fees and insurance are not included and must be factored in separately when comparing deals.
What rate should I enter if I do not yet have a mortgage offer?
Use the current average two-year or five-year fixed rate as a planning figure. As a stress test, enter a higher rate (such as 6–7%) to see what repayments would look like if rates rise when your current deal ends. Bank of England mortgage data is a reliable source for typical rates by loan-to-value.
Can I use this calculator for buy-to-let mortgages?
Yes — the repayment formula is the same. However, buy-to-let mortgages are often interest-only and lenders assess them differently, based on rental income coverage rather than personal income. Use the result alongside our rental yield calculator to check whether rental income covers the payment.
How much can I borrow?
Most UK mortgage lenders offer between 4x and 4.5x annual income, though some lend up to 5x or 5.5x for higher earners or certain professions. Deposit size, credit history, existing debts and monthly commitments all affect the final offer. Use our mortgage affordability calculator for an income-based estimate.
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Related guides
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Important information
This calculator is for general information and planning purposes only. Results are estimates based on the standard annuity formula applied to the inputs you provide. They do not constitute a mortgage offer, mortgage advice, or financial advice of any kind.
Actual mortgage repayments will depend on your lender's specific product terms, any fees added to the loan, the type of interest calculation used, and any changes to a variable or tracker rate. The calculator does not account for arrangement fees, valuation fees, broker fees, buildings insurance, or any other costs associated with a mortgage.
Always seek independent advice from a qualified, FCA-regulated mortgage broker or adviser before making any mortgage decision. Read our full Disclaimer.