Mortgage Interest Calculator
See the total interest you will pay over your mortgage term, your monthly payment, and a visual breakdown showing what proportion goes to interest vs capital.
Enter your loan amount (what you are borrowing), the annual interest rate, and the mortgage term in years. The calculator leads with the total interest paid over the full term — the headline number people most want when comparing deals. Below it you will also see the monthly payment, total amount repayable, and a bar chart showing how your total repayment splits between capital and interest.
This mortgage interest calculator assumes a standard repayment (capital + interest) mortgage. For interest-only structures, model your setup using the dedicated Interest-Only Mortgage Calculator.
Monthly payment comparisons can be misleading because they ignore the term. A 30-year mortgage has a lower monthly payment than a 20-year mortgage at the same rate — but it costs significantly more in total interest. This tool puts the total interest figure first so you can see the full cost of a deal, not just the monthly headline.
The bar in the results splits your total repayable amount into two segments: the blue portion is the capital you borrowed (principal), and the red portion is the interest cost on top. A high rate or long term shifts the bar toward red — making the relative cost of the deal immediately visible.
Total interest is calculated by multiplying your fixed monthly payment by the total number of months in your mortgage term, then subtracting the original loan principal. For example, a £200,000 mortgage at 5% over 25 years results in a monthly payment of £1,169.18. Multiplying this by 300 months gives a total repayment of £350,754, meaning the aggregate interest paid is £150,754.
While extending your mortgage term reduces your monthly commitment, it increases the total interest paid because the compounding interest rate acts on the outstanding balance for a longer duration. Capital is repaid at a slower pace, meaning the lender charges interest on a larger average balance over the lifetime of the loan.
The relationship between rates and lifetime costs is compounding. On a £200,000 loan over 25 years, a 2% rate costs approximately £54,000 in lifetime interest. Raising that rate to 6% increases the interest cost to roughly £186,000. Each single percentage point increase can add tens of thousands of pounds to your aggregate debt overhead.
A mortgage interest calculator focuses strictly on isolating the aggregate lifetime interest costs and standard monthly commitments. An amortization calculator provides a chronological table showing precisely how each monthly payment splits between interest and capital reducing over time, which varies as the outstanding balance falls.