Mortgage Overpayment Calculator

1. Enter your current mortgage balance in the Mortgage Balance field.
2. Enter your annual interest rate (e.g., 4.5 for 4.5%).
3. Enter the remaining term of your mortgage in years.
4. Enter the amount you plan to overpay each month in the Monthly Overpayment field.
5. Optionally enter a one-off lump sum overpayment amount.
6. View your total interest saved, time saved, and the comparison chart instantly.

How Mortgage Overpayments Work

When you make a mortgage overpayment, the extra money goes directly toward reducing your outstanding loan balance. Because interest is calculated on the remaining balance, a lower balance means less interest is charged each month. This creates a compounding effect: each overpayment reduces your balance, which reduces your interest, which means more of your regular payment goes toward the principal, which further reduces your balance. Over time, even modest overpayments can produce substantial savings.

The Compound Effect of Overpayments

The earlier you start overpaying, the greater the benefit. In the early years of a mortgage, most of your monthly payment goes toward interest rather than principal. By reducing the balance early on, you shift this ratio in your favour much sooner. A £200 monthly overpayment started in year 1 will save significantly more than the same overpayment started in year 10, because the interest savings compound over a longer period. Lump sum overpayments also have a greater impact when made early in the mortgage term.

Early Repayment Charges to Watch For

Before overpaying, check whether your mortgage has early repayment charges (ERCs). These typically apply during fixed-rate or tracker-rate introductory periods and can range from 1% to 5% of the overpayment amount. Most lenders allow penalty-free overpayments of up to 10% of your outstanding balance per year. Once your introductory deal ends and you move to your lender's standard variable rate, ERCs usually no longer apply. Always read your mortgage terms carefully or contact your lender to confirm the overpayment limits before committing to a plan.

When Overpaying May Not Be the Best Option

Overpaying your mortgage is not always the optimal use of spare cash. If you have higher-interest debts such as credit cards or personal loans, it is usually better to clear those first. You should also maintain an emergency fund covering 3 to 6 months of essential expenses. If savings interest rates exceed your mortgage rate, your money may work harder in a high-interest savings account or ISA. Additionally, maximising pension contributions (especially to capture employer matching) often provides a better long-term return than mortgage overpayments.

Calculate your standard mortgage payments with our Mortgage Calculator, check property purchase costs with the Stamp Duty Calculator, or explore how your spare cash could grow with the Savings Calculator.