Mortgage Overpayment Calculator

When you overpay your mortgage, the extra money reduces your outstanding balance faster. Because interest is calculated on your remaining balance, a lower balance means less interest accrues each month — so more of every future payment goes toward the principal. This snowball effect can save thousands in interest over the life of the loan.

Both approaches save you money, but they work differently. A lump sum immediately reduces your balance, cutting interest from day one. Regular monthly overpayments build up consistent savings over time and are often easier to budget for. If you have a windfall (like a bonus or inheritance), combining both strategies gives you the maximum benefit.

Most lenders allow you to overpay up to 10% of your outstanding balance per year without incurring an Early Repayment Charge (ERC). Some lenders, like NatWest, allow up to 20% fee-free. If you're on a Standard Variable Rate (SVR), there are typically no restrictions. Always check your mortgage terms before making large overpayments.

Depending on your lender, overpayments can either reduce your monthly payment (keeping the same term) or reduce your remaining term (keeping the same monthly payment). Most people choose to reduce their term, as this maximises interest savings. Check with your lender which option applies — or ask to choose.

Yes, most lenders accept ad-hoc lump sum payments at any time. You can log into your lender's mortgage management portal to make one-off overpayments. Our calculator lets you model a single lump sum alongside regular monthly overpayments so you can see the combined impact.

Overpaying your mortgage generally has a neutral to positive effect on your credit profile. Reducing your overall debt levels can improve your debt-to-income ratio, which lenders look at favourably. It does not negatively impact your credit score.

This depends on your mortgage interest rate versus available savings rates. If your mortgage rate is higher than what you'd earn in a savings account after tax, overpaying is usually the better financial move. If savings rates exceed your mortgage rate, putting money in a high-interest account may be more beneficial. Your personal circumstances, such as having an emergency fund, should also factor in.

Your monthly payment is calculated using the standard mortgage amortisation formula, which factors in your outstanding balance, annual interest rate, and remaining term. The formula ensures that each payment covers the interest accrued that month plus a portion of the principal, so the loan is fully repaid by the end of the term.