How much time and interest can you save by overpaying?
When you overpay your mortgage, the extra payment goes directly towards reducing the outstanding capital balance. Since interest is calculated on the outstanding balance, a lower balance means less interest accrues each month. This creates a compounding effect — less interest means more of each regular payment goes to capital, which reduces the balance faster, which reduces interest further, and so on.
The interest savings are particularly powerful early in the mortgage term when the outstanding balance is largest. A £200/month overpayment on a £200,000 mortgage at 5% in year 1 reduces the eventual interest bill by much more than the same £200/month overpayment in year 20.
The savings scale with the interest rate and how much you overpay. At 4.75% on a £200,000 mortgage over 25 years: a £100/month overpayment saves approximately £14,000 and reduces the term by 3 years; a £200/month overpayment saves ~£25,000 and cuts 5–6 years; a £500/month overpayment saves ~£47,000 and reduces the term by 10+ years. Use the calculator above for your exact figures, which depend on your specific balance, rate, and remaining term.
Most UK fixed-rate mortgages allow overpayments of up to 10% of the outstanding balance per year without an Early Repayment Charge (ERC). ERCs typically range from 1–5% of the amount overpaid above the limit. Once your fixed rate expires and you move to a tracker or SVR, unlimited overpayments are usually allowed. Always check your specific mortgage offer document or call your lender before making a large overpayment. Some specialist lenders allow 20% or even unlimited overpayments.
The mathematical answer depends on comparing your mortgage interest rate (a guaranteed, risk-free saving) against your expected investment return (variable and not guaranteed). At current UK mortgage rates of 4–5%, investing in a diversified global equity fund might reasonably be expected to outperform over a 10+ year horizon — but with market risk. Overpaying the mortgage reduces risk and provides a guaranteed return equal to your rate. Most financial advisers suggest: first, build an emergency fund (3–6 months expenses); second, maximise any employer pension matching; third, choose between mortgage overpayment and additional investment based on risk tolerance.
Both reduce outstanding balance and save interest. A lump sum has immediate impact — from the day you make it, interest is calculated on a lower balance. Monthly overpayments compound over time. If you have a windfall (work bonus, tax refund, sale of assets), a lump sum overpayment often makes more sense. If you can afford slightly more each month, regular overpayments build into substantial savings over time and are easy to set up as a standing order. The key is to start as soon as possible.
Most UK mortgage lenders keep monthly payments constant when you overpay — so the benefit goes to reducing the term (you pay off sooner). Some lenders will recalculate monthly payments to reflect the lower outstanding balance, reducing what you owe each month but not shortening the term. Reducing the term saves more interest in total. If your lender reduces monthly payments automatically, you may need to re-request higher payments to keep the term reduction benefit. Ask your lender explicitly which approach they use.