Mortgage Overpayment Calculator UK 2025 — How Much Interest Will You Save?
Overpaying your mortgage — even by a small amount each month — can save you thousands of pounds in interest and knock years off your mortgage term. Use this calculator to see exactly how much you could save, and check whether overpaying would trigger an early repayment charge.
Most fixed-rate mortgages allow overpayments of up to 10% of the outstanding balance per year without an early repayment charge (ERC). Exceeding this triggers ERC, typically 1–5% of the amount overpaid above the limit. Always check your mortgage offer documents or call your lender before making large overpayments.
Why Overpay Your Mortgage?
Every extra pound you pay off your mortgage reduces the outstanding balance, which in turn reduces the amount of interest you pay going forward. Because mortgage interest is calculated on the remaining balance, reducing it early has a compounding benefit — you save not just on the interest for that pound, but on all the interest that would have accrued on it over the remaining term.
On a typical £200,000 mortgage at 4.5% over 20 years, an extra £200/month saves over £25,000 in interest and knocks around 4 years off the mortgage. The earlier in the mortgage term you start overpaying, the greater the benefit — because interest is highest when the balance is largest.
The 10% Annual Overpayment Rule
Most fixed-rate and tracker mortgages allow you to overpay by up to 10% of the outstanding balance each calendar year (January to December) without incurring an early repayment charge (ERC). Some lenders define this as 10% of the original mortgage balance — check your mortgage offer carefully. Overpaying beyond this limit during a fixed or discounted rate period typically triggers an ERC of 1–5% of the excess.
Some mortgages — particularly flexible mortgages, offset mortgages, and standard variable rate (SVR) mortgages — allow unlimited overpayments with no ERC at all. If your deal has ended and you are on your lender's SVR, you can usually overpay as much as you want.
Lump Sum vs Monthly Overpayments
Both produce similar long-term results in terms of interest saved, but they work slightly differently. A single lump sum payment reduces the balance immediately and starts saving interest from that point. A monthly overpayment gradually builds up the same total while giving you more flexibility. If you receive a bonus, inheritance, or other windfall, a lump sum can be very effective — provided it stays within the 10% annual limit.
Should You Overpay or Save?
This depends primarily on the interest rate on your mortgage versus the interest rate available on savings. In a high interest rate environment, if your savings account pays more than your mortgage rate, it can make financial sense to save rather than overpay — especially in a cash ISA where interest is tax-free. However, many people prefer the certainty and guaranteed return of mortgage overpayment, and the psychological benefit of reducing debt. If you have high-interest debt (credit cards, personal loans), always pay those off before overpaying your mortgage.
Frequently Asked Questions
It depends on your lender and what you request. Most lenders by default will reduce your remaining term (keeping the monthly payment the same) when you overpay. Some allow you to choose to reduce your monthly payment instead, which lowers your regular outgoings but saves less interest overall. Shortening the term saves more money. Check with your lender which option applies and how to switch.
An early repayment charge (ERC) is a penalty charged by your lender if you repay more of your mortgage than allowed during a special rate period (fixed, discounted, or tracker). ERCs are typically 1–5% of the amount repaid above the annual allowance, reducing each year of the deal period. For example, a 2-year fix might have an ERC of 2% in year 1 and 1% in year 2. On large overpayments, ERCs can be substantial — always calculate the ERC against the interest saving before proceeding.
On most standard mortgages, no — once you overpay, you cannot withdraw that money. However, flexible and offset mortgages allow you to draw down overpayments you have made, essentially creating a pot of accessible money alongside your mortgage. This flexibility comes at a slightly higher rate. If access to funds is important, a high-interest savings account alongside a standard mortgage might be preferable.
Yes, significantly. Your loan-to-value (LTV) ratio determines the mortgage rates available to you. The lower your LTV, the better rates you can access. For example, reducing your LTV from 85% to 80% or from 75% to 70% can unlock substantially better rates. If you are close to an LTV threshold (80%, 75%, 60%, etc.), a targeted overpayment to cross that threshold before your deal ends can save considerably more than the interest saving from the overpayment itself.