Figures are illustrative only, ignore charges and tax, and are not a forecast or personal recommendation.
Even relatively modest extra monthly payments on a mortgage can shorten the loan term and cut the total interest paid by a surprising amount, because every pound of overpayment reduces the balance interest is calculated on for the rest of the mortgage. This calculator compares your current monthly payment against the same payment plus a proposed extra overpayment, showing the new payoff time and the interest saved.
How the calculation works
The calculator simulates your mortgage balance being paid down month by month at your current interest rate, first using only your stated current monthly payment, then again using that payment plus your proposed overpayment. Each month, interest is calculated on the remaining balance and added, then the monthly payment is applied, reducing the balance by whatever is left after that month's interest. The simulation continues until the balance reaches zero in each scenario, and the calculator reports how many years each took, the time saved, and the difference in total interest paid.
Why overpayments are so effective
A mortgage is structured so that, especially in its early years, a large share of each monthly payment goes toward interest rather than reducing the balance. An overpayment goes entirely toward reducing the balance (assuming no early repayment charge applies), which means it reduces the amount of interest charged in every single subsequent month for the rest of the mortgage — this compounding effect, working in your favour, is why even a modest sustained overpayment can save a disproportionate amount of interest over a long remaining term.
Worked example
A £180,000 mortgage balance at 4.5% interest with a £1,050 monthly payment, overpaid by an extra £150 a month, typically shortens the remaining term by several years and saves a meaningful five-figure sum in total interest — try adjusting the overpayment amount in the calculator above and notice how the interest-saved figure grows faster than the overpayment amount itself, particularly for mortgages with many years remaining.
Before you overpay, check the small print
Most UK mortgage lenders allow overpayments up to a set annual limit — commonly around 10% of the outstanding balance per year — without triggering an early repayment charge (ERC), but exceeding that limit, particularly during a fixed-rate deal period, can trigger a penalty that may outweigh the interest saved. Always check your specific mortgage terms before committing to a regular overpayment plan, and consider whether the money might be better used elsewhere first — for example, clearing higher-interest debt, ensuring you have an adequate emergency fund, or using available ISA or pension allowance if the after-tax return available there would likely exceed the interest rate on your mortgage.
Frequently asked questions
Is overpaying my mortgage always the best use of spare money?
Not necessarily — it depends on your mortgage rate compared with what you could otherwise earn (for example, in a pension with tax relief and employer matching, or a Stocks & Shares ISA), and on your broader financial priorities like an emergency fund or higher-interest debt.
Does overpaying reduce my monthly payment or shorten my term?
This depends on how your lender applies overpayments — some automatically shorten the term while keeping the monthly payment the same (which is what this calculator assumes and generally saves the most interest), while others let you choose to instead reduce the monthly payment while keeping the original term.
What if my mortgage rate changes during a fixed period?
This calculator assumes a constant interest rate throughout, which is realistic only for the remainder of a fixed-rate deal — once you remortgage onto a new rate, the real-world figures will differ from this simplified projection.
Are there limits on how much I can overpay penalty-free?
Most lenders cap penalty-free overpayments at around 10% of the outstanding balance per calendar year during a fixed or discounted deal — check your specific mortgage offer document, as this varies by lender and product.
Should I overpay or invest instead?
If your mortgage rate is higher than the after-tax return you'd realistically expect from investing, overpaying is the mathematically stronger choice; if the reverse is true (and you're comfortable with investment risk and have a long enough horizon), investing may produce a better outcome — see our Savings vs Investment Calculator for a related comparison.
Common mistakes when planning mortgage overpayments
A frequent mistake is not confirming with your lender exactly how an overpayment will be applied — some lenders automatically recalculate your required monthly payment downward after a lump-sum overpayment unless you specifically request the term be shortened instead, which would produce a very different real-world outcome from what this calculator (which assumes the original payment continues and the term shortens) illustrates.
A second mistake is overpaying a mortgage while carrying higher-interest debt elsewhere, such as credit cards or an unsecured loan — those debts are almost always more expensive than a mortgage's interest rate, so clearing them first typically produces a bigger financial benefit than mortgage overpayments, even though a mortgage overpayment can feel like the more emotionally satisfying priority for many homeowners.
Do all lenders allow overpayments?
The vast majority of UK mortgages allow some level of penalty-free overpayment, but the exact limit and mechanism (some require a minimum overpayment amount, others allow any extra amount at any time) varies by lender and product, so always check your specific mortgage terms first.
Is it better to overpay monthly or with occasional lump sums?
Both reduce the balance interest is calculated on and produce similar total savings for the same total amount overpaid over the same period — monthly overpayments tend to suit those who want a disciplined, automatic habit, while lump-sum overpayments suit those receiving irregular windfalls like bonuses.
Does overpaying affect my credit score?
Paying down debt faster than required, including a mortgage, is generally neutral to positive for your credit profile over time, though a mortgage being paid off entirely and closed can occasionally cause a small, temporary dip in some scoring models due to a reduced credit mix — this is typically minor and short-lived.
What if interest rates fall after I commit to overpaying?
Overpayments still reduce your balance and the interest charged on it regardless of subsequent rate changes, though the relative attractiveness of overpaying versus investing the same money elsewhere may shift if your mortgage rate becomes lower than what you could otherwise earn.