Mortgage Overpayment Calculator
Overpaying a mortgage — paying more than the required minimum each month, or adding a lump sum — reduces the balance faster than scheduled, which means less of your future payments go toward interest and more toward genuinely clearing the debt. This calculator simulates your mortgage with your chosen overpayment applied, showing exactly how much interest it saves and how much sooner the mortgage would be paid off.
Overpayment Calculator
See how much interest you save and years you knock off by overpaying your mortgage.
Why £100 a Month Saves More Than £100 a Month’s Worth of Interest
An overpayment does not simply save interest on the extra £100 itself — it reduces the balance the mortgage is charged interest on for every single month remaining in the term, which compounds considerably over a long mortgage. A pound overpaid in year one saves interest on that pound for potentially twenty or more remaining years, while a pound overpaid in year twenty only saves interest for the few years left. This is why overpaying earlier in a mortgage term produces a disproportionately larger total saving than the same overpayment made later, even though the monthly amount is identical either way.
This is exactly why starting an overpayment plan as early as realistically possible, rather than waiting until later in the mortgage when finances feel more settled, tends to produce a meaningfully better outcome — the same monthly amount, started a few years earlier, genuinely does more work over the life of the mortgage.
£100 a Month, Verified in Real Numbers
A £200,000 mortgage at 4.5% over 25 years would normally cost roughly £133,500 in total interest across the full term. Adding a £100 monthly overpayment throughout cuts that to around £112,360 in interest — a saving of roughly £21,140 — while also clearing the mortgage about three and a half years earlier than the original 25-year schedule. A modest, consistent overpayment produces a genuinely substantial result once run through the full term.
The 10% Ceiling Most People Never Check
Most mortgage deals allow overpayments up to a set annual limit, commonly 10% of the outstanding balance, without triggering an early repayment charge — but this is not universal, and exceeding it can genuinely cost more than the interest saved. This calculator checks your entered overpayment against your specified annual limit and flags clearly if either a lump sum or the annualised monthly overpayment would exceed it. This is worth checking directly against your own specific mortgage terms before committing to a large overpayment plan, since the exact limit and any charge for exceeding it varies meaningfully between lenders and even between different products from the same lender.
The limit is typically calculated against the balance at the start of each year, not a fixed lifetime figure, which means the actual pound amount you can overpay without a charge changes each year as the balance itself falls — worth recalculating annually rather than assuming a single fixed pound figure applies for the whole remaining term.
Lump Sum Now or Monthly Later — Does It Matter?
This calculator applies any lump sum immediately, reducing the starting balance before the monthly simulation begins, then applies the ongoing monthly overpayment on top of the standard required payment every month afterward. Timing matters here too: a lump sum paid in early reduces the balance for the entire remaining term, similar to the effect of an early monthly overpayment, while the same lump sum paid in later in the term has a smaller cumulative effect, simply because less time remains for the reduced balance to keep saving interest.
This is worth bearing in mind for anyone expecting a future lump sum, such as an inheritance, bonus or maturing savings product — applying it toward the mortgage as soon as it is genuinely available, rather than delaying, generally captures a larger overall saving than waiting until later in the term, even for the exact same lump sum amount.
Two Ways to Measure the Win: Years or Pounds
This calculator shows both how much total interest is saved and how much sooner the mortgage is paid off, and it is worth looking at both figures rather than just one — they tell genuinely different stories about the same decision. The interest saved figure shows the direct financial benefit in pounds; the time saved figure shows the more tangible, easier-to-visualise outcome of being mortgage-free years earlier than originally scheduled. Both are real, and which one resonates more is simply a matter of how you personally prefer to think about the decision. If overpaying does not suit your current situation and extending the term to lower monthly payments is more relevant instead, our Extend Mortgage Term Calculator covers that opposite strategy. Our Mortgage Calculator is useful for the standard repayment figures without any overpayment applied, as a baseline comparison.
Frequently Asked Questions
Does my monthly mortgage payment reduce after overpaying?
This depends on your lender — some automatically recalculate a lower required monthly payment after a significant overpayment, while others keep the original payment the same and simply shorten the remaining term instead. It is worth checking which approach your specific lender applies.
Is it better to overpay my mortgage or invest the money instead?
This depends on your mortgage rate compared with realistic investment returns, and your personal risk tolerance — overpaying guarantees a return equal to your mortgage rate, while investing offers potentially higher but genuinely uncertain returns, so there is no universally correct answer.
Can I stop overpaying if my circumstances change?
Yes, generally — voluntary overpayments are not a binding ongoing commitment in the way the required minimum payment is, so you can typically increase, reduce or stop them as your circumstances change, though it is worth confirming this flexibility with your specific lender.
Does overpaying affect my ability to borrow more later, like for home improvements?
It reduces your outstanding balance, which can improve your equity position and potentially your LTV band if you remortgage later, generally a positive factor rather than a limitation on future borrowing.
What happens to overpayments if I am on a fixed-rate deal?
Overpayment allowances typically apply throughout a fixed-rate deal’s term, though it is worth confirming the specific annual limit for your particular fixed deal, since this can occasionally differ from a lender’s standard variable rate terms.
Important Information
This calculator provides an estimate based on the figures you enter and does not constitute financial advice. It does not confirm your specific lender’s overpayment terms or limits. For advice specific to your circumstances, consult a mortgage broker or FCA-regulated adviser. See our Disclaimer for further information.