Free calculator
Mortgage overpayment calculator
Paying a bit extra each month is the highest-certainty return most households can get, and the clearest way to see it is the years it takes off your mortgage. Drag the slider to see how much sooner you would be mortgage-free and how much interest you would never pay. Also called overpaying, prepaying, or making extra mortgage payments.
Add lump sums, a start date, or your lender's allowance+
When the monthly overpayment runs
Month 1 is now. Use this to start once a fixed deal ends. Currently from the start.
For overpaying more as pay rises. The extra goes up by this much every 12 months.
One-off lump sums
A bonus, an inheritance or savings paid straight off the balance. Add as many as you like.
Your lender's annual allowance
Most UK fixed deals allow 10%. Enter 0 for no limit.
Lenders differ. Your offer will say which.
Charges usually only apply while you are tied in.
Mortgage-free sooner by
6y 2m
Cleared in 18y 10m instead of 25 years. That is your return on paying £200/mo on top of £1,169.18/mo
This plan stays inside a 10% annual allowance. Check the figure against your own mortgage offer, and check whether your lender measures it against the original or the current balance.
How overpaying works
A repayment mortgage charges interest on whatever you still owe. Your normal monthly payment covers that month's interest first, and only what is left over comes off the balance. That is why the early years of a mortgage feel like you are barely making a dent.
An overpayment skips that queue. It comes off the balance in full, immediately. From then on you are charged interest on a smaller debt every single month for the rest of the term, which is why the saving is so much larger than the overpayment itself.
The calculator above simulates your mortgage month by month, twice: once on your current payment, and once with the extra. The clearest measure of what you gain is the time it removes: the years the mortgage clears early. The interest saved is the same win counted in pounds.
A worked example
Take a £200,000 mortgage at 5% over 25 years. The normal payment is about £1,169 a month, and left alone it runs the full 25 years and costs roughly £150,800 in interest.
Now add £200 a month on top, paying about £1,369 instead of £1,169. The same mortgage is gone in a little under 19 years. That is roughly six years off the term, and about £41,800 of interest you never pay. A modest, steady overpayment buys back years, which is why the term saved is the number worth watching. Put your own figures into the calculator above to see your version of it.
Lump sums, rising overpayments and the 10% limit
A flat monthly figure is rarely the real plan. Most people overpay in a lumpier way than that: a bonus in March, a bit more each year as pay rises, or nothing at all until the fixed rate ends. The options panel under the slider covers all three, and every one of them is optional, so the basic answer stays a single slider.
Lump sums take an amount and the month it lands in. That month matters more than people expect, because a lump sum only saves interest on the years that are left after it. A yearly increase models an overpayment that grows, say £200 a month rising by £50 every year. A start month delays the overpayment, which is what you want if you are waiting for a fixed deal to end before overpaying at all.
Set your lender's annual allowance and the calculator checks each year of the plan against it, then names the year that goes over and by how much. It stops there on purpose. It does not put a number on the early repayment charge, because that depends on your lender, your product and how far into the deal you are, so any figure would be invented. The useful output is knowing which year to move money out of.
One thing worth noting about the allowance: if your lender measures 10% against the balance at the start of each year, the allowance shrinks as the mortgage does. Late in a term a steady overpayment can exceed 10% of a nearly-cleared balance on paper. That is why the panel asks how long is left on your deal, since early repayment charges normally only apply while you are tied in.
Overpay, or invest instead?
Overpaying is the guaranteed, tax-free option, but it is not the only one. If you are weighing it against putting the money into investments or a pension, see should you overpay your mortgage or invest? for the full trade-off.
Common questions
›How much does overpaying my mortgage actually save?
Every pound you overpay comes straight off the balance, so you never pay interest on it again. The saving compounds: an overpayment made in year two avoids interest for every remaining year of the term, which is why a modest monthly overpayment early on can save far more than a larger one later.
›How many years can overpaying take off my mortgage?
It depends on the size of the overpayment relative to the balance, but the effect is larger than most people expect because the payment stays the same while the debt shrinks faster. As a worked example, paying an extra £200 a month on a £200,000 mortgage at 5% over 25 years clears it in under 19 years and saves about £41,800 in interest. That is roughly six years early. The calculator above shows the years saved for your own numbers.
›Is it better to overpay my mortgage or save the money?
Compare the mortgage rate with the after-tax return you would get on savings or investments. If your mortgage rate is higher, overpaying is the guaranteed, tax-free win. If a savings rate beats it, saving may leave you better off. Even then, overpaying reduces risk, and the money is much harder to reach again once it is in the house.
›Can my lender charge me for overpaying?
Yes. Most fixed-rate deals allow overpayments of up to 10% of the balance per year without penalty, and charge an early repayment charge on anything above that. Check your mortgage offer before you set up an overpayment, and check whether the limit is measured against the original or the current balance.
›Does overpaying reduce my monthly payment or my term?
Both are possible, and you usually have to tell your lender which you want. This calculator models the option that saves the most interest: keeping the payment the same so the mortgage clears earlier. Reducing the payment instead keeps the same end date and saves less.
›Can I model a one-off lump sum rather than a monthly overpayment?
Yes. Open the options panel under the slider and add a lump sum with the month it lands in. A bonus, an inheritance or a chunk of savings comes off the balance in full on the month you choose, and you stop paying interest on it from that point. You can add as many lump sums as you like, and combine them with a monthly overpayment. Timing matters: the same lump sum saves noticeably more in month 12 than in month 120, because it has more remaining years to avoid interest over.
›Can I model an overpayment that increases over time?
Yes. The options panel has a yearly increase, so you can model something like £200 a month rising by £50 each year as your pay goes up. There is also a start month, for when you only plan to begin overpaying after a fixed-rate deal ends. Both are optional and switched off by default.
›How do I know if my plan breaks the 10% overpayment limit?
Set your allowance percentage in the options panel and the calculator checks every year of your plan against it. If any year goes over, it tells you which year, what the plan overpays that year and what the allowance actually is. It deliberately does not estimate the early repayment charge, because the rate and the way it is applied vary by lender, product and year, so any figure would be a guess. You can also choose whether the allowance is measured against the original balance or the balance at the start of each year, as lenders differ.
Where to go next
- Overpay your mortgage or invest? →
The interest you save is only half the question. Here is that same spare money weighed against investing it instead.
- Should I overpay my mortgage? →
When overpaying is clearly right, when it is not, and the limits and penalties worth checking before you start.
- UK personal finance flowchart →
See where clearing your mortgage actually sits in the wider order of what to do with spare money.
- Financial planning for couples →
A mortgage is usually a joint decision. See it against both incomes and both sets of plans, not just one.
This is not advice
This calculator is for illustration only and is not financial advice. It assumes your interest rate stays the same for the whole term. In reality most UK mortgages revert to a different rate when the fixed or tracker deal ends, which changes the numbers. Check your lender's overpayment allowance and early repayment charges before overpaying, and speak to a qualified mortgage adviser about your own circumstances.
Overpaying is one decision. It's never the only one.
A calculator answers a question in isolation. CrestCast forecasts your whole household, from the mortgage and the salaries to the bills and the car, so you can see what overpaying does to everything else you are trying to afford.
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