Mortgage overpayment calculator
See what overpaying would save in interest, how much sooner the mortgage clears — and how that compares with investing the same money.
Calculator
What overpaying saves, and the working behind it
What overpaying would be worth
Overpaying would save you
£49,509.24
of interest — £206,099.06 down to £156,589.82.
And it would be paid off
5 years and 3 months
sooner — 19 years and 9 months instead of 25 years.
| Figure | As it stands | With your overpayment |
|---|---|---|
| Contractual payment a month | £1,520.32 | £1,520.32 |
| Interest over the term | £206,099.06 | £156,589.82 |
| Overpayments actually made | £0.00 | £47,200.00 |
| Everything that leaves your account | £456,099.06 | £406,589.82 |
| Still owed at the end | £0.00 | £0.00 |
74.00% of your very next payment is interest.
Of the £1,520.32 leaving your account next month, £1,125.00 is interest and only £395.32 comes off what you owe. An overpayment is different: every penny of it comes off the balance, and it then stops that balance being charged interest for every month the mortgage has left to run. That is why the same £1,000 is worth several times as much paid now as paid in the final years, and the year-by-year table below shows the proportion moving.
Check your early repayment charge before you overpay.
Most fixed-rate UK mortgages allow you to overpay up to 10% of the balance a year without penalty, and charge an early repayment fee on anything above it — often a percentage of the amount repaid. That is a term of your particular deal: it varies by lender, by product and by which year of the deal you are in, so this calculator does not model it and no calculator can tell you what yours is. The figures above assume no charge is made. If you would exceed your allowance, the real saving is smaller than the one shown here — and possibly negative. Your mortgage offer or annual statement states the allowance and the charge.
A fixed-rate schedule is not a forecast. This is what would happen if 5.40% held for every month of the 25 years above. A rate fixed for two or five years is not fixed for 25 years; when the deal ends the rate changes, and so does every figure on this page. Re-run it with the rate you expect to be paying, and treat the answer as arithmetic on an assumption rather than as a prediction.
Year by year, and where each payment actually goes
| Year | Opening | Interest | Capital | Overpaid | Payments | Closing | Interest share |
|---|---|---|---|---|---|---|---|
| 1 | £250,000.00 | £13,320.50 | £4,923.34 | £2,400.00 | £18,243.84 | £242,676.66 | 73.01% |
| 2 | £242,676.66 | £12,915.13 | £5,328.71 | £2,400.00 | £18,243.84 | £234,947.95 | 70.79% |
| 3 | £234,947.95 | £12,487.28 | £5,756.56 | £2,400.00 | £18,243.84 | £226,791.39 | 68.45% |
| 4 | £226,791.39 | £12,035.75 | £6,208.09 | £2,400.00 | £18,243.84 | £218,183.30 | 65.97% |
| 5 | £218,183.30 | £11,559.24 | £6,684.60 | £2,400.00 | £18,243.84 | £209,098.70 | 63.36% |
| 6 | £209,098.70 | £11,056.35 | £7,187.49 | £2,400.00 | £18,243.84 | £199,511.21 | 60.60% |
| 7 | £199,511.21 | £10,525.62 | £7,718.22 | £2,400.00 | £18,243.84 | £189,392.99 | 57.69% |
| 8 | £189,392.99 | £9,965.51 | £8,278.33 | £2,400.00 | £18,243.84 | £178,714.66 | 54.62% |
| 9 | £178,714.66 | £9,374.38 | £8,869.46 | £2,400.00 | £18,243.84 | £167,445.20 | 51.38% |
| 10 | £167,445.20 | £8,750.50 | £9,493.34 | £2,400.00 | £18,243.84 | £155,551.86 | 47.96% |
| 11 | £155,551.86 | £8,092.17 | £10,151.67 | £2,400.00 | £18,243.84 | £143,000.19 | 44.36% |
| 12 | £143,000.19 | £7,397.36 | £10,846.48 | £2,400.00 | £18,243.84 | £129,753.71 | 40.55% |
| 13 | £129,753.71 | £6,664.07 | £11,579.77 | £2,400.00 | £18,243.84 | £115,773.94 | 36.53% |
| 14 | £115,773.94 | £5,890.19 | £12,353.65 | £2,400.00 | £18,243.84 | £101,020.29 | 32.29% |
| 15 | £101,020.29 | £5,073.48 | £13,170.36 | £2,400.00 | £18,243.84 | £85,449.93 | 27.81% |
| 16 | £85,449.93 | £4,211.54 | £14,032.30 | £2,400.00 | £18,243.84 | £69,017.63 | 23.08% |
| 17 | £69,017.63 | £3,301.92 | £14,941.92 | £2,400.00 | £18,243.84 | £51,675.71 | 18.10% |
| 18 | £51,675.71 | £2,341.94 | £15,901.90 | £2,400.00 | £18,243.84 | £33,373.81 | 12.84% |
| 19 | £33,373.81 | £1,328.79 | £16,915.05 | £2,400.00 | £18,243.84 | £14,058.76 | 7.28% |
| 20 | £14,058.76 | £298.10 | £12,458.76 | £1,600.00 | £12,756.86 | £0.00 | 2.34% |
20 years, rolled up from 237 monthly periods — the display would otherwise be 237 rows, which is a wall rather than a table. The CSV export contains every one of them. The schedule stops at month 237 because the balance reaches zero there; without the overpayment it would have run to month 300.
…or invest it instead?
Overpaying is an investment that returns your mortgage rate. Every pound of capital you repay is a pound the lender stops charging 5.40% a year on, for every month the mortgage had left to run. Nothing is deducted from that: no platform fee is charged on interest you were not charged, and no tax is due on it either. The return is 5.40%, and it is certain for as long as that is your rate.
You have assumed 5.00% a year for the investment. That is 0.40% a year less than the mortgage costs. On your own assumption the overpayment wins on the rate, and it also wins on the certainty — which is the one case where the two do not have to be traded off against each other.
| Figure | Overpay the mortgage | Invest it instead |
|---|---|---|
| Money in | £47,200.00 | £60,000.00 |
| What it produces | £49,509.24 | £57,147.03 |
| Which is | interest never charged | growth on the pot, before charges and before tax |
| Certain? | Yes, while 5.40% is your rate | No. Nobody can promise a return |
| And you end with | a mortgage cleared 5 years and 3 months early | £117,147.03 invested |
The two columns are not the same money, and the difference is stated rather than hidden. The overpayments stop when the mortgage clears, at month 237, so only £47,200.00 of them are ever made. The investment keeps receiving £200.00 a month for the full 25 years — £60,000.00 in total, which is £12,800.00 more. From month 237 the overpayer’s whole payment of £1,520.32 a month is free as well, and this panel does not invest it — pricing the certain route with an uncertain rate is exactly the move that would make this comparison useless.
The investment column is gross; the overpayment column is net. £57,147.03 is what 5.00% a year would produce before any platform fee, fund charge or tax — and charges compound against you exactly the way growth compounds for you. The investment calculator prices three separate kinds of charge on your own figures; the stocks and shares ISA calculator prices the tax a wrapper avoids. Both make the investment column smaller. Nothing makes the overpayment column smaller except an early repayment charge.
This is not a forecast, and it is not advice. The investment figure is a single deterministic path at one constant rate, which badly understates the spread of real outcomes — returns arrive as a sequence, and the order they arrive in changes the answer. This page will not tell you which column to choose. It knows nothing about your job security, your other savings, whether you would sleep better without the debt, or whether you can get 5.00% anywhere.
A worked example you can check
These are the figures the calculator above loads with, so every number in this section can be checked against it without typing anything. £250,000.00 outstanding at 5.40% with 25 years left to run, on a repayment mortgage, and an overpayment of £200.00 a month.
| Figure | As it stands | Overpaying £200.00 a month |
|---|---|---|
| Contractual payment a month | £1,520.32 | £1,520.32 |
| Interest charged over the term | £206,099.06 | £156,589.82 |
| Months the schedule actually runs | 300 | 237 |
| Overpayments made | £0.00 | £47,200.00 |
| Interest saved | — | £49,509.24 |
| Term shortened by | — | 63 months (5 years and 3 months) |
Three things in that table are worth pausing on, and none of them is obvious from the headline figure alone.
- 74.00% of the very next payment is interest. Of £1,520.32 leaving the account next month, £1,125.00 is interest and £395.32 comes off the debt. By the final payment of the same mortgage the interest is £6.82. The proportion moves the whole way down the table, and it is the reason an overpayment now is worth several times the same overpayment later.
- The contractual payment is identical in both columns. That is what “the payment stays the same and the term gets shorter” means arithmetically: the payment is fixed by the balance, the rate and the original term, none of which an overpayment changes. Some lenders will instead reduce your monthly payment and leave the term alone — that is a different question with a different answer, and this calculator does not model it.
- £47,200.00 of overpayments buys £49,509.24 of interest never charged. The overpayments stop at month 237, when the balance reaches zero — £200.00 a month for the full 300 months would have been £60,000.00, and the schedule never asks for it because there is nothing left to pay.
And the same money, twenty years apart. A single £10,000.00 lump sum against this mortgage, with no regular overpayment, saves £26,348.72 and 1 year and 11 months if it is paid in month 1 — and £2,815.94 and 8 months if it is paid in month 241. Identical money, 9.4× the effect, twenty years earlier. Nothing in the engine asserts that ratio; it falls out of interest being charged on the opening balance before anything is paid.
What Bank Rate was doing
The rate above is yours to enter, and the calculator takes it as given. For scale, the Bank of England’s official Bank Rate has been 3.75% since 2025-12-18. Over the 259 changes recorded here, going back to 1975-01-02, it has ranged between 0.10% and 17.00%.
Bank Rate is not a mortgage rate and it is not available to you. It is what the Bank pays commercial banks on their reserves. Every mortgage sits some way above it, and that gap is the lender’s — it moves for reasons no calculator can see, and it is not a discount anybody can ask for. Nothing here suggests a better rate exists, or predicts what the Bank will do next.
Contains Bank of England data licensed under the Open Government Licence v3.0. Official Bank Rate, series IUDBEDR. The series at the Bank of England.
Methodology: exactly what this calculator does
What happens in each month, in this order
There is one loop and it is the only place money moves. For each month: open at last month’s closing balance; charge interest on that opening balance; take the contractual payment, capped at what is actually owed; take the overpayment, capped at whatever is left; and close. Every figure is a whole number of pence and every row satisfies opening + interest − payment − overpayment = closing exactly. The totals above the table are column sums of that table, and the term is simply how many rows it has — no closed-form formula is evaluated alongside them, so the headline cannot disagree with the working.
Why the overpayment lands after the interest charge
This single ordering decides whether “you would save £X” is honest. Money you owed at the start of the month was owed for the whole month, so an overpayment does not reduce this month’s interest — it reduces the balance that next month’s interest is charged on. Crediting it first would hand you one free month of interest on every overpayment you ever make, and the error would grow with the exact figure you are varying. The upside of the same rule is the thing that makes overpaying worth doing: an overpayment saves the interest on itself for every remaining month, which is why the same amount is worth several times as much early as late.
The rate, and why it is divided by twelve
A UK lender quoting 5.40% charges 5.40% ÷ 12 = 0.4500% a month on the balance outstanding. That is a nominal annual rate on a monthly-rest convention, and it compounds to slightly more than the quoted figure over a year. That is not an error in the quote; it is what the contract says. The rest of this site treats an annual rate as an effective one — a fund’s quoted return and a savings AER both compound to exactly the rate quoted — and using that convention here would understate the monthly payment and the total interest, in the direction that makes a mortgage look cheaper than the contract makes it. So a mortgage schedule uses the mortgage convention and the investment comparison uses the investment one, and both are stated rather than assumed.
The final payment, and why the balance never goes negative
The monthly payment is rounded to whole pence and so is each month’s interest, so the payment multiplied by the number of months is not the debt plus its interest. Left alone that produces one of the two failures every naive amortisation table ships with: a final balance of a few pounds below zero — a mortgage the lender now owes you — or one extra row for £3. Two rules make both impossible. Every payment and every overpayment is capped at what is actually owed, so no closing balance in any schedule here is ever below zero; and the final scheduled payment settles the balance in full, in either direction. That last rule is the “your final payment may differ” line on every UK mortgage illustration.
The year-by-year table is rolled up from months
The schedule runs monthly — 300 rows on the default figures — and the table shows one row a year, each year’s columns summed from its own months. Three hundred rows is a wall rather than a table. Nothing is rounded to do it: a year’s opening is its first month’s opening, its closing is its last month’s closing, and the columns between are sums of whole pence, so the annual rows reconcile on the same identity the monthly ones do. The CSV export contains every month.
Early repayment charges and the 10% allowance are not modelled
Most fixed-rate UK deals let you overpay up to 10% of the balance a year without penalty and charge a fee above it. That is a product term rather than arithmetic: it varies by lender, by product and by which year of the deal you are in, and there is no published figure a calculator could look up. Hard-coding one would mean this page asserting a number about your contract that it cannot know. So the figures here assume no charge is made, the page says so next to the answer, and your mortgage offer or annual statement is the only place the real allowance is written down. If you would exceed it, your saving is smaller than the figure shown and can be negative.
What else it does not model
- Rate changes part-way through. One rate for the whole schedule. A two-year fix followed by a reversion rate is two schedules, and this page draws one — so a fixed-rate deal shorter than your remaining term makes every figure here conditional on a rate you have not been offered yet.
- “Reduce my payment” instead of “reduce my term”. Overpayments can be taken either way and lenders default to different ones. This models term reduction only: the payment stays fixed and the debt clears sooner. Mixing the two would give a saving that is partly a shorter term and partly a smaller payment — two answers wearing one number.
- Fees, and daily interest. No arrangement fee, valuation fee or product fee. Interest is charged monthly at rest rather than accrued daily, which moves the answer by pennies a month and is what makes every row reconcile in exact whole pence.
- Tax, of any kind, anywhere on the page. There is no tax on interest you were not charged, and the investment comparison is shown before any tax on the investment.
Sources, and why there is no rates table
Most calculators on this site cite gov.uk, because they implement statutory rates, bands and thresholds that can be checked against a published source. This one has no statutory figures at all. Your balance, your rate, your term and your overpayment are all yours, and the investment return is your assumption. There is nothing here an authority could confirm.
So this page carries no “verified against HMRC guidance” stamp and it will not borrow one. The claim it makes is arithmetic only: given these inputs and the conventions stated above, the schedule is right, every row reconciles exactly, and every headline figure is read off it.
What this is not
It is not a forecast. A schedule at a single fixed rate says what would happen if that rate held every month for the whole term, and a rate fixed for two or five years is not fixed for 25. The investment comparison is a fixed-rate projection too, and understates the spread of real outcomes for the same reason: returns arrive as a sequence, and the order they arrive in changes the answer. And it is information, not advice — it will not tell you whether to overpay. It cannot know your circumstances, and the decision involves things no calculator holds: your job security, your emergency fund, your other debts, and whether being free of the mortgage is worth more to you than a number.
Nothing you type here is transmitted or stored — there is no application server and no database. A share link is the exception: it carries your figures, including your balance and rate, in the URL. What that means.
Frequently asked questions
- How much would overpaying my mortgage by £200 a month save?
On this calculator's default figures — £250,000.00 outstanding at 5.40% with 25 years left — overpaying £200.00 a month saves £49,509.24 of interest and clears the mortgage 5 years and 3 months early. The overpayments themselves total £47,200.00, because they stop when the balance reaches zero at month 237 rather than running the full 300. Change the balance, rate, term or overpayment above and the figure changes; it is arithmetic on your inputs, not a prediction.
- Why is so much of my mortgage payment interest?
Because interest is charged on the whole balance outstanding, and at the start the balance is nearly all of what you borrowed. On the default figures the next payment is £1,520.32, of which £1,125.00 — 74.00% — is interest, and only £395.32 comes off the debt. That proportion falls every month as the balance does, and by the final payment the interest is £6.82. It is why an overpayment made early is worth so much more than the same amount made late: it removes capital from every future month's interest charge.
- Is it better to overpay my mortgage or invest the money?
Neither this calculator nor anyone else can answer that for you, but it can state the trade honestly. Overpaying returns exactly your mortgage rate — 5.40% on the defaults — with certainty, with no charges taken off it and no tax due on it, because there is no tax on interest you were not charged. Investing returns whatever the investment returns, which nobody can promise, and the figure is before platform fees, fund charges and any tax. On the default figures £200.00 a month invested at 5.00% a year for 25 years would grow by £57,147.03 — but note it receives £60,000.00 against the £47,200.00 the overpayments ever total, because the mortgage clears early and the overpayments stop. The comparison that decides it is the rate, not the total, and the difference between a certain rate and an assumed one is the whole of the question.
- Does overpaying reduce my monthly payment or my term?
This calculator models term reduction: the contractual payment stays exactly what it was and the mortgage clears sooner. That is the reading the overpayment question is usually asking about, and it is the one that saves the most interest. Lenders default to different behaviour and many will do either on request, so it is worth checking which yours applies — if your overpayment reduces the payment instead, the interest saving is smaller than the figure shown here, because the balance falls more slowly from then on.
- Will I be charged for overpaying my mortgage?
You might be, and this calculator cannot tell you. Most fixed-rate UK deals allow overpayments of up to 10% of the balance a year without penalty and charge an early repayment fee on anything above that, often a percentage of the amount repaid. The allowance and the charge are terms of your particular product — they vary by lender, by product and by which year of the deal you are in — so there is no published figure to look up and nothing here models one. Every figure on this page assumes no charge is made. Your mortgage offer or annual statement states your allowance; check it before overpaying, because exceeding it can wipe out the saving entirely.
- Does a lump sum save more than the same amount spread over the years?
Not because it is a lump sum, but because of when it lands. An overpayment saves the interest on itself for every month the mortgage has left, so timing is nearly everything. On the default mortgage, £10,000.00 paid in month 1 saves £26,348.72 and 1 year and 11 months; the identical £10,000.00 paid in month 241 saves £2,815.94 and 8 months. Same money, 9.4 times the effect, twenty years earlier.
- Does this work for an interest-only mortgage?
Yes, and it shows the thing that makes an interest-only schedule honest: what is still owed at the end. On interest only the payment covers the month's interest and nothing else, so the capital is untouched — £250,000.00 borrowed is £250,000.00 still owed on the last day of the term unless something else repays it. Overpayments do reduce the balance, and the payment is recomputed from the reduced balance each month rather than staying fixed, because charging the original figure would be charging interest on capital you had already repaid.
- Is this a prediction of what my mortgage will cost?
No. It computes what would happen if one rate held every month for the whole remaining term. Almost nobody has that: a rate fixed for two or five years is not fixed for twenty-five, and when the deal ends the rate changes and every figure here changes with it. The useful way to use it is to run it at more than one rate — the rate you pay now and the rate you might revert to — and look at the spread between the answers rather than trusting any single one.
- Is anything I type here sent anywhere?
Not by typing it. The calculator runs entirely in your browser, there is no application server and no database, and the share link and CSV export are both assembled in the tab you are reading this in — creating a link makes no request at all. The one exception is worth knowing, and it matters more here than on most pages: a share link puts your figures in the URL, including your outstanding balance and your rate, so if you send one and somebody opens it, their browser requests that address and the figures travel with it, the way any web address does. Nothing is stored either way.