The calculator
Build your plan on the left. Watch it pay off on the right.
No Calculate button — every number updates as you type, and each figure traces back to a row in your schedule.
Your loan
What you owe today
Your overpayments
Lump sums or top-ups
Choose the outcome
Shorter loan or lower monthly payment
Results, live
No Calculate button
Your loan
What you owe today
Computed from the numbers above.
Your overpayment plan
1 active · £47.4 K overpaid over the loan
from 05 Nov 26
On its own: saves £37.6 K and 5y 1m off your loan.
What should overpaying do?
The single most important choice here
Advanced optionsPayment frequency, rate changes, fees — optional
Payment mechanics
Interest rate changes
Your monthly payment is recomputed over the term remaining, so the end date does not move. Your lender's letter says which it does.
Fees & charges
Only if your deal has one — see your mortgage offer for the rate and allowance.
Of the balance at the start of each calendar year — check your offer.
Net benefit after fees
£37.6 K
No fees entered, so the whole saving is yours.
Where you stand after
Side by side
Your loan with your plan, and without it
Two futures for the same loan. The right-hand column is the one you're building.
Interest you keep
£37.6 K
23% less interest than doing nothing
Time you get back
5y 1m
earlier than Sep 2051
Every figure here is the last row of the matching schedule below — nothing is estimated separately.
Visual proof
Your balance reaches zero 5y 1m sooner
The shaded gap is what you no longer owe. At its widest it is £75.7 K.
Where each year's instalments go
Interest outweighs principal until 2034 — which is why an overpayment made before then removes the most.
What waiting costs you
What £30.0 K removes from your interest bill, by the date you pay it.
Paid now it removes £51.4 K. Wait twelve months and it removes £48.2 K — £3.2 K less for the same money.
Full detail
Every payment, to the penny
239 payments across 21 years, every penny accounted for
21 years match
| # | Date | Opening | Payment | Interest | Principal | Overpayment | Closing |
|---|---|---|---|---|---|---|---|
| £250,000 | £4,548 | £2,775 | £1,772 | £400 | £248,228 | ||
| £248,228 | £18,990 | £10,882 | £8,108 | £2,400 | £240,120 | ||
| £240,120 | £18,990 | £10,514 | £8,476 | £2,400 | £231,644 | ||
| £231,644 | £18,990 | £10,129 | £8,861 | £2,400 | £222,783 | ||
| £222,783 | £18,990 | £9,726 | £9,264 | £2,400 | £213,519 | ||
| £213,519 | £18,990 | £9,306 | £9,684 | £2,400 | £203,835 | ||
| £203,835 | £18,990 | £8,866 | £10,124 | £2,400 | £193,711 | ||
| £193,711 | £18,990 | £8,406 | £10,584 | £2,400 | £183,127 | ||
| £183,127 | £18,990 | £7,925 | £11,065 | £2,400 | £172,062 | ||
| £172,062 | £18,990 | £7,423 | £11,567 | £2,400 | £160,495 | ||
| £160,495 | £18,990 | £6,897 | £12,093 | £2,400 | £148,402 | ||
| £148,402 | £18,990 | £6,348 | £12,642 | £2,400 | £135,760 | ||
| £135,760 | £18,990 | £5,774 | £13,216 | £2,400 | £122,544 | ||
| £122,544 | £18,990 | £5,174 | £13,816 | £2,400 | £108,728 | ||
| £108,728 | £18,990 | £4,546 | £14,444 | £2,400 | £94,284 | ||
| £94,284 | £18,990 | £3,890 | £15,100 | £2,400 | £79,184 | ||
| £79,184 | £18,990 | £3,204 | £15,786 | £2,400 | £63,398 | ||
| £63,398 | £18,990 | £2,487 | £16,503 | £2,400 | £46,896 | ||
| £46,896 | £18,990 | £1,738 | £17,252 | £2,400 | £29,643 | ||
| £29,643 | £18,990 | £954 | £18,036 | £2,400 | £11,608 | ||
| £11,608 | £11,791 | £183 | £11,608 | £1,400 | £0 | ||
Compare strategies
Same money, very different outcomes
Ranked by interest saved against your loan. Tap one to make it your plan.
Lump sum, straight away
£25.0 K at your next payment date
Interest saved
£44.0 K
- Time saved
- 4y 1m
- Extra paid
- £25.0 K
- Debt-free
- Aug 2047
- Saved per £1
- £2
The same lump sum, a year later
£25.0 K after twelve more payments
Interest saved
£41.2 K
- Time saved
- 3y 11m
- Extra paid
- £25.0 K
- Debt-free
- Oct 2047
- Saved per £1
- £2
One bonus a year
£200 every December
Interest saved
£38.9 K
- Time saved
- 5y 2m
- Extra paid
- £48.0 K
- Debt-free
- Jul 2046
- Saved per £1
- £1
A little every month
£200 on top of every monthly payment
Interest saved
£37.9 K
- Time saved
- 5y 1m
- Extra paid
- £47.6 K
- Debt-free
- Aug 2046
- Saved per £1
- £1
How we calculate
No black box. Six rules produce every number here.
The same six a lender applies to your account each month. Read them, check them against your statement, then trust the schedule.
Interest accrues on your opening balance each period. On the default settings — monthly payments, monthly compounding — that is your annual rate divided by twelve, which is standard reducing balance. Change the payment frequency and the rate is converted so your effective annual rate stays the same.
An overpayment goes entirely to principal. Nothing is applied to interest, which is why the next period costs you less.
No period ever pays more than the balance plus its interest, so the loan settles at exactly zero and can never go negative.
On a rate change the monthly payment is recomputed over the term remaining — term held, monthly payment moves.
Both futures are run through the same engine on the same rate path, so the comparison is fair.
Every headline figure is the last row of its schedule. Nothing on this page is estimated a second way.
Questions people ask
Should I overpay or put the money in a savings account?
Compare your mortgage rate against the interest you would earn after tax. Overpaying gives you a guaranteed, tax-free return equal to your mortgage rate, but the money becomes illiquid unless your lender offers a borrow-back facility. Keeping an accessible emergency fund before overpaying is the usual order.
Is an overpayment worth more early or late?
Early. The same amount removes interest for every month that remains after it is paid, so an overpayment in year two of a twenty-five-year mortgage is worth far more than the same money in year eighteen. If your lender calculates interest daily, it starts working immediately rather than at your next annual review.
Will overpaying affect my next remortgage?
It generally helps: a smaller balance against the same property means a lower loan-to-value ratio, and LTV bands are what determine the rates you are offered. Crossing below a band boundary before you remortgage can be worth more than the interest the overpayment itself saves.
What should I enter as the outstanding balance?
What you still owe today, from your latest mortgage statement — not the amount you originally borrowed. This is the single most common mistake, and it changes every number on the page.
Will my lender charge me for overpaying?
Only if your deal has an early repayment charge, and usually only on the part above your allowance. During a fixed or discounted deal most lenders let you overpay a set amount each year without charge — commonly 10%, though some allow 20% — and the charge, often 1–5% and stepping down each year of the deal, applies to the amount above it. Lenders differ on the detail: some measure the allowance against your current balance and some against the original loan, and some reset it each calendar year and some on the deal's anniversary. On a standard variable rate, and on most trackers, there is usually no limit and no charge. Enter your deal's charge, allowance and end date under Advanced options and the net benefit updates. Checked against lenders' published terms on 10 September 2026.
Why does my lender's number differ slightly?
This calculator charges interest at your annual rate divided by twelve on the balance at the start of each month, and an overpayment takes effect from your next scheduled payment. Many lenders calculate interest daily and credit an overpayment on the day it arrives, so you will save a little more than shown — at most about one month of interest on the amount overpaid.
Where does my data go?
Nowhere. Every calculation runs in your browser. There is no account, no tracking of your mortgage details, and no server round-trip to compute any of this.
Why you can check this one
Independent
Not a lender, not a broker, not affiliated with anyone who is. Nothing here is ranked by what it pays us, because nothing pays us. There are no ads and no lead generation.
Checkable
Every figure is the last row of a schedule you can expand, read and download. The rules the engine follows — the rate conversion, the rounding points, how an overpayment is timed — are published in full, along with every place we knowingly differ from a lender.
Honest about limits
It models the loan, not your life. It does not know your lender's exact day-count conventions, your emergency fund or your tax position — and it will tell you when overpayment costs more than it saves rather than showing you a zero.
Nothing you type is sent anywhere. No account, no tracking of your loan details, nothing stored on a server. How that works.
Read the full methodology — the exact rate conversion, where rounding is applied, how we time an overpayment, and every choice we make differently from a lender, with the reasoning for each.
In plain terms
Overpayment allowances and early repayment charges
The arithmetic is universal. What is specific to a UK mortgage is how much you are allowed to overpay before it costs you.
Most fixed and discounted deals let you overpay a set amount each year without charge — commonly 10%, sometimes 20%. This is lender policy rather than law: the FCA requires only that an early repayment charge be a reasonable pre-estimate of the lender's costs. Go beyond the allowance during the deal and the charge applies to the part above it, typically 1–5% of that amount and stepping down each year of the deal; repay the whole mortgage during the deal and it applies to the redemption. On a standard variable rate, and on most trackers, there is usually no limit and no charge. Lenders differ on whether the allowance is measured against your current balance or the original loan, and on whether it resets each calendar year or on the deal's anniversary, so your mortgage offer is the authority.
Two mechanical details change the outcome and are worth checking. The first is how interest is charged: most lenders now calculate it daily, so an overpayment starts working the day it arrives. The second is what an overpayment does. There is no single default — some lenders lower your monthly payment, some shorten the term for regular overpayments, and some change neither until your next recalculation — so ask for the one you want. As in every market, shortening the term saves considerably more.
This calculator applies an overpayment from the next scheduled monthly payment on or after the date you set, and accrues interest monthly. If your lender calculates interest daily you will save slightly more than shown. Enter your early repayment charge, your allowance and the date your deal ends under Advanced options: the charge is applied only to each calendar year's overpayments above the allowance, and only until the deal ends, and the net benefit updates — including the case where the charge exceeds the saving, which the page reports rather than hides.
How it's calculated
The working behind the number
This mortgage overpayment calculator runs your repayment mortgage month by month with and without the lump sums and regular overpayments you enter, at your annual rate divided by twelve. Each overpayment reduces the balance directly, so interest is saved on it for every remaining month. If you enter an early repayment charge, it is applied only to each calendar year's overpayments above your allowance, and only until your deal ends.
Formula
Interestₖ = Balanceₖ₋₁ × r · Principalₖ = Monthly payment − Interestₖ + Overpaymentₖ · Balanceₖ = Balanceₖ₋₁ − Principalₖ
- Balanceₖ₋₁
- What you owe before payment k. The first is the outstanding balance you enter.
- r
- The monthly rate: your annual rate ÷ 12.
- Monthly payment
- The repayment from P × r × (1 + r)ⁿ ÷ ((1 + r)ⁿ − 1), or the figure you type.
- Overpaymentₖ
- Every overpayment falling due at payment k — never more than is left to repay.
- k
- The payment number, from 1 until the balance reaches zero.
Assumptions and limits
- A repayment mortgage, with interest charged monthly on the reducing balance at the annual rate ÷ 12.
- An overpayment takes effect from the next scheduled monthly payment on or after its date. Lenders that charge interest daily will save you slightly more.
- Interest, monthly payment, principal and balance are each rounded to the penny every month.
- An early repayment charge is paid out of pocket, never added to the balance, and applies only to each calendar year's overpayments above your allowance, until your deal ends.
- A rate change — the end of a fixed deal, say — applies from the first payment on or after its date.
- By default the monthly payment stays the same and the term shortens; the lower-payment alternative is costed alongside.
A worked example
Take £250,000 outstanding at 4.45% a year, with 25 years (300 monthly payments) of term left, and £200 extra every month from Nov 2026.
- Monthly rate r = 4.45% ÷ 12 = 0.3708%.
- Without overpayments the loan ends in Sep 2051 after £164.7 K of interest.
- Each overpayment goes straight to principal, so every later month charges interest on a smaller balance.
- With them it ends in Aug 2046, after £127.1 K of interest.
You keep £37.6 K of interest and finish 61 payments sooner, for £47.4 K paid early.
Computed by the calculator on this page, from the figures it opens with.
Sources
- FCA Handbook, MCOB 12.3: an early repayment charge must be a reasonable pre-estimate of the lender's costs — checked 10 September 2026
- Bank of England, Effective interest rates, July 2026: 4.45% effective rate on new UK mortgages — the rate this page opens on — checked 10 September 2026
By The CutYears team · Last reviewed 10 September 2026 · Full methodology