Free tool · nothing stored
How much can you still overpay this year — without a charge?
Every fixed deal comes with a penalty-free overpayment allowance, and almost nobody knows three things about theirs: the exact amount, how much of it they have already used, and the date it resets and the unused part vanishes. This works all three out — using your lender's published rule where we have verified one, and saying so plainly where we have not.
The allowance resets each year (reset date not confirmed by the lender's published pages — treat 1 January as the working assumption). Unused allowance does not carry over, so headroom left on 31 December 2026 is gone on 1 January 2027.
Source: NatWest, published overpayment policy, checked 2026-08-15. Not confirmed by that page: reset date.
Why the reset date is the useful part
Allowances do not carry over. If your lender resets on 1 January and you have headroom in December, that headroom expires — and a lump sum split across 31 December and 1 January can use two years' allowance in two days, entirely penalty-free. The same logic applies in reverse: overpay in January and you have eleven months of no headroom left if a windfall lands. The date turns a vague "I could overpay" into a plan.
A watch puts your allowance reset on the same timeline as your fix end, your early-repayment-charge step-downs and your lock window, and emails you before each one. £7.99, once, no card on file.
Run a site or blog? This calculator is free to put on your own pages — the code adds a small "Free calculator by WatchMyMortgage" link at the bottom of the widget, and that's the only thing we ask for in return.
Frequently asked questions
How much can I overpay on my mortgage without a penalty?
Most fixed deals allow 10% a year without an early repayment charge, but the details differ by lender: as published on their own pages (checked August 2026), NatWest and RBS allow 20% of the outstanding balance, Nationwide measures 10% against the original loan, and Halifax, TSB and Santander measure 10% against the balance at the start of the calendar year. Each lender page on this site shows the source and the date we checked it; your offer document states your exact rule.
When does my overpayment allowance reset?
For many lenders it is the calendar year, so the allowance resets on 1 January. Some lenders instead reset on the anniversary of your deal. Unused allowance does not carry over, so the reset date is worth knowing if you have spare cash.
What happens if I overpay more than the allowance?
The excess is usually charged at your deal's early repayment charge rate — commonly 1% to 5% of the amount over the limit. On a large lump sum that can wipe out years of the interest saving, so the allowance line matters.
Is it better to overpay or put the money in savings?
It depends on your mortgage rate against your after-tax savings rate, and on whether you might need the money back — overpayments are hard to reverse. When your mortgage rate is higher than your after-tax savings rate, overpaying wins on pure interest; the comparison is personal enough that this page sticks to the allowance itself.
Lender rules are quoted from each lender's published pages with the date we checked them; anything we could not verify is labelled as the market assumption. Your mortgage offer is the definitive statement of your allowance. This is information, not financial advice; for advice on your circumstances, speak to a mortgage adviser regulated by the FCA.