Should you overpay your mortgage or save?
Overpaying a mortgage earns exactly your mortgage rate, guaranteed and tax-free. Saving earns whatever your account pays, less any tax on the interest. Which is better comes down to those two numbers, what you'd have left in reach for emergencies, and your deal's overpayment rules. Here is how to compare them, with the sums shown.
Do these first
Money sent to the lender is hard to get back, so overpaying comes after the things that protect you or pay more:
- An emergency fund. Overpaying doesn't lower next month's payment, and if you lose your income the lender still expects it in full. Keep several months of essential spending in an easy-access account before anything else.
- Employer pension contributions. If your employer matches extra contributions, that match is an immediate return no mortgage rate comes close to. Pension contributions also get tax relief.
- Expensive debt. Credit cards, overdrafts and personal loans charging more than your mortgage rate should be cleared before the mortgage.
What overpaying earns you
Every pound of overpayment stops being charged interest for the rest of the mortgage. So the return is your mortgage rate, with no risk and no tax. Most UK lenders now calculate interest daily, so an overpayment starts saving interest from the day it lands, not at the next annual review. Check your offer or ask your lender if you're unsure how yours is calculated.
Take a £200,000 repayment mortgage at 4.5% over 25 years. The monthly payment is £1,111.66 and, left alone, the mortgage charges £133,501 of interest over its life. Overpaying £200 a month clears it 6 yrs 1 mo sooner, in Nov 2045 instead of Dec 2051, and saves £36,281 of interest. A one-off £10,000 paid with the first payment saves £19,197 and finishes the mortgage 2 yrs 2 mo early. To be mortgage-free five years early, in Dec 2046, you'd need to overpay £153.64 a month.
Calculated with the same engine as our calculators, rounding interest to the penny each month.
What saving has to beat
Savings interest is taxable once it goes above your Personal Savings Allowance, which is £1,000 for basic-rate taxpayers, £500 for higher-rate and nothing for additional-rate. Interest above that is taxed at your income tax rate: 20% at the basic rate and 40% at the higher rate in England, Wales and Northern Ireland (Scotland has different bands). So the comparison is:
- Interest within your allowance, or inside an ISA: compare the savings rate with the mortgage rate directly. On the 4.5% example, an account paying more than 4.5% wins; one paying less loses.
- Interest above your allowance, outside an ISA: a basic-rate taxpayer keeps 80% of the interest, so the account needs to pay more than 5.63% gross to match a 4.5% mortgage. A higher-rate taxpayer keeps 60%, so needs more than 7.5%.
You can put up to £20,000 into ISAs each tax year and pay no tax on the interest, which is why a cash ISA is the usual place to compare against. Fixed-rate savings deals also come with their own catch: the rate is fixed for a term, and so is your access to the money.
Overpay or save: the same money, two ways
Two households have the example mortgage and £200 a month to spare. One overpays, and once the mortgage is gone saves the whole payment it no longer has to make, until the date the mortgage would have ended. The other pays the normal amount and saves the £200 from day one. The savings rates below are assumptions, not quotes: read them as after-tax rates, or as ISA rates. Here is what each household has in Dec 2051:
| Savings rate after tax | Overpay, then save | Save from the start | Ends up ahead |
|---|---|---|---|
| 3% a year | £104,902 | £89,202 | Overpaying |
| 4% a year | £108,203 | £102,826 | Overpaying |
| 4.5% a year | £109,904 | £110,600 | Saving |
| 5% a year | £111,640 | £119,102 | Saving |
| 6% a year | £115,217 | £138,599 | Saving |
Interest compounds monthly and the rate is assumed to stay the same throughout; fees and inflation are left out so the comparison isolates the two rates. Both households own the home outright at the end.
The pattern is the whole answer: saving wins only when the after-tax rate is higher than the mortgage rate, and the margin either way is modest when the two are close. What tips it in practice:
- Savings rates move; your mortgage rate is fixed for the deal. An account that beats your rate today may not next year. Overpaying locks in its return for as long as the mortgage lasts.
- Saving keeps the money in reach. If a boiler fails or work dries up, savings are there. Overpayments usually aren't, unless your deal has a borrow-back or offset feature.
- A new deal changes the sums. When your fixed rate ends, a smaller balance means a smaller payment on the next deal, and a lower loan-to-value can unlock cheaper rates. See what happens when your fixed rate ends.
- Saving takes discipline. The saving household only wins if it really sets the money aside every month for decades and doesn't dip in. Many people find overpaying easier to stick with.
Mind your overpayment allowance
Most fixed and discounted deals let you overpay a set amount each year without an early repayment charge, commonly 10% of the balance; your mortgage offer states yours. On the example, a 10% allowance is £20,000 in the first deal year, or up to £1,667 a month. The £200monthly plus the £10,000 lump sum come to £12,400, comfortably within it. Go over, and you pay a charge on the excess, which eats into the saving and is usually avoidable with timing. The 10% overpayment rule guide explains which balance the allowance is taken from, when the charge bites, and how to overpay more without paying it. On a standard variable rate or most trackers there is normally no limit.
Reduce the term or reduce the payment?
When you overpay, your lender will usually ask whether to keep the monthly payment the same and shorten the term, or keep the term and lower the payment. Reducing the term saves the most interest: the £10,000lump sum above cuts 2 yrs 2 mo off the mortgage and saves £19,197. Reducing the payment instead would drop it from £1,111.66 to £1,055.98 a month, with the same end date, and save only £6,651. The lower payment is a reasonable choice if the point is breathing room in the budget; if the point is paying less interest, keep the payment and shorten the term. Some lenders default to one or the other, so say which you want.
A sensible middle path
This isn't all-or-nothing. A common approach is to keep the emergency fund topped up in an easy-access account or cash ISA, overpay a fixed amount each month within the allowance, and send a lump sum when the savings rate on the rest falls below the mortgage rate. Re-check the decision whenever your deal, your savings rate or your circumstances change.
Related tools
- Mortgage overpayment calculator: your new mortgage-free date and interest saved from monthly or lump-sum overpayments, and whether you stay within your allowance.
- Fixed rate ending calculator: what your payment becomes when the deal ends, and what overpaying before then does to it.
- The 10% overpayment rule and early repayment charges: how the annual allowance works and how to avoid the charge.
- What happens when your fixed rate ends: the standard variable rate, product transfers and remortgaging.
For education only; not financial, tax or investment advice. The savings rates in the table are assumptions, not predictions or quotes, and the mortgage in the examples is illustrative. Tax figures are for the 2026 to 2027 tax year and can change. Your lender's terms and figures are final. Sources checked October 15, 2026.