Should I overpay my mortgage or save instead?
The short answer
Compare your mortgage rate against the savings rate you could earn after tax. Whichever is higher is where your money works hardest.
If your mortgage is 4.5% and the best savings account pays 4.2% before tax, overpaying wins. If you can earn 5% in an ISA with no tax to pay, saving wins. It really is that simple as arithmetic.
What complicates it is everything around the arithmetic: overpayment limits, emergency savings, other debts, and the fact that money paid into a mortgage is very difficult to get back out.
Why the rate comparison works
Overpaying a mortgage does not earn interest, but it avoids interest, and avoiding interest at 4.5% is worth exactly as much as earning it at 4.5%. Better, in fact, because interest you avoid is never taxed, while interest you earn might be.
Every pound you overpay comes straight off the balance. The following month, interest is charged on a smaller amount, and that saving repeats every month for the rest of the term. It compounds in your favour, which is why overpaying early in a mortgage saves so much more than overpaying near the end.
Work it out Mortgage overpayment calculatorSee exactly what your overpayment would save in interest and how much sooner you would be mortgage free.
The tax point most comparisons miss
Savings interest may be taxable, and that changes the comparison meaningfully. Most people have a Personal Savings Allowance letting them earn a certain amount of interest tax free, but higher earners have a smaller allowance and additional rate taxpayers have none at all.
The practical effect: a savings account paying 5% is worth 5% to a basic rate taxpayer within their allowance, but only 4% to a basic rate taxpayer who has exceeded it, and 3% to a higher rate taxpayer in the same position. An account inside an ISA avoids this entirely, which is why ISAs frequently win this comparison despite headline rates that look unremarkable.
Always compare your mortgage rate against the savings rate after any tax you would pay. Check the current allowance on GOV.UK, since the thresholds change.
Four things to sort out first
Before either option, these usually take priority.
| Priority | Why it comes first |
|---|---|
| Expensive debt | A credit card at 22% costs roughly five times what a mortgage at 4.5% does, per pound owed. Clearing it saves far more than any overpayment. |
| An emergency fund | Three to six months of essential spending, somewhere you can reach quickly. Money in a mortgage cannot be withdrawn when the boiler fails. |
| Employer pension matching | If your employer matches additional contributions, that is an immediate return no mortgage rate can compete with. |
| Your overpayment limit | Most fixed deals cap overpayments at 10% of the balance a year. Exceeding it can trigger an early repayment charge that wipes out the saving. |
When saving wins even at a lower rate
The arithmetic is not the only consideration, and there are sound reasons to save despite a mortgage rate that is nominally higher.
- You might need the money. Overpayments are effectively locked away. Some lenders offer borrow-back facilities, but many do not, and remortgaging to release money is slow and not guaranteed.
- Your fixed rate ends soon. If your deal expires in a year and rates have fallen, saving the money and making one large overpayment when you remortgage keeps your options open.
- You are building a deposit for something. Home improvements, a car, a career change. Overpaying then borrowing again at a higher rate makes no sense.
- Your loan to value is close to a threshold. Sometimes a lump sum at remortgage time that pushes you from 85% to 80% loan to value unlocks a better rate on the whole mortgage, which is worth more than the interest saved along the way.
When overpaying wins by more than the maths suggests
Equally, overpaying has advantages that do not appear in a rate comparison.
- It is certain. The saving is guaranteed at your mortgage rate. Investment returns are not.
- It is tax free. Interest avoided is never taxed. Interest earned may be.
- It shortens the term. Being mortgage free years earlier changes what is possible in your fifties and sixties, and that is worth something no spreadsheet captures.
- It is a habit that sticks. Money that leaves your account automatically tends to stay saved. Money left in a savings account tends to get spent.
See what the same monthly amount would build up to if you saved it instead.
A worked comparison
Take a £200,000 mortgage at 4.5% with 22 years remaining, and £200 a month spare.
Overpaying: roughly £25,000 saved in interest, and mortgage free around three years early. The saving is certain and untaxed.
Saving at 4.5% in an ISA: after 22 years, around £90,000 built up, tax free, and fully accessible throughout.
Those look different because they measure different things: one is interest avoided, the other is a balance accumulated. The fair comparison is what you are left holding at the end. At identical rates with no tax, the two come out close to even, and the decision comes down to whether you value access to the money or freedom from the debt. Once tax enters, or the rates differ, one pulls clearly ahead.
Ask your lender one question
When you overpay, does the term shorten or does the monthly payment reduce?
This matters enormously and it is rarely explained. Shortening the term saves the most interest. Reducing the payment saves considerably less, because you are still paying for the full original term. Many lenders default to one but will do the other on request. Ask, and say which you want.
If money is tight
This whole question assumes spare money each month, which is a good position to be in. If you are struggling with mortgage payments or other debts, the priority is entirely different and help is free. StepChange, National Debtline and Citizens Advice all give free, confidential advice, and speaking to your lender early usually opens up more options than waiting does.