“Tax-free” is a very persuasive description. Put it next to a savings account and the decision can feel almost finished before you have looked at the interest rate. Nobody particularly enjoys paying tax on money they have carefully managed not to spend. An account that promises to keep the taxman away therefore starts with an emotional advantage.
But a cash ISA is not automatically the best home for your savings. An ordinary savings account can sometimes leave you with more money, even though its interest is potentially taxable. In other circumstances, a lower-paying ISA can produce the better result.
The answer depends on your savings balance, your tax position, the account terms and what you need the money to do. The useful question is not simply whether an account is tax-free. It is how much interest you will actually keep, and what you must give up to earn it.
Start with the money you keep
A cash ISA is a savings account with a particular tax treatment. Interest earned inside it is free of UK Income Tax. For 2026/27, the overall adult ISA subscription allowance is £20,000, shared across the relevant ISA types rather than available separately for each account. GOV.UK explains how ISAs work. This article compares ordinary adult cash ISAs with cash savings, rather than Lifetime ISAs or investments.
An ordinary savings account does not have the same protection, but that does not mean all its interest will be taxed. For 2026/27, the Personal Savings Allowance is £1,000 for basic-rate taxpayers and £500 for higher-rate taxpayers. Additional-rate taxpayers receive no Personal Savings Allowance. People with lower incomes may also have an unused Personal Allowance or qualify for the starting rate for savings. Your income, including savings interest, affects the calculation. HMRC sets out the available allowances.
That distinction matters. If your interest is already covered by an allowance, moving it into an ISA may not save any tax this year. You would then need another reason to accept a lower interest rate. Equally, if your allowances are already used, comparing the two headline rates can produce the wrong answer. One figure is yours to keep; the other may need a deduction before it becomes useful.
What £10,000 actually earns in each account
Consider an illustrative comparison between an ordinary savings account paying 5% and a cash ISA paying 4.5%. These are example annual returns, not current offers or recommendations.
Assume £10,000 remains in either account for a full year, with no fees, withdrawals or rate changes. For the tax examples, assume all the interest falls within the 2026/27 tax year, the saver’s tax band does not change, and any taxable interest falls entirely within the stated rate. For the ISA examples, assume sufficient unused subscription allowance and eligibility to contribute.
The ordinary account produces £500 of gross interest: £10,000 × 5%. The ISA produces £450: £10,000 × 4.5%. If the ordinary account’s entire £500 is covered by an available allowance, the saver keeps £500. The ordinary account finishes £50 ahead.
Now suppose the saver has already used their available allowances elsewhere and this additional interest is taxed at 40%. The tax on £500 is £200, leaving £300. The ISA’s £450 is now £150 ahead.
| Tax treatment | Savings retained | ISA retained |
|---|---|---|
| £500 covered by allowances | £500 | £450 |
| All £500 taxed at 20% | £400 | £450 |
| All £500 taxed at 40% | £300 | £450 |
Ordinary savings wins by £50 when all the interest is covered by allowances. The ISA wins by £50 at 20% tax and £150 at 40% tax. These are interest amounts retained after a full year, not the total account balances.
The accounts have not changed. Neither has the £10,000. The saver’s circumstances change which option works better. That is why advice based only on “ISAs are tax-free” or “this savings rate is higher” is incomplete. Both statements can be correct while pointing different people towards different decisions.
Partly taxable interest needs a proper calculation
Real comparisons are often less tidy than the table above. Some interest may be covered by an allowance while the rest is taxable. Imagine a higher-rate taxpayer with the full £500 Personal Savings Allowance still available and no other savings interest. They place £20,000 into the same illustrative ordinary account at 5%.
The gross interest is £1,000. Under the assumptions used here, £500 is covered by the allowance and £500 is taxed at 40%. The tax is therefore £200, leaving £800. The cash ISA at 4.5% produces £900. In this example, the ISA finishes £100 ahead.
Notice what would go wrong if we simply reduced the entire 5% savings rate by 40%. That would suggest a net return of £600, understating the result because it ignores the available allowance. Assuming that all £1,000 is tax-free would make the opposite mistake. The useful calculation is to identify the interest covered by your remaining allowances, then apply the relevant tax treatment to the rest.
You also need to look beyond the account you are currently comparing. HMRC considers interest across your savings accounts together; opening another account does not create another Personal Savings Allowance. See HMRC’s guidance on multiple savings accounts. A spreadsheet with one beautifully organised account and three forgotten ones is still an incomplete spreadsheet.
Put a pound value on the difference
Percentage differences can sound either more impressive or less important than they really are. A gap of 0.5 percentage points is £50 a year on £10,000 before allowing for tax and changing balances. On £2,000, it is £10. On £40,000, it is £200.
Those amounts deserve different levels of attention. Spending an afternoon opening an awkward account to earn an extra £10 may be a poor use of your time. Ignoring a persistent £200 difference because the rates look nearly identical is another matter.
There is also a useful break-even calculation when all the additional ordinary savings interest would be taxable at one rate:
Required ordinary savings rate = cash ISA rate ÷ (1 − tax rate)
Use the tax rate as a decimal in that formula: 20% becomes 0.20. For an ISA paying 4.5%, a saver facing 20% tax on all the competing interest would need an ordinary rate of 5.625% to match it. At 40%, the required rate would be 7.5%.
These are mathematical comparisons, not suggestions that those rates are available. They also do not apply unchanged when part of the interest remains covered by an allowance. The purpose is to translate an attractive headline into a fair comparison. A rate only becomes meaningful when you know what reaches your pocket.
Access to your savings is part of the return
The tax comparison is only one part of the decision. You should also compare accounts that let you use the money in a similar way. An easy-access account and an account that restricts withdrawals are not offering precisely the same service. Where a higher rate comes with restrictions, consider what you are accepting in exchange for the extra interest.
Suppose an account pays 0.3 percentage points more on £10,000. That is an extra £30 over a full year before tax. If the money is your emergency reserve, ask whether £30 is enough to justify the particular access conditions. Your boiler will not postpone its retirement because you selected a competitive fixed rate.
This does not make restricted-access accounts unsuitable. Money put aside for a known future expense may have a different job from money reserved for an unexpected bill. The important thing is to decide what the money is for before choosing the account. It is the practical reason accessible savings matter when you actually need them, even when leaving the balance alone feels uneventful.
The same principle appears in the decision about overpaying your mortgage or keeping cash in savings: improving one number can be less useful if it leaves the household short of accessible cash. Read the withdrawal conditions, any penalties and the interest terms together. A tax advantage cannot make an unsuitable account convenient.
Easy access and ISA flexibility are different things
An ISA can allow withdrawals without necessarily letting you replace those withdrawals without using more of your annual allowance. A flexible ISA permits qualifying cash withdrawals to be replaced during the same tax year without reducing the current year’s allowance. Not every ISA offers this feature, so check the provider’s terms, including where replacement money must go. GOV.UK explains flexible ISA withdrawals.
That can matter if your savings balance moves around during the year. Perhaps you temporarily use some savings for a large bill and intend to replenish them after being paid. The practical question is therefore not only, “Can I take my money out?” It is also, “What happens if I put it back?”
An account can be easy to withdraw from and still have consequences for replacing the money. The descriptions address different needs, even though they can sound reassuringly similar on a product page. For someone who rarely withdraws and has plenty of unused allowance, flexibility may make little difference. For someone regularly moving substantial amounts, it could be an important part of the comparison.
Moving an existing ISA needs a different approach
If you already hold money in an ISA, moving it to a better-paying ISA is not the same exercise as deciding where to put new savings. Use the receiving provider’s formal ISA transfer process. Simply withdrawing the money and paying it into another account can affect its tax-protected status and your ability to replace it. Check transfer restrictions and any charges before proceeding. GOV.UK explains how to transfer an ISA.
This matters particularly when an ISA contains savings accumulated over several years. The balance may be much larger than the amount you could newly subscribe in a single year. An old ISA paying an uncompetitive rate does not present only two choices: leave it untouched or abandon the protection. A suitable transfer may let you improve the rate while retaining the tax treatment.
There is still a calculation to do. If leaving a fixed account incurs a charge, compare that cost with the extra interest you expect to earn over the relevant period. Moving money is not automatically progress; it needs to improve the position after costs.
Separate this year’s rules from next April’s changes
Timing matters because the rules are changing. For 2026/27, the overall adult ISA allowance remains £20,000. HMRC’s published changes introduce a £12,000 annual cash ISA subscription limit for people under 65 from 6 April 2027, within the £20,000 overall ISA allowance. The annual cash ISA limit remains £20,000 for those aged 65 or over. These are subscription limits, not a £12,000 cap on an existing ISA balance. HMRC explains the cash ISA limit change.
Separately, savings Income Tax rates are due to become 22%, 42% and 47% from 6 April 2027. The calculations earlier in this article use the 2026/27 rates, not those later rates. The government’s technical note sets out the savings-tax changes.
This does not mean everyone should rush to move money. It means a comparison extending into the next tax year needs the correct assumptions. In particular, a full year starting today is not automatically a single tax year. Check when interest becomes taxable and whether the calculation crosses a rule change. The examples above deliberately isolate one set of rules so that the underlying comparison remains clear.
You do not have to put everything in one place
The decision is often framed as though you must become either an ISA person or an ordinary-savings person. Your money does not need that kind of identity. You might keep emergency cash in an accessible account, use an ISA for money you expect to retain for longer, and place a known future expense in a separate account whose access terms fit the date. An easy-access cash ISA can itself hold emergency savings; tax treatment does not dictate the purpose of the money.
What matters is that each arrangement earns its place. Splitting money can help organise different goals, but opening more accounts does not itself improve the return. Nor does it solve the tax calculation. It can simply create additional passwords and a vague feeling of financial activity.
Before dividing the money, write down the purpose of each portion, the likely time before you need it and the net interest you expect. If the arrangement becomes so complicated that you are unlikely to maintain it, simplicity has value too. The best structure is one you can understand and keep under review.
Choose the account for the whole decision
Before opening or moving an account, compare the interest you expect to retain over the same period. Use your remaining allowances, not an assumption that all interest is either taxable or tax-free. Then check access, fees, rate conditions and any transfer requirements. Finally, consider whether keeping money inside an ISA has value for future years, particularly if you expect your savings or taxable income to grow.
That future value is worth considering, but it should not become an excuse to ignore the rate indefinitely. Tax protection and a competitive return are both worth looking for. If the difference between suitable accounts is small, the account you can manage confidently may be the better practical choice. If the difference is substantial, it deserves attention even when your existing provider feels familiar.
“Tax-free” tells you something important about an account. It does not tell you everything you need to know about the decision. Choose the place that leaves you with a sensible return, the access you need and terms you understand. The point of saving carefully is to improve your position, not simply to collect the most reassuring labels.
Rules checked on 7 October 2026. Example rates are illustrative, not advertised products. This article provides general information rather than a personal recommendation.
