A Cash ISA does not automatically give you a better return than an ordinary savings account. The better option depends on the interest rate, how much interest you already earn elsewhere, and whether your savings interest exceeds your tax-free allowances. A taxable savings account paying a higher rate can still leave you with more money after tax, especially if you are within your Personal Savings Allowance.
For the 2026/27 tax year, a basic-rate taxpayer can generally receive up to £1,000 of savings interest within the Personal Savings Allowance, while a higher-rate taxpayer gets £500 and an additional-rate taxpayer gets no Personal Savings Allowance. Interest inside a Cash ISA is tax-free and does not use this allowance.
This comparison uses UK tax rules for the 2026/27 tax year and also explains important changes due from 6 April 2027.
Cash ISA vs Savings Account: Quick Comparison
| Feature | Cash ISA | Ordinary Savings Account |
|---|---|---|
| Interest tax | Tax-free | Potentially taxable |
| Uses Personal Savings Allowance | No | Yes |
| 2026/27 ISA subscription limit | Part of £20,000 overall ISA allowance | No ISA contribution limit |
| Best headline rate | Varies by provider | Can sometimes be higher than Cash ISA rates |
| Access | Depends on account terms | Depends on account terms |
| Tax reporting | ISA interest normally does not need to be declared | HMRC normally receives interest information from providers |
| FSCS deposit protection | Potentially eligible | Potentially eligible |
The key mistake is comparing only the advertised interest rates. What matters is the net interest you actually keep.
How Savings Interest Is Taxed in 2026/27
HMRC provides several ways in which savings interest may be received without an Income Tax charge. These can include unused Personal Allowance, the starting rate for savings and the Personal Savings Allowance.
For many employed savers, the Personal Savings Allowance is the most relevant part of the calculation. According to HMRC's guidance on tax on savings interest, the allowance for 2026/27 is:
- Basic-rate taxpayer: £1,000
- Higher-rate taxpayer: £500
- Additional-rate taxpayer: £0
Your tax band is determined after taking relevant income into account, including savings interest, so somebody close to a tax-band boundary may need a more careful calculation.
There is also a starting rate for savings of up to £5,000 for some people with relatively low non-savings income. HMRC says that if your other income is £17,570 or more, you do not qualify for the starting rate for savings under the current rules.
A Cash ISA works differently. According to the government's ISA guidance, interest on cash held inside an ISA is not subject to Income Tax and does not need to be declared on a tax return.
The After-Tax Calculation That Actually Matters
For an ordinary savings account, a simplified calculation is:
Gross interest = savings balance × annual interest rate
Then work out how much of that interest exceeds any available savings allowance.
Taxable interest = gross interest − available tax-free savings allowance
If the result is below zero, treat taxable interest as zero.
Then:
Tax = taxable interest × applicable savings Income Tax rate
And finally:
Net interest = gross interest − tax
For a Cash ISA, assuming there are no account fees or penalties affecting the return:
Net interest = gross ISA interest
Worked Example: £20,000 in Cash
Consider an illustrative saver with £20,000 available. These rates are hypothetical examples, not claims about current market-leading accounts.
- Ordinary savings account: 4.50%
- Cash ISA: 4.00%
- Period: one year
- No withdrawals
- Simple annual comparison before considering compounding timing
Ordinary savings account
£20,000 × 4.50% = £900 gross interest.
Cash ISA
£20,000 × 4.00% = £800 tax-free interest.
At first glance, the savings account wins by £100. Tax can change the result, but not for everyone.
Basic-rate taxpayer with the full £1,000 PSA available
The £900 interest is below the £1,000 Personal Savings Allowance.
- Gross savings interest: £900
- Taxable interest: £0
- Tax: £0
- Net savings-account return: £900
- Cash ISA return: £800
In this example, the ordinary savings account leaves the saver £100 better off because no tax is actually due.
Higher-rate taxpayer with the full £500 PSA available
The first £500 of interest falls within the Personal Savings Allowance. The remaining £400 is taxable.
Using the 2026/27 savings higher rate of 40%:
£400 × 40% = £160 tax.
- Gross savings interest: £900
- Tax: £160
- Net savings-account return: £740
- Cash ISA return: £800
Here, the lower-paying Cash ISA actually produces £60 more after tax.
Additional-rate taxpayer
An additional-rate taxpayer has no Personal Savings Allowance in 2026/27. Using the current 45% savings additional rate:
£900 × 45% = £405 tax.
That leaves:
£900 − £405 = £495 net interest.
Against the illustrative £800 Cash ISA return, the tax shelter becomes much more valuable.
How to Find the Break-Even Savings Rate
You can also work backwards and ask: how high would an ordinary savings account rate have to be to beat a particular Cash ISA?
If every extra pound of interest is taxable, a rough break-even formula is:
Required taxable savings rate = Cash ISA rate ÷ (1 − tax rate)
Suppose a Cash ISA pays 4.00%.
| Marginal tax rate on the interest | Approximate taxable rate needed to equal a 4.00% ISA |
|---|---|
| 0% | 4.00% |
| 20% | 5.00% |
| 40% | 6.67% |
| 45% | 7.27% |
For example, if all of your marginal savings interest were taxed at 40%, a 6.67% taxable rate would leave roughly 4.00% after 40% tax.
This shortcut should not be used blindly when some of your interest is still covered by the Personal Savings Allowance. In that situation, calculate the actual pounds of tax instead.
When an Ordinary Savings Account Can Be Better
A conventional savings account may produce the higher net return when:
- its interest rate is meaningfully higher than available Cash ISA rates;
- your total taxable savings interest remains within your Personal Savings Allowance;
- you qualify for the starting rate for savings;
- you need account features that are better suited to your cash-flow needs;
- you have already used your relevant ISA subscription allowance; or
- the Cash ISA imposes withdrawal restrictions or penalties that outweigh its tax advantage.
For smaller balances, tax can be almost irrelevant. A saver should therefore avoid accepting a substantially worse interest rate merely because an account carries the ISA label.
When a Cash ISA Becomes More Valuable
The Cash ISA becomes more compelling as the amount of otherwise taxable interest increases.
That can happen when you:
- hold a large cash balance;
- already earn interest from several other accounts;
- are a higher-rate or additional-rate taxpayer;
- expect interest rates or your savings balance to increase;
- want to protect cash from future tax as well as this year's tax; or
- are building ISA assets over several tax years.
The last point matters because ISA protection is not merely a one-year calculation. Once qualifying money is inside the ISA wrapper, interest on that cash can continue to be sheltered from Income Tax while the money remains within the ISA rules.
How Much Can You Put Into a Cash ISA?
For the 2026/27 tax year, the government states that the overall ISA subscription allowance is £20,000. The tax year runs from 6 April to the following 5 April.
You can divide the allowance between eligible ISA types subject to their individual rules. See the government's official ISA overview for current eligibility and subscription rules.
Important Changes From 6 April 2027
The comparison changes materially from the next tax year.
HMRC confirmed in September 2026 that from 6 April 2027, the annual Cash ISA subscription limit for people under age 65 will fall to £12,000, while the overall annual ISA limit remains £20,000. People aged 65 or over will retain a £20,000 Cash ISA limit.
You can read the details in HMRC's Cash ISA limit reduction guidance.
The government has also announced higher Income Tax rates specifically for savings income from 6 April 2027:
- Savings basic rate: 22%
- Savings higher rate: 42%
- Savings additional rate: 47%
The Personal Savings Allowance is scheduled to remain £1,000 for basic-rate taxpayers and £500 for higher-rate taxpayers, with no PSA for additional-rate taxpayers.
That combination creates an interesting trade-off: people under 65 will have less annual capacity for new Cash ISA subscriptions, but taxable savings interest above their allowances will face higher tax rates.
Someone planning a substantial cash position should therefore distinguish between the 2026/27 rules that apply now and the rules beginning on 6 April 2027 rather than relying on a generic ISA comparison.
Do Not Ignore Access and Withdrawal Rules
Tax is only one part of the decision.
A top-paying fixed Cash ISA may be less useful for an emergency fund if withdrawals trigger an interest penalty. Likewise, an ordinary fixed-rate savings bond may restrict access despite not being an ISA.
Compare like with like:
- easy-access Cash ISA vs easy-access savings account;
- notice ISA vs notice savings account; or
- fixed-term Cash ISA vs fixed-term savings account of a similar term.
Also check whether a Cash ISA is flexible. Government guidance explains that a flexible ISA may allow you to withdraw money and replace it within the same tax year without using additional allowance, subject to the provider's terms.
What About FSCS Protection?
Tax-free status does not determine deposit protection.
Eligible deposits with UK-authorised banks, building societies and credit unions can fall within Financial Services Compensation Scheme protection. This includes qualifying ordinary savings accounts and cash ISAs.
The FSCS deposit protection limit increased on 1 December 2025 to £120,000 per eligible person, per authorised firm.
The phrase “per authorised firm” matters. Two different banking brands can share the same banking authorisation, in which case balances across those brands may share one £120,000 protection limit.
Do not assume that opening accounts under two different brand names automatically doubles your protection.
A Simple Decision Table
| Your situation | What to compare first |
|---|---|
| Interest remains comfortably within your tax-free allowances | Headline rate and access terms |
| You are close to exhausting your PSA | Actual pounds of tax under each option |
| You already exceed your PSA | After-tax savings rate versus ISA rate |
| You are a higher-rate taxpayer | Lower £500 PSA plus marginal tax on excess interest |
| You are an additional-rate taxpayer | Cash ISA tax shelter because there is no PSA |
| You have substantial cash | Tax, ISA limits and FSCS exposure |
| You may need the money suddenly | Withdrawal terms before small rate differences |
Common Mistakes When Comparing the Two
Comparing gross rates instead of net returns
A 5% taxable account is not necessarily better than a 4.5% Cash ISA. It depends on how much of the interest will actually be taxed.
Assuming all ordinary savings interest is taxable
Many savers pay no tax on savings interest because the interest falls within their available allowances.
Looking at only one savings account
HMRC generally considers interest across your taxable savings rather than giving a separate Personal Savings Allowance for every account.
Forgetting about future interest
A balance that sits comfortably below your PSA today may exceed it after you add more money, rates rise or interest compounds.
Using last year's ISA rules
This is particularly important now because the Cash ISA rules change from April 2027.
Ignoring banking licences
If deposit protection matters for a large balance, check the authorised firm behind each savings brand rather than counting brand names.
Bottom Line
Choose based on after-tax pounds, not the account label. If an ordinary savings account pays a better rate and your interest stays within your available tax-free allowances, it can beat a Cash ISA even though the ISA is tax-free.
Once your taxable savings interest exceeds those allowances, however, a Cash ISA can outperform an account with a noticeably higher headline rate. The advantage tends to increase for higher-rate and additional-rate taxpayers.
A practical approach is to total the interest you expect from all taxable savings for the tax year, subtract any allowance available to you, estimate the resulting tax and then compare that net figure with the interest available from a Cash ISA with similar access terms.
For decisions extending beyond 5 April 2027, redo the calculation using the new Cash ISA limit and savings tax rates rather than carrying forward the 2026/27 assumptions.
This article is for general educational purposes and does not constitute personal tax or financial advice. Tax treatment depends on individual circumstances and may change. Check current HMRC guidance or seek professional advice where necessary.
