Guide · 4 minute read

Cash ISA vs savings account: which pays more?

For most savers, the account with the best interest rate pays more, and that is often an ordinary savings account rather than a cash ISA. The ISA only wins once your interest would be taxed, and thanks to the personal savings allowance, most people's interest is not.

The rate does most of the work

A cash ISA and an ordinary savings account are the same product underneath: you deposit cash, the bank pays interest. The only difference is the tax wrapper. Inside an ISA, interest is always tax-free. Outside, interest is tax-free up to your personal savings allowance, then taxed at your income tax rate.

That means the comparison is mostly about rates, not wrappers. ISA versions of an account often pay a slightly lower rate than the ordinary version, because providers know the word "ISA" sells itself. If you will not pay tax on the interest anyway, taking a lower ISA rate is just giving money away.

The personal savings allowance, and when you actually breach it

In 2026/27 the personal savings allowance lets basic rate taxpayers earn £1,000 of interest a year outside an ISA before tax, and higher rate taxpayers £500. Additional rate taxpayers get nothing. Interest above the allowance is taxed at your marginal rate: 20%, 40% or 45%.

Run the numbers and the allowance is bigger than it sounds. At a 4.5% rate, a basic rate taxpayer needs more than £22,000 in ordinary savings before any interest is taxed. A higher rate taxpayer breaches it at just over £11,000. Below those balances, the wrapper saves you exactly £0, so pick whichever account pays the higher rate, ISA or not.

Above them, tax bites quickly. Take £20,000 at 4.5%: that is £900 of interest. A basic rate taxpayer pays nothing (it is under the £1,000 allowance). A higher rate taxpayer has £400 above their £500 allowance, taxed at 40%, which is £160 of tax. Suddenly a cash ISA paying 4.2% (£840, all kept) beats the taxed 4.5% account (£740 kept). The ISA and savings rate checker does this sum for your actual balance, rates and tax band.

When the wrapper earns its keep

The cash ISA case gets stronger the longer your horizon. You can pay in up to £20,000 per tax year (the allowance is shared across all ISA types and resets every 6 April, unused allowance does not carry over). Money inside stays sheltered for good: the interest never counts towards your personal savings allowance, in this tax year or any future one.

That matters if your savings are growing, if you expect a pay rise into the higher rate band (which halves your allowance to £500), or if rates climb. £15,000 that is safely tax-free today can quietly become taxable in three years. Filling ISA allowance you would otherwise lose is cheap insurance against your future self paying 40% on interest.

It also matters for larger pots. A higher rate taxpayer with £40,000 in cash at 4.5% earns £1,800 of interest, £1,300 of it taxable, costing £520 a year. Moving £20,000 of that into a cash ISA each April claws most of that back.

Rate chasing beats wrapper loyalty

Whichever side of the allowance you are on, the biggest losses come from idle cash in a legacy account. The gap between a high street account paying 1.5% and a competitive one paying 4.5% on £10,000 is £300 a year, which dwarfs the tax saving most basic rate savers could ever get from a wrapper. Check your rate once or twice a year and move when it slips.

The same applies inside ISAs. You are not married to your provider: an ISA transfer (use the new provider's transfer process, never withdraw the cash yourself, or it loses its tax-free status) moves old ISA money to a better rate without touching this year's £20,000 allowance.

And before optimising the wrapper at all, make sure the cash itself is in the right place. Money you might need within weeks belongs in easy access, whatever the tax treatment; our emergency fund guide and emergency fund calculator cover how much that should be.

Common questions

Can I have a cash ISA and an ordinary savings account at the same time?

Yes, and most savers should. A common setup is a competitive easy-access account for the emergency fund and short-term cash, with the cash ISA used for money that would otherwise push your interest above the personal savings allowance.

Do I lose my ISA allowance if I do not use it?

Yes. The £20,000 allowance applies per tax year and resets on 6 April. There is no carry-over, so allowance you do not use by 5 April is gone for good. That is why people with large cash balances drip £20,000 into ISAs each April.

How does HMRC actually collect tax on savings interest?

Banks report the interest they pay you to HMRC automatically. If you are employed, HMRC usually adjusts your tax code to collect any tax due; if you complete Self Assessment, you declare it there. You do not need to do anything for interest inside an ISA, it is never reported as taxable.

Can I move an old cash ISA to a better rate without losing the tax benefit?

Yes, via an ISA transfer. Open the new ISA and ask the new provider to transfer the old one; they handle it between themselves. Never withdraw the money and re-deposit it yourself, because withdrawn cash loses its ISA status and re-paying it in uses up this year's £20,000 allowance.

Does interest earned inside a cash ISA use up my personal savings allowance?

No. ISA interest is ignored entirely for tax, so your full allowance (£1,000 basic rate, £500 higher rate) remains available for interest from ordinary accounts. The two tax breaks stack.

Guidance and education, not regulated financial advice.