Cash ISA or Ordinary Savings Account: How to Choose Now That Interest Rates Have Changed the Maths

An ISA is a wrapper, not a product in its own right

It helps to stop thinking of an ISA as a "thing" you buy and start thinking of it as packaging. An Individual Savings Account is a tax-free wrapper you place around cash or investments. Inside it, interest, dividends and capital gains are shielded from tax. Outside it, the same money is just an ordinary account or investment, taxed under the normal rules.

Every tax year — 6 April to 5 April — adults get a £20,000 ISA allowance. You can split it between a cash ISA, a stocks and shares ISA, and a couple of other variants, but you cannot carry unused allowance forward, so it genuinely is "use it or lose it".

The wrapper itself doesn’t make your money grow faster. A cash ISA earning 3% still earns 3%. What the wrapper does is protect whatever you earn from being added to your taxable income. That distinction matters more than it used to, because of what’s happened to interest rates.

Why the tax question got sharper

For most of the 2010s, savings rates were so low that almost nobody needed to think about tax on interest. The Personal Savings Allowance (PSA), introduced in 2016, let basic-rate taxpayers earn £1,000 of interest tax-free, higher-rate taxpayers £500, and gave additional-rate taxpayers nothing at all. When easy-access accounts paid around 1%, you’d have needed roughly £100,000 in savings as a basic-rate taxpayer to even brush against that limit.

Rates have since climbed a long way from their post-financial-crisis lows, and even after recent falls they remain well above where they sat a few years ago. With top easy-access rates now in the low single digits, a basic-rate taxpayer can breach their £1,000 allowance with somewhere around £20,000 saved, and a higher-rate taxpayer with roughly half that. Savers who once assumed "ISAs aren’t worth the hassle" may now be quietly handing some of their interest to HMRC without realising it.

Two other allowances can also change the picture, and they’re easy to miss. If your total income is low enough, the starting rate for savings can let you earn up to an extra £5,000 of interest tax-free, on top of your Personal Allowance and PSA — though it shrinks as other income rises and disappears once that income reaches £17,570. And on joint accounts, interest is normally split equally between holders for tax purposes, which can matter if one partner pays a higher tax rate than the other.

Cash ISA versus ordinary savings account

Neither option is automatically better — the right one depends on how much you’re saving, your tax band, and how soon you might need the money.

The practical rule of thumb that falls out of this: if you’re not close to using up your Personal Savings Allowance, chasing the highest rate — ISA or not — usually serves you better, because top ordinary savings rates can beat top cash ISA rates. But once your interest starts exceeding your PSA, the maths tilts. A basic-rate taxpayer over the allowance keeps only £80 of every £100 earned outside an ISA; a higher-rate taxpayer keeps just £60; an additional-rate taxpayer keeps £55. At that point, a decent cash ISA rate can beat an even higher-paying taxable account, simply because none of it gets taxed away.

A simple way to work out where you stand

flowchart TD
 A[Add up your total savings interest for the year] --> B{Is it near or over your PSA and starting rate?}
 B -->|No, well under| C[Pick whichever pays the best rate]
 B -->|Yes, close or over| D{Do you want easy access or a fixed term?}
 D --> E[Compare top cash ISA rates against taxable equivalents]
 E --> F[Choose the option that leaves you with more after tax]

Working through this doesn’t require precision to the penny — it’s about noticing which side of the line you’re likely to be on, and checking your actual bank statements if you’re unsure.

What changes from 2027, and what it means for you

At Autumn Budget 2025, the government confirmed that from 6 April 2027 the cash ISA allowance for people under 65 will fall to £12,000, while the overall £20,000 ISA allowance stays the same — the remaining £8,000 can still go into a stocks and shares ISA or other eligible ISA. Anyone 65 or over keeps the full £20,000 cash ISA allowance. This is a change to future deposits, not to money already sitting in a cash ISA, so existing savings aren’t affected retroactively.

Alongside this, a set of anti-circumvention rules is being introduced to stop the lower cash ISA cap being sidestepped. From April 2027, transfers from a stocks and shares ISA into a cash ISA will no longer be permitted (transfers the other way will still be allowed), and interest earned on cash left sitting inside a stocks and shares ISA will face a 22% charge, paid by the ISA manager rather than declared by the saver. Money market funds held as a genuine part of a diversified portfolio won’t count as "cash" for this purpose, provided they’re not the entire portfolio. The government has also signalled that Lifetime ISAs will eventually be replaced by a new first-time-buyer product, though the timing hasn’t been set.

None of this is a reason to rush into a decision now — nothing changes until April 2027, and the details could still be refined during technical consultation. But it is a nudge to check, over the next year or so, whether your savings goals still fit comfortably within a £12,000 cash ISA allowance, or whether you’ll want to plan ahead if they don’t.

A short checklist before you decide

• Add up interest received across all your ordinary (non-ISA) savings for the tax year.
• Compare that total against your Personal Savings Allowance for your tax band, and check whether the starting rate for savings applies to you.
• If you’re on a joint account, remember the interest is typically split between holders for tax purposes.
• If you’re over or close to your allowance, compare the actual after-tax return of a top ordinary account versus a top cash ISA, not just headline rates.
• If you’re comfortably under your allowance, don’t assume the ISA wins by default — check the numbers.

The takeaway

A cash ISA isn’t a magic trick that makes your money grow faster; it’s a shield against tax that only earns its keep once there’s tax to shield against. For many savers with modest balances, an ordinary account paying a slightly better rate remains perfectly sensible. For others — especially as rates and balances have both risen — the wrapper is now doing real work. The honest answer to "do I need a cash ISA?" is: check your interest against your allowances first, then let the rate, not the label, make the final call.

Sources

1. How do Isas work? The essential guide for cash savers and investors
2. Tax on savings interest