How Does A Cash ISA Work? All You Need To Know

A cash ISA is a savings account where you never pay tax on the interest. You can put up to £20,000 into ISAs each tax year (6 April to 5 April), and you can put all of it into cash if you want. Once the money’s in, it stays tax-free for as long as you leave it there.

Budget update, 28 October 2026: the planned cut to the cash ISA limit has been scrapped. From April 2027 everyone aged 18 or over can still put the full £20,000 into a cash ISA. I’ve explained what changed further down.

How does a Cash ISA work?

You open a cash ISA with a bank, building society or savings app, pay money in, and it earns interest just like a normal savings account. The difference is the ISA wrapper: the interest is free of Income Tax, and it doesn’t use up your Personal Savings Allowance (the interest you can earn tax-free outside an ISA).

The interest is added to your balance, so next time you earn interest on your interest too. That’s compound interest, and it’s how a cash ISA grows if you leave it alone.

Cash ISAs mainly differ on two things: the interest rate, and how easily you can get your money out. Generally, the longer you lock it away, the higher the rate.

With a fixed rate you can sometimes still get at your money early, but you’ll be hit with a penalty, often a set number of days’ interest (90 days, for example). Some fixed-rate ISAs don’t allow early withdrawals at all, so check before you lock in.

Who can open a cash ISA?

You must be 18 or over and live in the UK. Crown servants posted overseas (such as diplomats and members of the armed forces) and their spouses or civil partners can open one too. The minimum age rose from 16 to 18 in April 2024; anyone who already had a cash ISA at 16 or 17 could keep it. Under-18s can have a Junior ISA instead.

You can’t hold an ISA jointly with someone else, so couples each get their own. If you move abroad, you can keep your ISA open and it stays tax-free in the UK, but you can’t pay any more in while you’re not UK resident. Tell your provider when you go.

How much can I pay into my Cash ISA?

Your ISA allowance is £20,000 in the 2026/27 tax year, and it stays £20,000 in 2027/28. That’s one allowance shared across all your ISAs, so you can put the whole lot into a cash ISA or split it with a stocks and shares ISA, a Lifetime ISA or both. My guide to how much you can put in an ISA breaks the limits down by type.

Since April 2024 you can pay into more than one cash ISA in the same tax year, as long as your total across every ISA stays within £20,000. Interest you earn doesn’t count towards the allowance, so a £20,000 balance that earns £800 of interest hasn’t gone over the limit.

What does count is money you take out and put back. Here’s how it works with the government’s own example: you pay in £10,000 this tax year, then take out £3,000.

• Flexible cash ISA: you can still pay in £13,000 this tax year (your remaining £10,000 plus the £3,000 you took out).
• Not flexible: you can only pay in another £10,000. Putting the £3,000 back uses up allowance.

Nothing carries over when your allowance resets on 6 April, so it’s a use-it-or-lose-it situation. But money you’ve already paid in stays tax-free year after year, and your balance can grow well past £20,000 over time.

What changes are being made to Cash ISAs?

At the Autumn Budget 2025, the government announced that from 6 April 2027 people under 65 could only put £12,000 a year into cash ISAs. Rules followed to stop people getting round it: a ban on moving money from a stocks and shares ISA into a cash ISA, and a 22% charge on interest earned on cash sitting inside a stocks and shares ISA.

The Budget on 28 October 2026 reversed all of that:

• The cash ISA limit stays at £20,000 for all savers aged 18 and over from 6 April 2027. There’s no separate limit for under-65s.
• The extra rules are gone too. HMRC has confirmed they won’t take effect, so you can still transfer from a stocks and shares ISA into a cash ISA, and there’s no charge on cash held in a stocks and shares ISA.
• Savings tax outside ISAs goes up. From 6 April 2027, interest above your tax-free allowances will be taxed at 24% (basic rate), 44% (higher rate) and 49% (additional rate), up from 20%, 40% and 45% today.

The regulations that brought in the £12,000 limit are due to be revoked, with the change laid before Parliament in November 2026. That’s the formal paperwork; the government and HMRC have both confirmed the policy. My full rundown of the 2027 ISA changes has the detail.

Do you need a cash ISA, or is a normal savings account fine?

It depends on how much interest you earn. Outside an ISA, the Personal Savings Allowance lets you earn some interest tax-free each year:

If your other income is under £17,570, you may also get the starting rate for savings, which lets you earn up to £5,000 of interest tax-free on top. Both allowances stay the same after the Budget.

Here’s what tax could look like on £40,000 in a normal savings account earning an example 4% a year, which pays £1,600 of interest:

If your interest is comfortably inside your allowance, an ISA won’t save you tax today, and a normal savings account paying a better rate can be the better deal. But balances grow, rates move and the tax rates are going up, while money in a cash ISA stays tax-free for good. I compare the two properly in ISA vs savings account.

Read nextHow To Add Bitcoin To Your ISA – Is It Possible in 2026?

Is my money safe in a cash ISA?

Yes, up to a limit. Cash ISAs with UK-authorised banks, building societies and credit unions are covered by the FSCS (Financial Services Compensation Scheme). If the provider goes bust, you get back up to £120,000 per person. That limit rose from £85,000 on 1 December 2025.

The limit is per banking licence, not per brand. Just because two providers have different names doesn’t mean anything, as some share one licence, and then your £120,000 is shared across both. If you’ve got more than that saved, check who holds the licence on the FSCS website and spread your money so it’s all covered.

Temporary high balances, like money from a house sale or an inheritance, are protected up to £1.4 million for six months.

What are the benefits of a Cash ISA?

A cash ISA is great if you want the assurance of growth. It might not be huge growth, but growth nonetheless, and your balance can’t fall in pounds.

For example, £20,000 in a cash ISA earning 4% a year would be £24,333 after five years, and that’s without adding any more to it yourself. Rates change, so treat that as an example, not a promise.

Cash ISA rates broadly follow the Bank of England’s Bank Rate, which is 4% (the next decision is due on 17 December 2026). When Bank Rate sat near zero for most of the 2010s, cash ISAs barely paid anything, so they’re far more useful now than they were.

Yes, you might be able to get a higher return by using something like a stocks and shares ISA, but there’s no guaranteed growth in that, as investments can go down in value as well as up. Over long periods, investing has historically done better than cash, which I go through in cash ISA vs stocks and shares ISA.

Pros and cons of a Cash ISA

Pros

• Tax efficiency: the interest is free of Income Tax and doesn’t use up your Personal Savings Allowance. That matters more once savings tax rises in April 2027.
• Your balance can’t fall: in pounds, at least. It only goes up as interest is added.
• Flexibility: easy access cash ISAs let you withdraw whenever you need to, and flexible ones let you put the money back in the same tax year.
• Simplicity: they’re as easy to open and run as any normal savings account.
• Allowance stacking: you get a fresh allowance every tax year, so your tax-free pot can keep growing.
• Transferability: you can move your ISA to another provider for a better rate without losing the tax-free status or using up allowance.

Cons

• Inflation risk: if the interest rate is lower than inflation, your savings lose buying power even though the balance grows. Prices rose 3.5% in the 12 months to September 2026.
• Rates can beat ISAs elsewhere: some normal savings accounts and regular savers pay more, which can be worth it if you’re inside your Personal Savings Allowance.
• Annual limit: you can only pay in £20,000 a year across all your ISAs.
• Fixed terms: the best rates often mean locking your money away, with a penalty to get it out early.
• Variable rates move: if Bank Rate falls, easy access rates usually follow.
• Opportunity cost: over the long term, money in cash has usually grown less than money invested.

Other considerations

You need to keep an eye on the interest rate you’re getting. Sometimes providers offer a higher rate for the first year or so to pull you in, and then that rate drops. If you’re not watching, you can forget about the account and have a stretch where your money’s making very little.

If that happens, you can transfer your cash ISA to another provider paying more. Follow the ISA transfer process properly: ask your new provider to move it for you. If you simply withdraw the cash and pay it into a new account, you’re eating into this year’s allowance and could lose the tax-free status on money from earlier years. A transfer between cash ISAs should take no more than 15 working days, and it doesn’t touch your allowance. Check for exit penalties on fixed rates first.

It’s also worth comparing against other places for your cash. If you’re well inside your Personal Savings Allowance, a regular saver or a current account paying a high rate on a small balance can sometimes beat an ISA. It’s more work than one simple cash ISA, as you might need a few accounts, but you can earn more. Some banks also pay a bonus for switching your current account to them.

How to open a cash ISA

1. Decide what you need: easy access for money you might need soon, or a fixed rate if you know you won’t touch it.
2. Compare rates, and check whether the account is flexible and what any early withdrawal penalty is.
3. Have your National Insurance number and personal details ready. Most providers also check your ID and address.
4. Apply online or in the provider’s app. It usually takes a few minutes.
5. Pay in, up to your £20,000 allowance. If you’re moving an existing ISA, use the transfer form instead of withdrawing.

If it’s your first ISA of any kind, my beginner’s guide to opening an ISA walks through every step.

Are Cash ISAs worth it?

The main benefit of a cash ISA is tax-free growth. When you sign up, you know what rate you’ll get on your money.

Of course, you might get a letter in the post further down the line telling you that rate is reducing. But you’re in control of your money: you can transfer to another provider if you’re unhappy and find the best possible home for your savings, tax-free. Using your allowance every year, you can grow your money year after year and take advantage of compound interest.

If you paid the full £20,000 allowance into a cash ISA at the start of every year for 20 years and it earned 4% a year, it would be worth about £619,384. That’s £400,000 you paid in plus about £219,384 of interest, all tax-free. Most of us can’t save £20,000 a year, and rates won’t stay the same for 20 years, but the same maths works on smaller amounts. Try your own numbers:

For an emergency fund and short-term goals, a cash ISA is one of the best homes going, especially with savings tax rising from April 2027. If you’re building one, here’s how much should be in your emergency fund.

Cash ISA FAQs

Conclusion

So, how does a cash ISA work? Fairly easily, actually. You open one, pay in up to £20,000 a year, and every penny of interest is yours to keep.

The sooner you start saving into a cash ISA, the sooner you can start earning that interest and seeing that growth. Put your work bonus in, or your side hustle money, and put it to work.