ISA Calculator: See How Your Tax-Free Savings Could Grow

Try the ISA calculator

Use the calculator above to model your own ISA. Enter your starting balance, monthly or annual contribution, an estimated growth rate and how long you plan to save. The result updates instantly so you can compare scenarios, for example saving £200 a month versus £300, or assuming a cautious 3% return versus a more optimistic 7%.

What an ISA actually is

An ISA, or Individual Savings Account, is a wrapper that sits around your savings or investments and keeps the taxman out. Interest, dividends and capital gains earned inside an ISA are completely free of UK income tax, dividend tax and Capital Gains Tax. You don't declare ISA income on a Self Assessment return, and there's no tax to pay when you withdraw.

The main flavours are the Cash ISA (like a tax-free savings account), the Stocks & Shares ISA (where you invest in funds, shares or bonds), the Lifetime ISA and the Innovative Finance ISA. This ISA calculator works for both Cash and Stocks & Shares ISAs: the only difference is the growth rate you plug in, since cash pays interest while investments aim for higher, but less certain, returns.

The £20,000 ISA allowance and how it works

Every UK adult gets a £20,000 ISA allowance per tax year, running from 6 April to 5 April the next year. That's the total you can pay in across all your ISAs combined, not £20,000 per account. You could put the whole lot in one Stocks & Shares ISA, split it across a Cash ISA and a Stocks & Shares ISA, or any mix you like.

The allowance does not roll over. If you only use £8,000 of it by 5 April, the unused £12,000 is gone for good when the new tax year starts. That's why many people make a final top-up in late March or early April. The £20,000 figure is set by HM Treasury and applies UK-wide, so there's no Scotland, Wales or Northern Ireland variation to worry about here. The ISA allowance is one of the few tax limits that is genuinely the same wherever you live.

How the ISA calculator works

Behind the scenes, an ISA calculator is a compound growth model. The plain-English formula is:

Future value = (starting balance grown at the return) + (each contribution grown for the time it stays invested)

In other words, every payment you make starts earning growth from the moment it lands, and that growth then earns its own growth. For a monthly contribution, the calculator effectively runs this loop each year:

• Step 1. Take your balance at the start of the year and add this year's contributions.
• Step 2. Apply your annual growth rate to the balance.
• Step 3. Carry the new total forward as the opening balance for the next year, and repeat.

Because everything happens inside the ISA wrapper, there's no tax deduction at any step. In a normal savings account or General Investment Account, you might lose a slice of the interest or gains to tax each year, which quietly drags down compounding. Strip that drag away and the same contributions grow faster. The longer you leave it, the bigger that gap becomes.

Two things to keep in mind when reading the output. First, the growth rate is an assumption, not a promise. A Cash ISA rate can change at any time, and investment returns go up and down, so treat the projection as a guide. Second, the calculator usually assumes contributions stay within your £20,000 allowance. If you try to model paying in more than that in a single tax year, the excess wouldn't be allowed in a real ISA.

Worked example: Priya saves £300 a month

Priya is a 32-year-old nurse in Leeds. She opens a Stocks & Shares ISA with a £2,000 lump sum and sets up a £300 monthly direct debit, which is £3,600 a year, comfortably inside the £20,000 allowance. She assumes an average 5% annual return and wants to see where she'd be after 10 years.

Here's how the pot builds in the early years, rounded for clarity:

• Start: £2,000 opening balance.
• Year 1: £2,000 plus £3,600 contributions, grown at 5%, lands at roughly £5,880.
• Year 2: add another £3,600, grow at 5%, and she's near £9,950.
• Year 5: the balance is around £23,300, of which roughly £3,300 is growth.
• Year 10: the pot reaches roughly £49,500.

Over the decade Priya has paid in £38,000 of her own money (£2,000 plus £36,000 of contributions). The remaining roughly £11,500 is pure compound growth, and not a penny of it is taxable. Had she earned that same growth as interest or dividends in a taxable account, a higher-rate taxpayer could have lost a meaningful share to tax along the way. Inside the ISA, she keeps the lot.

Try the same numbers at a 3% return and the 10-year figure drops to around £43,500; at 7% it climbs to roughly £56,500. That spread shows why the growth-rate assumption matters so much, especially over longer periods.

Cash ISA vs Stocks & Shares ISA

The tax treatment is identical, so the choice comes down to risk and time horizon. A Cash ISA pays a set or variable interest rate, your balance can't fall, and it suits short-term goals or money you can't afford to lose. A Stocks & Shares ISA invests in the market, so the value can drop as well as rise, but historically it has tended to beat cash over long periods. It generally suits goals five or more years away.

Plenty of people hold both: a Cash ISA for the emergency fund and a Stocks & Shares ISA for long-term growth. As long as the combined paid-in amount stays within £20,000 for the year, you're fine. If you want to compare the two side by side, run a low growth rate (say 3-4%) for the cash scenario and a higher one for the investment scenario in the calculator above.

Why ISA growth is tax-free, and what that's worth

To see the value of the wrapper, it helps to know what you're being sheltered from outside an ISA. In a normal account:

• Savings interest above your Personal Savings Allowance (£1,000 for basic-rate, £500 for higher-rate, £0 for additional-rate taxpayers) is taxed at your income tax rate.
• Dividends above the £500 dividend allowance are taxed at 10.75% (basic), 35.75% (higher) or 39.35% (additional rate) for 2026/27.
• Gains above the £3,000 Capital Gains Tax annual exempt amount are taxed at 18% or 24%.

Inside an ISA, all three of those charges simply don't apply. You can read more about how dividend tax is worked out and what Capital Gains Tax you'd pay on a taxable account to see exactly what an ISA saves you. For a hands-on look at the underlying compounding, the compound interest calculator shows the raw maths without the wrapper.

ISA reference figures

Source: gov.uk guidance on Individual Savings Accounts. Figures checked for the 2026/27 tax year. Tax rules can change at Budgets, so confirm the current allowance before you commit a large sum.

How to get the most from your ISA

A few practical moves make a real difference to the numbers in your projection:

• Start early in the tax year. Money paid in on 6 April has nearly 12 extra months to grow versus a March top-up. Over many years that head start compounds.
• Automate contributions. A monthly direct debit smooths out market timing and means you actually use the allowance rather than meaning to.
• Use flexible ISAs carefully. Some Cash ISAs are flexible, letting you withdraw and replace money in the same tax year without it counting twice against your allowance. Non-flexible ISAs don't allow this, so check your provider's terms.
• Mind the long game. For a Stocks & Shares ISA, the biggest gains usually come from leaving it invested through ups and downs rather than dipping in and out.

If your savings are part of a wider plan, it can help to map your ISA alongside your other goals using the savings calculator or to stress-test a lump-sum investment with the investment calculator.

Common mistakes to watch for

A few traps catch people out every year:

• Paying into two of the same type with the wrong provider. Since April 2024 you can pay into more than one ISA of the same type in a tax year, but older accounts and some provider rules still vary. The hard limit is always the combined £20,000, so keep a running total.
• Assuming the allowance carries over. It doesn't. Unused allowance is lost at midnight on 5 April.
• Treating the projected return as guaranteed. A Stocks & Shares ISA can fall in value. The calculator's growth rate is an assumption; markets don't pay a fixed percentage each year.
• Forgetting platform and fund charges. Investment ISAs carry fees that nibble at returns. If your provider charges, say, 0.5% a year, model a slightly lower net growth rate to stay realistic.
• Withdrawing and re-paying in a non-flexible ISA. Take £5,000 out of a non-flexible ISA and put it back the same year, and that £5,000 uses up allowance a second time.

Disclaimer: these figures are estimates for guidance only and are not personal tax or financial advice. Investment values can fall as well as rise, and tax treatment depends on your circumstances and may change.

Related calculators

Carry on planning with these tools: the savings calculator for goals outside an ISA, the compound interest calculator to see growth maths in detail, and the Capital Gains Tax calculator to understand the tax an ISA spares you.

Related tools

• Savings interest tax calculator, see the tax on savings held outside an ISA.

Related guides

Not sure how many accounts you can open? Read how many ISAs you can have in 2026/27.