ISA Calculator
ISA savings calculator with annual allowance tracking, monthly deposits and year-by-year growth projection table
Summary
Final Balance
£25,947
Total Contributions
£22,000
Interest Earned
£3,947
ISA Annual Allowance: £20,000 per tax year
Year-by-Year Breakdown
| Year | Opening | Interest | Closing |
|---|---|---|---|
| 1 | £10,000 | £510 | £12,910 |
| 2 | £12,910 | £643 | £15,953 |
| 3 | £15,953 | £783 | £19,136 |
| 4 | £19,136 | £929 | £22,465 |
| 5 | £22,465 | £1,082 | £25,947 |
What Could £20,000 a Year in an ISA Become?
Maxing the £20,000 ISA allowance every year for 20 years at 5% real growth produces a pot of around £660,000, all of it tax-free at withdrawal. At 6% real growth the same contributions produce roughly £736,000; at 7%, £820,000. Real growth means after inflation, so these are pots in today's money. Cash ISAs paying around 4.5% in 2026 grow more slowly in nominal terms and barely keep pace with inflation in real terms.
The shape of compound growth means the last few years of contributions do most of the heavy lifting. A 25-year-old who fills the ISA for ten years and then stops will retire with more in real terms than a 35-year-old who fills it for twenty years, because the early contributions have an extra ten years to compound. This is why financial planners obsess about starting early, even with smaller amounts. £200 a month from age 22 reaches a larger pot at 65 than £400 a month from age 35.
ISA Allowance Rules That Trip People Up
The £20,000 annual allowance for 2026/27 covers all your ISAs combined, not £20,000 per ISA. You can spread it however you like across Cash, Stocks and Shares, Innovative Finance, and Lifetime ISAs (the LISA has its own £4,000 sub-allowance which counts towards the overall £20,000). If you put £15,000 into a Stocks and Shares ISA in April, you have £5,000 left for the rest of the tax year across all your ISAs combined.
Partial-year transfers are the rule that catches most savers. You can transfer existing ISAs between providers without it counting towards your annual allowance, but you cannot withdraw from an ISA and re-contribute the same money in the same tax year unless your provider offers a 'flexible' ISA (most cash ISAs do, most stocks-and-shares ISAs do not). If you take £5,000 out of a non-flexible ISA in May to cover an emergency, you have lost that £5,000 of allowance for the year. The [compound interest calculator](/compound-interest-calculator) is useful for stress-testing whether breaking into ISA capital is worth the lost growth on the outflow.
Cash ISA vs Stocks and Shares ISA
Cash ISAs in 2026 pay around 4.5% on the best easy-access deals and 5% to 5.3% on one to two-year fixes. With CPI inflation hovering around 2.5% to 3%, that is roughly 1.5% to 2.5% real return. Stocks and Shares ISAs invested in a global tracker have averaged 6% to 7% real return over the last 30 years (FTSE All-World total return data), with significant volatility year to year. Over 20-year holding periods, stocks have outperformed cash 100% of the time in UK historical data; over 5-year periods, stocks beat cash roughly 75% of the time but with notable drawdowns in adverse periods. The choice usually comes down to time horizon: cash for goals under 5 years, stocks for goals over 10 years, a blended split for the awkward middle.
Frequently Asked Questions
What is the ISA allowance for 2026/27?
£20,000 per person, per tax year. That figure has not changed since 2017/18 and is currently frozen with no scheduled increase. Couples can each use their full £20,000, giving a household allowance of £40,000 a year. Children get a separate £9,000 Junior ISA allowance up to age 18.
Can I have multiple ISAs?
Yes. Since April 2024 you can pay into multiple ISAs of the same type within the same tax year (previously you had to pick one Cash ISA and one Stocks and Shares ISA per year). The combined contributions across all ISAs still cannot exceed £20,000. You can also hold ISAs from previous tax years across multiple providers without restriction.
Do I pay tax on ISA gains or interest?
No. ISAs are completely shielded from income tax and capital gains tax inside the wrapper, and there is no further tax when you withdraw. This contrasts with general savings (where interest above £1,000 a year is taxed for basic-rate payers, £500 for higher-rate, £0 for additional-rate) and general investment accounts (where dividends above £500 are taxed and capital gains above £3,000 are taxed).
Should I move my ISA to a higher-paying provider?
Often yes, especially with cash ISAs. Use a provider's official ISA transfer process - never withdraw and re-pay, because that would count as a fresh contribution against your annual allowance. Stocks and shares ISA transfers between platforms typically take two to four weeks; cash transfers are quicker, normally five to seven working days. Some fixed-rate cash ISAs charge an early-exit penalty equivalent to 90 to 180 days' interest, so check before transferring.