ISA Calculator UK
How does an ISA calculator work?
An ISA calculator projects how much your tax-free savings will grow. Enter your starting balance, monthly contribution, expected return and how long you will save. For 2026/27 you can pay in £20,000 across Cash, Stocks & Shares, Innovative Finance and Lifetime ISAs, with £4,000 the Lifetime ISA limit and £9,000 for a Junior ISA.
Individual Savings Accounts (ISAs) let you save and invest tax-free — no income tax on interest, no Capital Gains Tax on growth, and no tax on dividends. With the right strategy, the tax savings alone can be worth thousands over a decade.
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Many people underestimate how much their ISA could grow, or don't realise they can split their £20,000 annual allowance across different ISA types. From 6 April 2027 the overall allowance stays at £20,000 but only £12,000 of it can go into a Cash ISA if you're under 65. A common mistake is keeping long-term savings in a Cash ISA when a Stocks & Shares ISA would likely deliver significantly better returns over 10+ years — even accounting for market volatility.
Choose your ISA type below to project how your savings could grow. The Lifetime ISA option includes the 25% government bonus automatically. Whether you're saving for a first home, building a child's nest egg, or growing long-term wealth, this tool shows you exactly what to expect.
Based on 2026/27 ISA allowances (£20,000 overall, £4,000 Lifetime ISA, £9,000 Junior ISA). Last reviewed: September 2026.
Tax-free savings at a fixed or variable interest rate.
Cash ISA Details
That's £416.67/month
Based on 2026/27 ISA allowances. Projections are estimates only.
Projected Value
Estimated Value After 10 Years
£64,206
ISA Type Comparison
| Type | Limit | Bonus | Access |
|---|---|---|---|
| Cash ISA | £20,000 | None | Anytime |
| S&S ISA | £20,000 | None | Anytime |
| Lifetime ISA | £4,000 | 25% | 60 / 1st home |
| Junior ISA | £9,000 | None | Age 18 |
The £20,000 overall ISA allowance is shared across all ISA types (except Junior ISA).
Coming 6 April 2027: the overall allowance stays at £20,000, but savers under 65 will only be able to pay £12,000 of it into a Cash ISA each year — the rest must go into stocks & shares (or a LISA). Savers aged 65+ keep the full £20,000 cash limit.
Cash ISA Growth Over Time
What this means for your wealth
The £20,000 ISA allowance is the most generous tax shelter most people will ever get — every penny inside grows free of income tax, CGT and dividend tax forever. Over 20 years, a Stocks & Shares ISA filled at £500/month at 6% growth becomes ~£232,000, with around £35k+ saved in tax compared to a standard investment account.
Cash ISAs make sense for short-term savings (under 5 years) or as your emergency fund. For anything longer-term, history strongly favours Stocks & Shares. If you're under 40 and saving for a first home or retirement, the Lifetime ISA adds a 25% government bonus on top — worth up to £1,000 a year free money.
ISA calculator: 2026/27 allowances and limits
Every adult gets one overall ISA allowance of £20,000 for the 2026/27 tax year (6 April 2026 to 5 April 2027). You can split it across as many ISA types as you like, but the Lifetime ISA has its own £4,000 sub-limit, and the Junior ISA sits outside your own allowance entirely.
| ISA type | 2026/27 limit | Age | Government bonus | Key point |
|---|---|---|---|---|
| Cash ISA | £20,000 | 18+ | None | Falls to £12,000 a year for under-65s from 6 April 2027 |
| Stocks & Shares ISA | £20,000 | 18+ | None | Gains and dividends are free of CGT and dividend tax |
| Lifetime ISA (LISA) | £4,000 | 18–39 to open, pay in to 50 | 25% (up to £1,000/yr) | First home up to £450,000, or from age 60 |
| Innovative Finance ISA | £20,000 | 18+ | None | Peer-to-peer lending — capital at risk, not FSCS protected |
| Junior ISA (JISA) | £9,000 | Under 18 | None | Separate allowance; converts to an adult ISA at 18 |
Overall allowance £20,000 across all adult ISAs. Junior ISA allowance is separate.
How much could an ISA grow? Projections at 2%, 4% and 6%
The table shows the projected pot from regular monthly contributions, with returns compounded monthly and no charges deducted. A 2% return is roughly a poor cash account, 4% a competitive Cash ISA in autumn 2026, and 6% a long-run diversified equity assumption.
| Monthly saving | Growth rate | After 5 years | After 10 years | After 20 years | After 30 years |
|---|---|---|---|---|---|
| £250 | 2% | £15,762 | £33,180 | £73,699 | £123,181 |
| £250 | 4% | £16,575 | £36,812 | £91,694 | £173,512 |
| £250 | 6% | £17,443 | £40,970 | £115,510 | £251,129 |
| £500 | 2% | £31,524 | £66,360 | £147,398 | £246,363 |
| £500 | 4% | £33,149 | £73,625 | £183,387 | £347,025 |
| £500 | 6% | £34,885 | £81,940 | £231,020 | £502,258 |
| £1,666 (full allowance) | 2% | £105,037 | £221,111 | £491,132 | £820,880 |
| £1,666 (full allowance) | 4% | £110,454 | £245,318 | £611,047 | £1,156,286 |
| £1,666 (full allowance) | 6% | £116,237 | £273,023 | £769,760 | £1,673,522 |
Illustrative only. Investment returns are not guaranteed and platform or fund charges will reduce these figures.
Worked example: £500 a month into a Stocks & Shares ISA
Sarah pays £500 a month (£6,000 a year) into a Stocks & Shares ISA, leaving £14,000 of her £20,000 allowance unused. She assumes 6% annual growth.
- Total contributions over 20 years: £120,000
- Projected value after 20 years at 6%: £231,020
- Investment growth: £111,020
- Capital Gains Tax due inside the ISA: £0 — outside an ISA a higher-rate investor could face 24% CGT on gains above the £3,000 annual exemption.
If Sarah is 30 and saving for a first home instead, redirecting £4,000 a year to a Lifetime ISA adds a £1,000 government bonus each year — £10,000 of free money over a decade, provided she buys below £450,000.
Sources and methodology
Allowances and rules are taken directly from HMRC and gov.uk guidance for the 2026/27 tax year (ISA / LISA allowances last verified 2026-04-06). Projections use monthly compounding of level contributions, ignore charges and inflation, and assume the allowance is not exceeded.
ISA Types Explained
Cash ISA
Works like a savings account but interest is completely tax-free. Best for short-term savings, emergency funds, or risk-averse savers. Rates track base rate — with rates currently around 4-5% in autumn 2026, you could earn roughly £800-£1,000 a year tax-free on a full £20,000 allowance. From April 2027 the annual Cash ISA limit drops to £12,000 for under-65s.
Stocks & Shares ISA
Invest in funds, shares, bonds, and ETFs with all gains and dividends tax-free. Historical UK equity returns average around 7% per year, but values can fall. Best for goals 5+ years away where you can ride out market volatility.
Lifetime ISA (LISA)
Available to 18-39 year olds, with a 25% government bonus on contributions up to £4,000/year (max £1,000 bonus). Money can be used for a first home (up to £450,000) or withdrawn penalty-free after age 60. Early withdrawal for other reasons incurs a 25% charge.
Junior ISA (JISA)
Tax-free savings for under-18s with a £9,000 annual limit. The child can't withdraw until they turn 18, when it automatically converts to an adult ISA. Grandparents, relatives, and friends can all contribute. Available as cash or stocks & shares.
Common ISA Mistakes
- 1.
Leaving your allowance unused
You can't carry over unused ISA allowance. Even small regular contributions add up significantly over time thanks to compound growth.
- 2.
Using cash ISAs for long-term goals
Over 10+ years, stocks & shares ISAs have historically outperformed cash significantly. If you won't need the money for 5+ years, investing usually delivers better returns.
- 3.
Withdrawing from a LISA for non-qualifying purposes
The 25% withdrawal charge means you lose more than the bonus — you actually lose 6.25% of your original contribution too. Only use a LISA if you're confident about buying a first home or saving until 60.
- 4.
Not comparing ISA rates and fees
Cash ISA rates vary enormously between providers. For S&S ISAs, platform fees and fund charges can significantly erode returns over time. Always compare before committing.
- 5.
Withdrawing and re-depositing (non-flexible ISA)
Unless your ISA is 'flexible', withdrawing and putting money back counts as a new contribution against your annual allowance. Check if your provider offers a flexible ISA.
Frequently Asked Questions
What's Next?
Stocks and Shares ISA Guide
How a stocks and shares ISA works, fees and platform choice.
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