Your ISA is one of the most powerful tax-free wealth-building tools in the UK. See how your ISA could grow over 10, 20, or 30 years with our free calculator.
An ISA (Individual Savings Account) is a tax-free wrapper around your savings or investments. Inside a Cash ISA, your money earns interest. Inside a Stocks & Shares ISA, your money is invested in the stock market through funds, ETFs, or individual shares. In both cases, all growth is completely free from UK tax.
This means no capital gains tax on your profits (normally 18% for basic-rate taxpayers or 24% for higher-rate), and no tax on dividends (above the £500 annual dividend allowance). Over decades, this tax-free compounding creates a substantial advantage compared to investing in a general investment account where you would owe tax on gains and dividends.
The annual ISA allowance is £20,000, shared across all ISA types you hold (Cash ISA, S&S ISA, Lifetime ISA, Innovative Finance ISA). Since April 2024, you can open multiple ISAs of the same type in a single tax year, but the total £20,000 limit still applies. Every pound you invest inside an ISA today will compound tax-free for the rest of your life — there is no time limit on holding ISA investments.
| Time period | Cash ISA (4%) | S&S ISA (7%) | You put in |
|---|---|---|---|
| 10 years | £73,600 | £86,700 | £60,000 |
| 20 years | £183,400 | £246,600 | £120,000 |
| 30 years | £347,000 | £566,800 | £180,000 |
Based on £500/month contributions compounded monthly. Cash ISA rate assumed constant at 4% (will vary in practice). S&S ISA assumes 7% average annual returns. Investment returns are not guaranteed and capital is at risk. Past performance does not guarantee future results.
After 30 years, the Stocks & Shares ISA has produced roughly £220,000 more than the Cash ISA — despite both receiving the same £180,000 in contributions. That difference is entirely due to the higher average return on investments compounding over three decades. The gap widens with every additional year.
After 15 years
~£504,000
£300k contributed
After 20 years
~£820,000
£400k contributed
After ~25 years
~£1,000,000+
ISA millionaire
Based on investing the full £20,000 ISA allowance (£1,667/month) at 7% average annual returns. This requires a high income and savings rate. Even investing half this amount builds a substantial tax-free portfolio.
The £20,000 annual ISA allowance expires on 5 April each year and cannot be carried forward. Every pound you invest inside your ISA is sheltered from tax permanently. Even if you can only invest a few hundred pounds per month, use the ISA wrapper — the tax savings compound alongside your returns.
Fund fees eat directly into your returns. A global index fund with a 0.07-0.23% annual fee will outperform the majority of expensive actively managed funds over the long term. On Trading 212, you can buy ETFs like VWRL (Vanguard FTSE All-World) or VUSA (S&P 500) with zero commission and no platform fee.
Dividends reinvested into your ISA buy more shares, which generate more dividends, which buy more shares. This dividend compounding loop is a significant driver of long-term returns. Choose accumulation (Acc) fund variants where dividends are automatically reinvested, or manually reinvest if using income (Inc) variants.
Frequent buying and selling does not improve returns for the vast majority of investors — in fact, it typically reduces them. The best ISA strategy for most people is to buy a diversified index fund regularly and leave it alone for decades. Resist the urge to react to market dips or chase trends. Time in the market beats timing the market.
Holding too much in cash. Many people open a Stocks & Shares ISA but leave the money as uninvested cash inside the account. Your ISA cash balance earns little to no interest and misses out on market returns. Once you deposit money into your S&S ISA, make sure you actually invest it in funds or ETFs.
Paying high fees. Some platforms charge 0.25-0.45% annually on top of fund fees of 0.5-1.5%. Over 20 years, the difference between a total cost of 0.2% and 1.5% is enormous — potentially tens of thousands of pounds. Compare total costs (platform fee + fund fee) before choosing a provider.
Not investing the full allowance. If you have cash sitting in a standard savings account while your ISA allowance goes unused, you are paying unnecessary tax on interest (above the £1,000 personal savings allowance for basic-rate taxpayers) and missing the chance to shelter future growth. Prioritise filling your ISA before using taxable accounts.
Withdrawing during market dips. Markets drop regularly — roughly every 7-10 years there is a significant correction. If your time horizon is 10+ years, these dips are buying opportunities, not reasons to sell. Selling during a downturn locks in losses and destroys compound growth. Stay invested and keep contributing.
See how your ISA could grow with our free compound interest calculator.
Open ISA growth calculatorOpen a free Stocks & Shares ISA and invest from just £1. Zero commission, no platform fees — meaning more of your ISA grows through compound interest.
Capital at risk. This is not financial advice. Affiliate link — we may earn a commission at no extra cost to you.
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