Our Stocks and Shares ISA calculator shows how regular monthly contributions can grow over time within a tax-free ISA wrapper. Investing 200 pounds per month at 7% annual growth could be worth approximately 34,600 pounds after 10 years and over 104,800 pounds after 20 years, all completely free from capital gains tax and dividend tax. The earlier you start and the more consistently you invest, the greater the benefit of compound growth.
How Much Could Your Stocks and Shares ISA Grow?
The tables below show how different monthly contributions could grow inside a Stocks and Shares ISA at various average annual growth rates. All figures are estimates for illustration purposes. Actual returns will vary and investments can fall in value.
Growth at 5% Per Year
| Monthly | 5 Years | 10 Years | 15 Years | 20 Years | 30 Years |
| 100 pounds | 6,800 | 15,500 | 26,700 | 41,100 | 83,200 |
| 200 pounds | 13,600 | 31,100 | 53,400 | 82,100 | 166,500 |
| 500 pounds | 34,000 | 77,600 | 133,600 | 205,400 | 416,100 |
| 1,000 pounds | 68,000 | 155,300 | 267,200 | 410,800 | 832,300 |
| 1,667 pounds (max ISA) | 113,400 | 258,900 | 445,500 | 684,800 | 1,387,500 |
Growth at 7% Per Year
| Monthly | 5 Years | 10 Years | 15 Years | 20 Years | 30 Years |
| 100 pounds | 7,200 | 17,300 | 31,700 | 52,100 | 122,700 |
| 200 pounds | 14,300 | 34,600 | 63,400 | 104,800 | 245,400 |
| 500 pounds | 35,800 | 86,500 | 158,500 | 260,500 | 613,500 |
| 1,000 pounds | 71,600 | 173,100 | 317,100 | 521,000 | 1,227,000 |
| 1,667 pounds (max ISA) | 119,300 | 288,600 | 528,600 | 868,500 | 2,045,300 |
Growth at 9% Per Year
| Monthly | 5 Years | 10 Years | 15 Years | 20 Years | 30 Years |
| 100 pounds | 7,500 | 19,400 | 37,800 | 66,800 | 183,100 |
| 200 pounds | 15,100 | 38,700 | 75,700 | 133,600 | 366,100 |
| 500 pounds | 37,700 | 96,800 | 189,300 | 334,100 | 915,400 |
| 1,000 pounds | 75,400 | 193,700 | 378,500 | 668,100 | 1,830,700 |
| 1,667 pounds (max ISA) | 125,700 | 322,900 | 631,000 | 1,113,600 | 3,051,800 |
These projections assume: consistent monthly contributions, no fees deducted, growth compounded monthly, and no withdrawals. The 1,667 pounds per month figure represents using the full 20,000 pound annual ISA allowance spread evenly across 12 months.
How Does Compound Growth Work in a Tax-Free ISA?
The power of a Stocks and Shares ISA comes from compound growth: your returns generate their own returns, and all of it is tax-free. In a General Investment Account, you would pay capital gains tax and dividend tax on your returns, which reduces the amount available to compound.
The Tax-Free Advantage
Consider 500 pounds per month invested for 20 years at 7% annual growth:
| Scenario | Value After 20 Years | Tax Paid | Net Value |
| Inside ISA (tax-free) | 260,500 pounds | 0 pounds | 260,500 pounds |
| Outside ISA (basic rate) | 260,500 pounds gross | ~8,900 pounds* | ~251,600 pounds |
| Outside ISA (higher rate) | 260,500 pounds gross | ~22,400 pounds* | ~238,100 pounds |
*Estimated tax based on typical CGT and dividend tax at 2025/26 rates, assuming annual crystallisation of gains above the 3,000 pound CGT exemption and dividends above the 500 pound allowance. Actual tax will depend on individual circumstances.
For a higher-rate taxpayer, the ISA saves an estimated 22,400 pounds in tax over 20 years on a 500 pound monthly investment. The savings are even larger for bigger portfolios.
What Growth Rate Should You Use for ISA Projections in 2026?
The growth rate you use depends on your investment approach and risk level:
| Portfolio Type | Typical Average Return | Risk Level |
| Cash ISA | 2% to 4% | None (capital protected) |
| Cautious (mostly bonds) | 4% to 5% | Low |
| Balanced (mix of shares and bonds) | 5% to 7% | Medium |
| Growth (mostly global equities) | 7% to 9% | Medium-high |
| Aggressive (100% equities) | 8% to 10% | High |
Historically, the UK stock market (FTSE All-Share) has returned approximately 7% to 8% per year over the long term, while global equities have averaged around 8% to 10%. These are nominal returns before adjusting for inflation (typically 2% to 3% per year). Past performance does not guarantee future results.
(Source: Barclays Equity Gilt Study, long-term UK equity returns data.)
A commonly used planning figure for a diversified global equity portfolio is 7% per year. For a more conservative estimate, use 5%. For an optimistic projection, use 9%.
How Do Platform Fees Affect Your ISA Growth?
Platform fees and fund charges reduce your effective growth rate. A platform charging 0.45% per year with fund costs of 0.20% would reduce a 7% gross return to approximately 6.35% net. Over 20 years, even a 0.5% annual fee difference can reduce your final pot by tens of thousands of pounds.
| Monthly Amount | 20 Years at 7% | 20 Years at 6.5% | Fee Impact |
| 200 pounds | 104,800 pounds | 97,500 pounds | -7,300 pounds |
| 500 pounds | 260,500 pounds | 243,700 pounds | -16,800 pounds |
| 1,000 pounds | 521,000 pounds | 487,500 pounds | -33,500 pounds |
This is why choosing a low-cost platform matters. Free platforms like Trading 212 and InvestEngine eliminate platform fees entirely for DIY investors. See our best trading platforms UK comparison.
How Can You Maximise Your ISA Growth?
Start early: The longer your money is invested, the more time compound growth has to work. Starting 10 years earlier can more than double your final pot.
Invest regularly: Setting up a monthly direct debit smooths out market volatility through pound-cost averaging. You buy more units when prices are low and fewer when prices are high.
Use your full allowance: The 20,000 pound annual allowance does not roll over. Every year you do not use it is a lost opportunity for tax-free growth.
Keep fees low: Choose low-cost platforms and funds. Even small fee differences compound significantly over decades.
Reinvest dividends: Accumulation funds automatically reinvest dividends, which accelerates compound growth.
Stay invested: Trying to time the market usually reduces returns. Staying invested through volatility has historically rewarded patient investors.
Frequently Asked Questions
How much will my ISA be worth in 10 years?
It depends on how much you invest and the growth rate. At 7% annual growth, investing 200 pounds per month would be worth approximately 34,600 pounds after 10 years. See the tables above for different amounts and rates.
Is 7% a realistic growth rate?
For a diversified global equity portfolio, 7% is a reasonable long-term planning assumption. The MSCI World Index has averaged approximately 8% to 10% per year over the past 30 years. However, returns in any given year can vary widely.
(Source: MSCI World Index factsheet, historical annualised returns.)
Should I invest a lump sum or monthly?
Historically, investing a lump sum earlier tends to outperform monthly contributions because your money is in the market longer. However, monthly investing reduces the risk of investing at a market peak and is psychologically easier for most people.
How much tax do I save with an ISA?
The tax savings depend on your tax bracket and portfolio size. A higher-rate taxpayer with a £100,000 portfolio generating £3,000 in dividends and £5,000 in capital gains could save roughly £1,300 per year in tax by holding investments in an ISA, based on 2025/26 dividend tax and Capital Gains Tax allowances and rates. Actual savings depend on your circumstances.
What is the maximum I can invest in an ISA?
The annual ISA allowance is 20,000 pounds for the 2025/26 tax year. There is no lifetime limit on ISA holdings. You can build up an ISA pot of any size over time.
Do these calculations include inflation?
No. The figures show nominal returns. To estimate real (inflation-adjusted) returns, subtract approximately 2% to 3% from the growth rate. For example, 7% nominal growth with 2.5% inflation gives approximately 4.5% real growth.
Can I withdraw from my ISA and keep the tax benefits?
Yes. Withdrawals from a Stocks and Shares ISA are always tax-free. If you have a flexible ISA, you can replace withdrawn amounts within the same tax year without losing allowance. See Stocks and Shares ISA rules.
How do fees affect my ISA growth?
A 0.5% annual fee on a 500 pound monthly investment reduces your pot by approximately 16,800 pounds over 20 years at 7% growth. Choosing low-cost providers can save you tens of thousands over your investing lifetime.
Related Reading
Is a Stocks and Shares ISA Worth It?
