ISA Retirement Calculator
Model your Stocks & Shares ISA growth using real S&P 500 and FTSE 100 historical returns going back 50 years. All growth inside an ISA is 100% tax-free.
ISA Settings
Existing savings to invest now
£6,000/yr — ISA limit £20,000/yr
Historical total return (dividends reinvested)
BoE target: 2% | Historical CAGR since 1975 (inc. 70s/80s): 4.9%
Final Value (Real £)
£596,929
In today's purchasing power
Total Contributed
£155,000
Your actual money in
Investment Growth
£951,674
614% gain on contributions
Monthly Income (4% SWR)
£1,990
Tax-free from ISA
Portfolio Growth Projection
Portfolio value shown in today's purchasing power (real £). Contributions are nominal (actual £ paid in).
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About the Historical Data
This calculator uses real, compounded annual total return data (price appreciation + dividends reinvested) for the S&P 500 (1975–2024) and FTSE 100 (1984–2024). UK CPI inflation figures from the ONS are used to calculate real (inflation-adjusted) returns.
Monte Carlo Simulation
The Monte Carlo mode runs 1,000 independent retirement scenarios by randomly sampling from the historical pool of annual returns (bootstrap resampling). This captures the effect of sequence-of-returns risk — the fact that when market crashes happen matters as much as how big they are.
UK ISA Rules (2024/25)
- Annual ISA allowance: £20,000 per tax year
- All growth and income within the ISA is tax-free
- No capital gains tax on withdrawals
- Flexible ISA rules: some providers allow you to replace withdrawn funds in the same tax year
- Allowance resets each April (UK tax year)
Important:Past performance is not a reliable indicator of future returns. All projections are illustrative only. Markets can fall as well as rise. This is not financial advice. For personalised guidance, consult a regulated financial adviser. Investments in a Stocks & Shares ISA can go down in value.