Investment & ISA Calculator
See exactly how your money grows with compound returns — monthly contributions, lump sums, ISA vs taxable, year-by-year.
Calculated using compound interest formula — see our methodology
How much will your investment grow?
Compound growth is the single most powerful force in personal finance. Money invested today does not just earn a return — it earns a return on all previous returns. The longer you stay invested, the more dramatic the effect. This calculator shows you exactly what compound growth looks like for your specific numbers.
ISA vs taxable account: does the wrapper matter?
In an ISA, all gains and dividends are free of UK income tax and capital gains tax. In a taxable account, gains above the annual CGT allowance (£3,000 in 2025/26) are taxed at 18% (basic rate) or 24% (higher rate). For long-term investment over 10+ years, the tax drag on a taxable account is significant — it functions as negative compounding on your returns. The calculator's ISA mode shows growth untouched by tax; the taxable mode applies an approximate annual tax drag.
What return rate should I use?
Global equity index funds have historically returned 7–10% annually in nominal terms (before inflation). After inflation, real returns are typically 5–7%. A balanced portfolio (60% equities, 40% bonds) has historically returned 5–7% nominally, or 3–5% in real terms. Use 7% for a realistic equity-only projection, 5% for a balanced portfolio, or 4% for a conservative scenario. The calculator shows nominal returns — to think about real purchasing power, subtract your inflation assumption.