A UK-localised calculator pre-filled in GBP. Use it to project growth inside a Stocks & Shares ISA, LISA, or SIPP — all of which compound tax-free up to the annual allowances.
A £10,000 ISA balance plus £500/month at 7% for 25 years compounds to roughly £415,000 — entirely tax-free because growth inside an ISA escapes capital gains and dividend tax.
The 2026/27 ISA allowance is £20,000 per tax year, of which up to £4,000 can go into a Lifetime ISA (which adds a 25% government bonus on contributions until age 50).
Compound interest is interest earned on interest. Each period, your balance grows by the previous balance times the periodic rate, then your contribution is added. Over decades the compounded portion dwarfs the contributed portion — the "hockey stick" curve every personal-finance article keeps showing you.
P is your starting amount. PMT is each monthly contribution. r is the monthly rate (annual ÷ 12). n is the total number of months.
In a UK context: ISA and LISA growth is tax-free, so the nominal number this calculator shows is also close to your real take-home. Outside an ISA, capital gains tax (currently 10–20%) and dividend tax reduce the effective return — model the outcome with a rate ~1–2% lower if you're saving in a General Investment Account.
Important: figures are nominal — they don't subtract UK inflation. To see real growth, subtract a 2–3% CPI estimate from your return assumption.
Compound interest is interest earned on both your original deposit AND on the interest already credited. It causes balances to grow faster over time — the longer you leave money invested, the more dramatic the effect.
Future Value = Principal × (1 + r)n + PMT × ((1 + r)n − 1) / r, where r is the periodic interest rate (annual rate ÷ 12 for monthly compounding) and n is the total number of periods.
For a globally-diversified equity index fund inside a Stocks & Shares ISA, 6–8% annualised is a common historical assumption. Cash ISAs run much lower (typically 4–5%). The calculator does not promise any specific return — model multiple scenarios.
No — figures are nominal. To see real (after-inflation) growth, subtract a UK CPI estimate (typically 2–3%) from your assumed return rate.
Inside an ISA or LISA, growth and withdrawals are tax-free, so the nominal number this calculator shows is also your real take-home. SIPP contributions get income tax relief on the way in, but withdrawals (after the 25% tax-free lump sum) are taxed as income. Outside any wrapper, capital gains tax (10–20%) and dividend tax apply — model with a return rate about 1–2% lower.
ProFinanceCast turns one-off calculations into a living 10-year forecast that updates as your income, expenses, and goals change. Free forever for the core forecast.