Compound Interest & ISA Investment Calculator
Visualise how lump sums and monthly contributions compound over time with year-by-year charts.
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Projected Final Balance ยฃ343,777 Compounded monthly
Total Cash Contributed ยฃ130,000 37.8% of final pot
Total Compound Interest Earned +ยฃ213,777 62.2% free compound growth
Year-by-Year Stacked Growth (Principal vs. Compound Returns) โ Contributions ยท โ Compound Interest
Year-by-Year Compounding Schedule
| Year | Total Contributed | Compound Interest | Ending Balance |
|---|
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How to Use Compound Interest & ISA Investment Calculator
- Set your Starting Balance, Monthly Contribution, Annual Return Rate (%), and Investment Time Horizon (Years).
- Watch the stacked visual bar chart and summary cards update instantly to show Total Principal Paid vs. Total Compound Interest Earned.
- Inspect the year-by-year schedule below or download the projection table as a CSV spreadsheet.
The Mathematics of Compound Growth & Regular Investing
Compound interest generates exponential wealth because your investment returns begin earning returns of their own. For a starting principal P, monthly contribution PMT, annual rate r, and 12 monthly periods over t years, future value FV is computed via:
FV = P ร (1 + r/12)^(12t) + PMT ร [((1 + r/12)^(12t) โ 1) รท (r/12)]
Even modest monthly contributions inside a tax-efficient wrapper (such as a UK ยฃ20,000 annual Stocks & Shares ISA or SIPP) frequently result in total compound interest overtaking total cash deposits around year 10 to 12 at a 7โ8% historical equity return.