Compound Interest Calculator
Drag your savings, rate and timescale, and watch compounding do its work — every figure and the growth curve update live as you move the controls. No forms, no submit button.
Growth assumptions
Long-term global equity returns of 5–7% a year after fees are a common planning assumption; cash savings rates are typically lower.
Path to your final balance
Projected balance at the end of each year, updating as you adjust the controls.
Balance after 20 years
£129,885Total paid in
£70,000Interest earned
£59,885Where your £129,885 comes from
- Starting balance£10,000
- Contributions£60,000
- Growth£59,885
Share of final balance from growth
46.1%
Assumes monthly compounding · how we calculate this
How this calculator works
Your starting balance grows using the compound interest formula A = P(1 + r/n)^(nt), compounded monthly. Regular monthly deposits are valued with the future value of an annuity formula — PMT × (((1 + r)^n − 1) / r) — so each contribution earns interest from the month it is made. Any additional yearly contribution and annual increases to your monthly deposits are applied year by year.
The projection assumes a constant rate of return and does not account for fees, taxes or inflation. Returns on real investments vary from year to year, so treat the figures as an illustration of compounding rather than a forecast.
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Disclaimer: This calculator provides estimates only and should not be considered financial advice. The calculations are based on the information you provide and assume a constant rate of return, which may not reflect actual market conditions. Investment returns are not guaranteed and can go down as well as up. The calculator does not account for fees, taxes, or inflation, which can significantly impact actual returns. Past performance is not indicative of future results. Please consult with a qualified financial advisor for personalized advice regarding your investment strategy.
Frequently Asked Questions
Disclaimer: These calculators provide estimates for informational purposes only and do not constitute financial advice. Calculations use 2026/27 UK tax rates and thresholds sourced from HMRC and GOV.UK. Results are illustrative and may not reflect your personal circumstances. For personalised advice, consult an FCA-authorised financial adviser.