About the Compound Interest Calculator
Compound interest is interest paid on interest. The classic illustration is a single deposit left to grow for decades — but the calculation that matters in modern personal finance is compounded growth with regular contributions, the way a retirement account or a child's education fund actually grows. This calculator handles both: a one-off lump sum, periodic contributions, and the interaction between them.
Mathematically, the future value of a growing investment with regular contributions is the sum of two terms: the lump-sum future value FV_lump = P × (1 + r/n)^(n·t), and the future value of a contribution stream FV_stream = C × ((1 + r/n)^(n·t) − 1) / (r/n). The calculator computes both and adds them.
Why time matters more than rate
Compound growth is exponential, which means doubling the time matters more than doubling the rate. £10,000 at 6% for 20 years grows to about £32,071; at 6% for 40 years it grows to over £102,857. The mathematical reason: in the first half of the period the lump grew to ~£32k, and in the second half that £32k grew to over £100k. The lesson is to start early. Even small contributions, started decades before retirement, beat much larger contributions started ten years before.
Real returns vs nominal returns
A "7% return" in nominal terms is not the same as 7% in purchasing power. Long-term inflation in most developed economies has been 2–3% per year. Real returns are roughly nominal minus inflation. When planning for goals decades in the future, model conservatively — use a real return assumption (4–5%) rather than a long-run stock-market nominal return (8–10%).
How to use the Compound Interest Calculator
Enter the starting amount
The lump sum you already have invested (zero is fine if you are starting from nothing).
Enter the monthly or yearly contribution
Realistic regular addition.
Choose rate and term
Annual nominal rate and time horizon in years.
Read the growth curve
Final balance, total contributed, and total interest earned are shown alongside a year-by-year breakdown.
Worked examples
Example 1
Input: £10,000 lump, £200/month, 7%, 30 years
Result: Final balance £319,170; total contributed £82,000; interest £237,170
A realistic long-term retirement contribution scenario.
Example 2
Input: £0 lump, £500/month, 5%, 20 years
Result: Final balance £205,517; contributed £120,000; interest £85,517
Starting from zero with steady contributions.
Real-world use cases
- Estimating retirement balance under different contribution levels.
- Planning a child's university savings.
- Stress-testing how a market downturn (lower rate) would affect long-term goals.
- Comparing a higher-yield account against a familiar option.
- Visualising why early contribution beats catch-up contributions.
Tips & common mistakes
- Use a conservative real-return assumption (4–5%) for planning, not the long-run nominal stock return.
- Increase contributions as income grows; small annual increases (3% per year) significantly boost final balance.
- Tax-advantaged accounts (ISA, 401k, IRA, Roth, Super) generally compound faster than taxable accounts — model them separately.
- Pay attention to fees. A 1% expense ratio over 30 years can consume a quarter of your final balance.
Frequently asked questions
What compounding frequency does it use?
Monthly compounding by default, which matches most savings and retirement accounts. The mathematical impact of daily vs monthly is small at typical interest rates.
Does it account for inflation?
Not directly. You can simulate inflation-adjusted returns by entering a "real" rate (nominal minus expected inflation).
Are taxes included?
No. Use a tax-advantaged-account assumption, or model post-tax separately by reducing the rate.
How accurate is the projection?
It is mathematically exact for the inputs given. Real life adds variability — market returns are not steady — so treat the result as a planning estimate, not a guarantee.
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Last updated: June 2026 · All processing happens locally in your browser.