Overview
Compound interest is interest earning interest: each period's gains are added to the balance, so the next period earns on a larger base. The difference from simple interest looks small at first and enormous later — the curve bends upward, which is why time matters more than timing. This calculator shows the bend: future value, total interest earned, and a year-by-year table.
You can model an initial lump sum alone, or add monthly contributions to see how steady saving plus compounding builds over decades. The table makes the key insight visible: in the later years, growth comes mostly from interest, not from what you put in.
How does compounding actually work?
The formula for a lump sum is FV = P × (1 + r/n)^(n×t): principal times one-plus-the-periodic-rate, raised to the number of periods. More frequent compounding (monthly vs yearly) earns slightly more because gains start earning sooner — though the difference is modest compared to the effect of time itself.
The dramatic part is the exponent. Double the years and you more than double the result, because growth multiplies rather than adds. That is the whole secret behind “start early”: ten extra years at the beginning beats a higher rate later. Contributions add a second engine — each deposit starts its own compounding clock.
Features
- Future value for any principal, rate, time and compounding frequency.
- Monthly contributions modelled alongside the lump sum.
- Interest-vs-principal bar showing where the final balance came from.
- Year-by-year table with balance and yearly interest.
- Four compounding frequencies: monthly, quarterly, yearly, daily.
- Private: your numbers never leave the browser.
Benefits
- See the curve. The table shows compounding accelerating over time.
- Compare scenarios. Rate vs time vs contributions, side by side in seconds.
- Motivation, quantified. Small monthly amounts become large numbers.
- Honest framing: labelled as illustration, not advice.
Who can use it
Savers planning long-term goals, students learning the time value of money, parents modelling education funds, and anyone who has heard “compound interest is the eighth wonder” and wants to see the numbers. Remember: this illustrates the maths of compounding — it is not financial advice, and real returns vary with markets, taxes and fees.
How to use
- Enter the initial amount you are starting with.
- Enter the annual interest rate as a percentage.
- Enter the number of years and how often interest compounds.
- Optionally add a monthly contribution.
- Read the future value, the interest earned, and the yearly table.
Frequently asked questions
Why does the growth accelerate in later years?
Because interest is calculated on an ever-larger balance. In year one, 7% of 10,000 is 700. In year twenty, 7% applies to a balance swollen by two decades of gains — so the same rate produces far more interest. The rate never changed; the base did. That is compounding.
Does compounding frequency matter much?
Less than people think. Monthly vs yearly compounding on the same nominal rate adds only a fraction of a percent per year. Time in the market and the rate itself dominate. Frequency is worth knowing about, but years and contributions move the needle.
What is the Rule of 72?
A mental shortcut: divide 72 by the annual rate to estimate doubling time. At 7%, money doubles in roughly 10 years (72/7 ≈ 10.3). It is approximate — it assumes annual compounding and ignores contributions — but it is handy for sanity-checking calculator results.
Why doesn't the calculator include inflation or tax?
To keep the maths transparent: it shows nominal growth before those real-world haircuts. Inflation quietly reduces purchasing power (3% inflation halves it in ~24 years), and taxes take a slice of gains. Treat the output as the gross figure, then mentally discount for reality — or consult a financial professional.
Are my figures stored?
No. Everything is computed locally in your browser and vanishes when you leave the page.
Conclusion
Compounding rewards patience more than brilliance: rate matters, contributions matter, but time matters most. Run the numbers, respect the caveats, and let the later years do the heavy lifting.