Compound Interest Calculator
See how compounding turns steady contributions into serious wealth — with a year-by-year growth chart.
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Total balanceYour contributions
See how compounding turns steady contributions into serious wealth — with a year-by-year growth chart.
Compound interest means you earn returns on your returns. In year one, a 7% return on £10,000 earns £700. By year thirty, that same 7% applies to a balance swollen by decades of growth and contributions — so a single year can add more than your entire first decade of deposits. The chart above shows this visually: the gap between your contributions (grey) and your total balance (teal) is pure compounding, and it accelerates the longer you leave money alone.
The most important input is not the rate — it is the years. Starting ten years earlier at a modest return beats starting later at a brilliant one, because compounding is exponential. This is the mathematical argument for starting retirement saving in your twenties even with small amounts: money invested at 25 has forty years to compound, while money invested at 35 has only thirty. Try it above: cut the years from 30 to 20 and watch the future value collapse far more than the one-third reduction in time suggests.
A handy shortcut: divide 72 by your annual return to estimate how many years it takes money to double. At 7%, money doubles roughly every 10 years (72 ÷ 7 ≈ 10.3). At 10%, every 7 years. It is approximate, but it makes the abstract concrete — and it is another way to see why even a 1–2% difference in fees or returns compounds into life-changing money over decades.
Interest calculated on both your original money and all previously earned interest. Over long periods it grows balances exponentially rather than linearly.
Monthly. Contributions are assumed to be made at the end of each month, and the annual rate is divided into twelve monthly periods.
For long-term stock market investing, 7% per year after inflation is a commonly used planning figure in the US (roughly 10% nominal minus 3% inflation). For cash savings, use your account’s actual rate.
No — the projection is nominal and pre-tax. To think in today’s money, subtract expected inflation (around 2–3%) from your assumed return.
Both benefit from compounding. A lump sum invested earlier usually wins mathematically, but regular monthly contributions build the habit — and habit beats optimisation.
Enter your starting lump sum (or 0), your monthly contribution, the expected annual return and the number of years. Contributions are assumed to be added at the end of each month and compounded monthly. For example, $500 a month for 30 years at 7% grows to about $610,000 — only $180,000 of which is money you put in; the other $430,000 is compounding at work. If you want the biggest impact, increase the years first: time moves the result more than the rate does.