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Work out how your investment grows with compound interest and find out how long it takes to double your money.
This compound interest calculator estimates how an investment grows from an initial amount, a monthly contribution and an expected annual return, compounding monthly, and how many years it takes to double your money (rule of 72).
The S&P 500's historical return has been roughly 10% a year. 7% is a conservative estimate that strips out inflation.
Total
54.714 €
| Año | Contribution | Cumul. contrib. | Interest for the year | Int. Acum. | Total capital |
|---|---|---|---|---|---|
| 0 | 10.000 € | 10.000 € | +0 € | 0 € | 10.000 € |
| 1 | 2400 € | 12.400 € | +801 € | 801 € | 13.201 € |
| 2 | 2400 € | 14.800 € | +1033 € | 1834 € | 16.634 € |
| 3 | 2400 € | 17.200 € | +1281 € | 3115 € | 20.315 € |
| 4 | 2400 € | 19.600 € | +1547 € | 4662 € | 24.262 € |
| 5 | 2400 € | 22.000 € | +1833 € | 6495 € | 28.495 € |
| 6 | 2400 € | 24.400 € | +2138 € | 8633 € | 33.033 € |
| 7 | 2400 € | 26.800 € | +2467 € | 11.100 € | 37.900 € |
| 8 | 2400 € | 29.200 € | +2818 € | 13.918 € | 43.118 € |
| 9 | 2400 € | 31.600 € | +3196 € | 17.114 € | 48.714 € |
| 10 | 2400 € | 34.000 € | +3600 € | 20.714 € | 54.714 € |
Limit: 40 years maximum. Processed locally in your browser (no server resources used).
Report: Downloads a .zip with the Excel file (parameters + yearly breakdown) and 3 charts as PNG.
Albert Einstein called it "the eighth wonder of the world". Compound interest is, put simply, the effect of earning interest on the interest you have already made.
Unlike simple interest, where you only earn on your initial capital, here your gains are reinvested automatically, creating a snowball effect that accelerates your wealth exponentially over time.
"My life has been a product of compound interest."
— Warren Buffett
The secret is not starting with a lot of money, it is time. As you can see in the calculator, growth is slow in the early years, but from year 10 or 15 the curve turns almost vertical.
The earlier you start, the less effort you need. That is the real power of putting your money to work for you.
Each month, the balance is multiplied by the monthly return (the annual rate divided by twelve) and the month’s contribution is added. One month’s interest becomes part of the next month’s capital: that is why growth speeds up over time.
Month balance = previous balance × (1 + annual rate / 12) + monthly contribution
Interest earned is the final balance minus everything you contributed. The calculator does not deduct taxes or fees, and the return is an assumption: the real one changes every year.
Four scenarios worked out with the calculator’s own formula, at 7% a year. Notice how interest goes from a small share to most of the balance.
| Scenario | 10 years | 20 years | 30 years |
|---|---|---|---|
| €100 a month, at 7 % | €17,308interest: €5,308 | €52,093interest: €28,093 | €121,997interest: €85,997 |
| €300 a month, at 7 % | €51,925interest: €15,925 | €156,278interest: €84,278 | €365,991interest: €257,991 |
| €10,000 lump sum, at 7 % | €20,097interest: €10,097 | €40,387interest: €30,387 | €81,165interest: €71,165 |
| €10,000 + €200 a month, at 7 % | €54,714interest: €20,714 | €144,573interest: €86,573 | €325,159interest: €243,159 |
Assumptions: monthly compounding, constant return, no taxes or fees. It is not a forecast: it is the arithmetic of compound interest applied to those inputs.
Divide 72 by the annual return and you get, roughly, the years it takes a lump sum to double with no contributions. It is a mental shortcut, not an exact figure; the calculator gives you the precise number.
It is interest calculated on the initial capital and also on the interest accumulated in earlier periods. By reinvesting what you earn, each period starts from a larger base and growth accelerates over time.
Simple interest is always calculated on the initial capital, so it grows in a straight line. Compound interest is calculated on capital plus interest, so it grows along a curve: over 30 years the difference is huge.
The one you expect from your investment. A global index fund has historically returned around 7% a year after inflation, but nothing is guaranteed. Try several values: the calculator does not predict, it compares scenarios.
Monthly: the annual return is divided by twelve and applied every month, together with the monthly contribution. That is what real recurring contributions to a fund or an automatic investment plan do.
Yes. The download button builds a ZIP with an Excel file (inputs and year-by-year breakdown) and the three charts as PNG. Everything is computed in your browser: we store no data.
No. It is an educational tool that applies a formula to the figures you enter. It ignores your situation, taxes and fees. For specific decisions, consult an adviser registered with the CNMV.