Compound interest calculator
Compound interest is why time matters more than timing when investing. Play with the numbers and watch how sharply the result changes in the final years.
Scroll down and watch: 200 € a month at an assumed 7 % annual return. A plain assumption, not a forecast.
How this is calculated
What it calculates
What a starting amount plus a monthly contribution grows to over a given period when returns keep earning returns of their own, and how much of that is money you paid in yourself.
Your inputs
- Starting amount: invested once at the start and growing for the whole period.
- Monthly contribution: added every month, at the start of the month.
- Assumed annual return: a constant assumption, not a forecast. The calculator divides it by 12 and compounds month by month.
- Time horizon: the number of years, counted in months (years × 12).
The calculation
With the monthly rate i = return ÷ 12 ÷ 100 and the number of months n = years × 12:
Final value = starting amount × (1 + i)n + contribution × ((1 + i)n − 1) ÷ i × (1 + i)
The first part grows the starting amount for n months. The second part adds up every contribution: each one grows from the month it is paid in, so early contributions grow for much longer than late ones. The extra factor (1 + i) reflects that each contribution already earns a return in the month it is paid. At a 0% return, the result is simply contribution × months.
Paid in yourself = starting amount + contribution × n
Return = final value − paid in yourself
Assumptions: a constant return, no costs, no taxes, no inflation. Because returns compound monthly, the effective annual return is slightly above what you enter (6% works out to about 6.17%).
What the result means
The result shows an order of magnitude, not a prediction. The larger the share that comes from returns, the more time has done the work for you, which is why the final value grows fastest in the last years. Real returns fluctuate, and costs and taxes take a cut (see the fee calculator).
Example
Default values: €1,000 starting amount, €200 a month, 6% a year, 25 years. That gives i = 0.5% and n = 300 months. You pay in €1,000 + €200 × 300 = €61,000. The final value comes to about €143,757, of which €82,757 is return: 57.6% of the final value.
This assumes a constant rate of return. In reality it fluctuates a lot. The result shows a rough order of magnitude, not a forecast.
The full explanation is in the lesson Goal and time horizon.
Frequently asked questions
How long does it take to double your money?
As a rule of thumb: 72 divided by the interest rate. At 8 percent annual return, doubling takes about nine years.
Why is the gain in the final years the largest?
Because the annual gain is calculated on capital that's already grown, and so it grows exponentially in its own right: not linearly.
Why do fees have such a strong effect over long periods?
Because they use the same exponential mechanism, just against you: even 1 percent less in annual costs can mean a five-figure difference in the final value over decades.
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