Understanding compound interest
Compound interest is often described as an almost magical force. The reality is simpler, and more interesting: it is a repeated arithmetic operation. Understanding how it works mainly helps you read your own wealth correctly, and avoid misjudging what to expect from it.
Simple interest and compound interest
With simple interest, gains are always calculated on the starting amount. Put 1,000 aside at 4% a year and you receive 40 every year, indefinitely.
With compound interest, each period's gains are added to the capital and go on to produce gains of their own. In the second year, the 4% applies not to 1,000 but to 1,040. In the third, to 1,081.60. The gap looks trivial at first: that is exactly what makes the mechanism counter-intuitive.
Duration matters more than the rate
The effect comes not from the return itself but from its repetition. Capital growing steadily takes a certain time to double, then takes the same time to double again. Each doubling applies to a base larger than all the previous ones combined.
That is why most of the growth sits at the end of the period rather than the beginning. A projection cut short gives a misleading picture of the mechanism.
A worked example, to be read as an illustration
Take 200 set aside every month for 20 years, with a hypothetical 5% annual growth. The payments alone add up to 48,000. With compounding, the capital would reach roughly 82,000.
This is an arithmetic illustration, not a forecast: it assumes a constant rate, which no investment guarantees. Real returns vary, sometimes negatively, and past performance says nothing about the future.
What works against it
Three things pull the other way, and they compound too.
Fees first: an annual fee reduces the base on which everything else is calculated, year after year. Then inflation: it erodes purchasing power, so nominal growth can hide real stagnation. Finally withdrawals: taking capital out also removes every future gain it would have produced.
Observing it on your own wealth
The theory is easy to check against real figures. In Finvygo, your net worth chart shows the growth actually recorded, payments included, rather than a projection.
The compound interest simulator is there to explore assumptions: vary the duration, the amount paid in or the rate used, and see how the result shifts. It is a tool for understanding, not for predicting.
Compound interest is neither a promise nor a guarantee: it is a calculation whose effect depends entirely on duration, consistency and the fees involved. The best way to form an accurate view of it remains to look at your own figures.
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