Compound Interest, Plainly Explained
The mechanism is simple enough to explain in a paragraph. What is hard is believing how much of the outcome it controls.
Compound interest is interest that earns interest. That is the entire idea, and it fits in one sentence. The reason it is worth writing about anyway is that the consequences of that sentence are consistently underestimated, including by people who can state it correctly.
The Mechanism
If you put money somewhere that pays 7% a year, then after one year you have 7% more. After the second year you do not have 14% more — you have 7% more than you had at the end of the first year, which is 14.49%.
The extra 0.49% is interest on the interest. It is small. It stays small for a long time. Then it stops being small.
Why The Early Years Feel Pointless
Compounding is not linear and it is not even close to linear at the start. For the first several years, the growth is dominated by what you contribute, not by what the contributions earn. This is the part that breaks people's resolve: the chart looks like a straight line, so it feels like the strategy is not working.
It is working. The line is a curve, and it is shaped like one everywhere except where you are standing.
The first decade of compounding is a rumour. The third is arithmetic you can no longer ignore.
The Two Variables That Dominate
Given a fixed contribution schedule, the outcome is governed mostly by:
- The rate, which is the part people argue about and can least control.
- The time, which is the part people control entirely and argue about least.
Time is also the only one of the two that cannot be bought later. Every year spent waiting is a year removed from the end of the curve, where nearly all of the growth is.
What It Does Not Mean
Compounding is not a claim that returns are guaranteed, or that volatility does not exist. A sequence of bad years early is meaningfully worse than the same years later, and no amount of patience fixes that arithmetic.
It is a claim about the shape of the thing: that small amounts, left alone for a long time, behave in a way that small amounts left alone for a short time do not. Nothing about that is intuitive. That is why it keeps needing to be explained.