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How compounding actually works — and what quietly cancels it

Compounding is not magic; it is arithmetic that rewards time and punishes leaks. Here is what each leak costs over a couple of decades.

By Sankha NagChoudhurySeptember 8, 20267 min read

People are told to 'let compounding work' without ever seeing what makes it work or what breaks it. Three things drive the result: how much you put in, how long it stays in, and how much leaks out along the way. Only the third is usually ignored.

Time does the heavy lifting

Growth on growth is slow at the start and then obvious. The early years feel like nothing is happening because contributions dominate; the later years feel dramatic because prior growth dominates. That shape is why starting earlier beats contributing more later, and why a pause in your thirties costs more than it looks.

  • Contributions dominate the first decade; growth dominates after that.
  • A gap of a few years early is not just those years' contributions — it is their growth too.
  • Automatic, boring contributions beat perfectly timed occasional ones.

The leaks

Every percentage point that leaves the account compounds against you exactly the way returns compound for you. Fees, taxes on frequent trading, and cashing out during downturns are the three common leaks, and they are largely controllable.

  • Fund costs come out every year, in good markets and bad.
  • Frequent selling turns unrealized growth into taxable events, often at short-term rates.
  • Selling during a decline converts a temporary drop into a permanent loss.
  • Account type matters: tax-advantaged accounts shelter growth, taxable accounts do not.

What to actually do with this

Once you see the arithmetic, the behavioral rules stop being slogans. Keep costs low, keep the money invested, and keep the money you may need in the next few years out of the market entirely so you are never forced to sell.

  • Know what you are paying in total fund costs each year.
  • Automate contributions so they do not depend on how you feel that month.
  • Match the account to the goal and the time horizon.
  • Run your own numbers in the free planner rather than trusting a round example.

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Educational guidance, not personal advice. Outputs are illustrative, may contain errors, and should be independently verified before material decisions.

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