In short: Einstein probably never said compound interest is the eighth wonder of the world, but the numbers make the point anyway: small monthly deposits plus time beat large late deposits almost every time.
Updated August 13, 2026, after a fresh review, with updated visuals and links.
Play with the tool below: change the years slider and watch the curve do its thing. The examples below show why starting early wins.
Why time matters more than amount
Compounding pays interest on interest: each cycle's gains join the pot and earn their own gains next cycle. Growth bends upward, slowly at first, then dramatically.
A concrete comparison: $200 a month at 7% for 30 years becomes about $244,000. The same contributions for only 20 years reach about $104,000. Ten extra years more than doubled the result, with just $24,000 more deposited.
Frequency matters at the margins: monthly compounding edges out yearly compounding at the same stated rate, because the interest starts earning sooner.
Worth remembering
- Automate the deposit the day after payday: willpower is the most fragile part of any savings plan.
- Total market index funds are the boring classic the 7% example is modeled on; inflation-adjusted returns over long spans historically sit near that neighborhood.
Frequently asked
What is compound interest in simple words?
Interest earning interest: instead of paying out gains, you reinvest them, so each period grows from a bigger base than the last.
How much will $10,000 grow in 10 years?
At 7% compounded monthly with nothing added, about $20,100. With $200 added monthly, about $54,700. The monthly habit outweighs the starting pile.
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