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Understanding the Power of Compound Interest

Work through these steps in order.

  1. Simple interest vs. compound interest

    Simple interest is calculated only on the original principal, while compound interest adds each period's interest back onto the principal, so future interest is calculated on a growing balance.

  2. How the compounding effect grows with time

    Compound interest looks similar to simple interest at first, but as the investment period stretches out, the effect of interest earning interest builds up, and the gap between the two widens more and more.

  3. Using the Rule of 72 to estimate doubling time

    Dividing 72 by the annual rate of return gives a rough estimate of how many years it takes for the principal to roughly double β€” a quick approximation, not an exact formula.

  4. Why reinvestment matters

    To capture the full benefit of compounding, interest or dividends need to be reinvested rather than withdrawn each time they're paid out.

  5. Why starting early matters

    For the same amount invested, starting earlier gives compounding more time to work, which can meaningfully change the final result compared with starting later.

  6. A note before investing

    This is general educational material explaining the basic principle of compound interest, not investment advice β€” actual returns depend heavily on the investment itself and market conditions.

Why long-term investing gets emphasized so often

Understanding how compounding works makes clear that how long you stay invested can matter more to the outcome than any single period's return. This is general financial education about the basic principle of compounding, not investment advice, and actual returns will vary depending on what you invest in and market conditions.

A simple example of compounding in action

Investing $1,000 at a steady 7% annual return grows to roughly $1,967 after 10 years, about $3,870 after 20 years, and around $7,612 after 30 years β€” the same starting amount and rate, but the last ten years alone add nearly as much as the first twenty combined. That's the practical shape of the "interest earning interest" effect described above.

Frequently Asked Questions

Does compounding work the same way on losses?

Yes β€” compounding applies the same underlying math to losses as it does to gains, so a loss on top of an existing loss can require an even larger percentage gain just to get back to even.

Is the Rule of 72 an exact calculation?

No, it's a simplified approximation for quickly estimating compound growth β€” for a precise figure, use an actual compound interest calculation.