Albert Einstein famously called compound interest the 'eighth wonder of the world: he who understands it, earns it; he who doesn't, pays it.' While simple interest calculates returns solely on the initial principal, compound interest reinvests earned returns so that future interest is calculated on a continuously expanding sum.

Simple Interest vs Compound Interest

If you invest $10,000 at an 8% annual return:

  • With Simple Interest: You earn a flat $800 each year. After 25 years, you have your $10,000 principal plus $20,000 in interest ($30,000 total).
  • With Compound Interest: In year one you earn $800. In year two, you earn 8% on $10,800 ($864). After 25 years, your investment grows to over $68,480—more than double the simple interest return!

The Critical Element: Time

Because the compounding curve is exponential, the lion's share of growth occurs in the latter decades. Starting to invest $200 a month at age 25 yields substantially more wealth by retirement than investing $500 a month starting at age 40.