How Compound Interest Works

Albert Einstein supposedly called compound interest the eighth wonder of the world. Whether or not he said it, the math is genuinely powerful — and it's the single most important idea in long-term saving and investing.

Simple vs compound

Simple interest is paid only on your original amount. Put $1,000 in at 5% simple interest and you earn $50 every year — forever $50.

Compound interest is paid on your original amount plus all the interest already earned. Year one you earn $50. Year two you earn 5% on $1,050 — $52.50. Year three, more again. The balance grows by a larger amount each period because the base keeps getting bigger.

See it move with your own numbers in the compound interest calculator.

Time is the real engine

The striking part isn't the rate — it's the years. Because growth builds on itself, money invested early does far more work than money invested later. The U.S. Securities and Exchange Commission's investor education site, Investor.gov, has a calculator and a clear primer showing how the curve bends upward over decades.

A rule of thumb that captures it: the Rule of 72. Divide 72 by your annual return to estimate the years it takes money to double. At 8%, that's about 9 years; at 4%, about 18.

What makes it work in real life

Three levers, in order of impact for most people:

1. Start early. A decade of head start often beats a larger amount invested later.
2. Keep contributing. Regular monthly additions compound too, and smooth out market ups and downs.
3. Keep costs low. Fees compound against you. Low-cost index funds keep more of the growth in your pocket — the SEC explains how fees erode returns over time.

The flip side

Compounding works just as hard against you on debt. Credit card interest compounds monthly, which is why a balance can feel like it never shrinks — see the credit card payoff calculator. The same force that builds wealth can quietly drain it.

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This is general information, not investment advice. Investment returns vary and are not guaranteed.