Understanding Compound Interest

What compound interest is

Simple interest is earned only on your original money; compound interest is earned on your money plus the interest it has already earned, so it grows on itself. Over a year or two the difference is small, but over decades it's the gap between a modest sum and a large one. Our compound growth calculator shows the curve for your own numbers, splitting what you contribute from what growth adds.

The Rule of 72

A quick way to grasp compounding is the Rule of 72: divide 72 by your annual return to estimate the years it takes money to double. At 8% that's about nine years; at 6%, twelve. The Rule of 72 calculator makes this instant. It also shows why the rate you earn matters so much, and how inflation, at 3%, halves your money's purchasing power in about 24 years.

Time beats timing

The biggest driver of compound growth isn't picking the perfect moment to invest, it's the number of years your money stays invested. Because each year's gains earn gains of their own, the curve bends upward most in the later years. That's why starting early is so powerful and why trying to time the market usually backfires. Staying invested through downturns, and contributing when prices are lower, captures the compounding that market-timers miss.

Fees are compounding in reverse

The same math that rewards a higher return punishes fees, because a 1% annual fee is a 1% lower return every year, compounded. Over a lifetime, a small expense ratio can consume a quarter or more of your final balance. This is why low-cost index funds have become the default for long-term investors, minimizing costs is one of the few reliable ways to keep more of the growth compounding for you.

Reinvest to keep it compounding

Compounding only works if earnings stay invested. Reinvesting dividends and interest automatically buys more shares or adds to the balance, so those earnings begin compounding too, historically a large share of total stock-market returns. Spending the income instead breaks the chain. Setting investments to reinvest automatically, and leaving them alone, is one of the simplest ways to let compounding do its work.

Invest steadily and stay the course

Most people build wealth not with a lump sum but by investing a fixed amount on a regular schedule, dollar-cost averaging, which buys more shares when prices are low and removes the temptation to time the market. The harder part is behavioral: staying invested through scary headlines is what lets compounding work. Automating contributions and checking balances rarely both help you leave the account alone for the decades compounding needs.

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