What is compound interest and how does it work?
Interest on your interest. A one-sentence definition, a worked example, and the three dials (rate, time, consistency) that decide which way it runs.
By Priya Raman Investing and Savings Writer· Updated Sep 6, 2026· Last reviewed Sep 6, 20262 min read0 views
- Compound interest is interest earned on past interest; the snowball that grows a balance faster every year.
- Time is the dominant dial; the same money started earlier outruns bigger money started later.
- The same machinery runs against you on credit-card balances; cheap debt matters for this reason.
Compound interest is interest earned on interest; a snowball that makes a balance grow faster every year it survives. On investments it's the quiet engine of wealth; on debt it's the quiet engine of ruin. Which direction it runs is mostly a timing problem.
The best way to believe it isn't a definition; it's five minutes with a compounding calculator watching year three spin year four. Below is the worked version, tied to the three dials you actually control.
The example that explains everything
Start with $1,000 earning 7% a year. Year one adds $70. Year two adds 7% of $1,070 (about $75. Year three, 7% of $1,145) about $80. The extra few dollars each year look tiny; the pattern is the point. Leave it ten years and you're near $2,000; doubled, with interest doing part of the work. Leave it thirty and the earnings dwarf what you put in.
The three dials, ranked
- Time; the multiplier that beats everything. Start at 25 and the same percentage beats starting at 40, even with less total contributed.
- Consistency; automation; money in every market, up years and down. The down years are where compounding buys its cheap shares.
- Rate; important, but mostly set by the broad low-cost market average you choose. Hunting for rate usually subtracts from it.
Divide 72 by your annual rate to estimate doubling time: 72 ÷ 7% is about ten years; 72 ÷ 20% (a card) is about three and a half. Same machinery, whichever side of the ledger you're on.
Why it works against you so quietly
Debt compounds on a tighter schedule (daily or monthly statements) so a 22% card balance grows noticeably, fast. This is why 'pay the high-interest debt, invest the rest' is the whole debt-versus-invest answer, and why the cheapest time to start compounding on your side was yesterday.
Related reading: how much to invest monthly once you're compounding, and the 401(k)-versus-IRA frame for where to let it snowball.
Compound interest isn't a hack or a loophole; it's arithmetic, paid in the coin of patience.
FAQ
Frequently asked questions
What is compound interest in simple words?
Interest earned on interest. Year one you earn on your original money; year two you earn on the original plus last year's earnings, so growth feeds on itself. Left alone it snowballs; interrupted, each new start is a smaller snowball.
Why is time so important in compounding?
Because the snowball doubles through years, not amounts. Growth at 7% doubles money in about a decade. Money invested at 25 has roughly double the decades of money invested at 35; the extra decade is worth more than extra dollars.
Does compound interest hurt me with debt?
Exactly the same math, inverted. Credit cards compound against you monthly or daily, so at 20%+ a carried balance doubles in a few years if untouched. That's why the debt-payoff order starts with the toxic balances.
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