Compound interest is interest calculated on both the money you originally invested and the interest that money has already earned. Instead of earning a flat amount every year, your gains start generating their own gains, which is why compound growth accelerates over time instead of moving in a straight line.
Here’s a simple example: you invest $10,000 at a 7% annual return. After year one, you have $10,700. In year two, you earn 7% on the full $10,700, not just the original $10,000, so you end up with $11,449. That extra $49 compared to simple interest doesn’t sound like much yet, but over 20 or 30 years, this snowball effect becomes the single biggest driver of long-term growth.
Three things determine how much compounding works in your favor: the rate of return, how often interest is compounded (annually, monthly, or daily), and time. Time matters more than most people expect. Someone who invests for 30 years at a modest rate can end up with more money than someone who invests a larger amount for only 10 years at a higher rate, simply because compounding needs time to build momentum.
Compounding works the same way in reverse for debt: credit card balances and loans grow the same way if interest isn’t paid off, which is why high-interest debt can escalate so quickly.
Want to see how your own numbers grow over time? Try our Compound Interest Calculator to run the math instantly.
Official Source and Assumptions
Investor.gov describes compound interest as interest earned on principal and previously accumulated interest and provides an official Compound Interest Calculator. Examples here assume a fixed rate and regular compounding; real investment returns are not guaranteed and may fluctuate or be reduced by fees and taxes.